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Helium compounds
Helium compounds
Helium compounds
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Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions.[1] Helium's first ionization energy of 24.57 eV is the highest of any element.[2] Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero.[2] The helium atom is small with the radius of the outer electron shell at 0.29 Å.[2] Helium is a very hard atom with a Pearson hardness of 12.3 eV.[3] It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point (4.2 K) of any known substance.

Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen.

Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionised helium (He+), also known as He II, is a very high energy material able to extract an electron from any other atom. He+ has an electron configuration like hydrogen, so as well as being ionic it can form covalent bonds. Excimers do not last for long, as the molecule containing the higher energy level helium atom can rapidly decay back to a repulsive ground state, where the two atoms making up the bond repel. However, in some locations such as helium white dwarfs, conditions may be suitable to rapidly form excited helium atoms. The excited helium atom has a 1s electron promoted to 2s. This requires 1,900 kilojoules (450 kcal) per gram of helium, which can be supplied by electron impact, or electric discharge.[4] The 2s excited electron state resembles that of the lithium atom.

Known solid phases

[edit]

Most solid combinations of helium with other substances require high pressure. Helium does not bond with the other atoms, but the substances can have a well defined crystal structure.[citation needed]

Disodium helide

[edit]
Main article: Disodium helide

Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above 113 gigapascals (1,130,000 bar). Disodium helide was first predicted[5] using USPEX code and was first synthesised in 2016.[2][6] It was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa. Na2He has a cubic crystal structure, resembling fluorite. At 300 GPa the edge of a unit cell of the crystal has a = 3.95 Å. Each unit cell contains four helium atoms on the centre of the cube faces and corners, and eight sodium atoms at coordinates a quarter cell in from each face. Double electrons (2e−) are positioned on each edge and the centre of the unit cell.[7] Each pair of electrons is spin paired. The presence of these isolated electrons makes this an electride. The helium atoms do not participate in any bonding. However the electron pairs can be considered as an eight-centre two-electron bond.[2] Disodium helide is predicted to be an insulator and transparent.[2]

Silicates

[edit]

Helium was first observed to enter into a silicate in 2007. The mineral melanophlogite is a natural silica clathrate (clathrasil) that normally would contain carbon dioxide, methane or nitrogen. When compressed with helium, a new clathrate forms. This has a much higher bulk modulus, and resists amorphization. Helium was taken up around 17 GPa, enlarging the unit cell, and given off again when pressure dropped to 11 GPa.[8]

Cristobalite He II (SiO2He) is stable between 1.7 and 6.4 GPa. It has a rhombohedral space group R-3c with unit cell dimensions a = 9.080 Å, α = 31.809° and V = 184.77 Å3 at 4 GPa.[9]

Cristobalite He I (SiO2He) can be formed under higher helium pressures over 6.4 GPa. It has a monoclinic space group P21/C with unit cell dimensions a = 8.062 Å, b = 4.797 Å, c = 9.491 Å, β = 120.43° and V = 316.47 Å3 at 10 GPa.[10]

Helium penetrates into fused silica at high pressure, reducing its compressibility.[11]

Chibaite, another natural silica clathrate has its structure penetrated by helium under pressures higher than 2.5 GPa. The presence of guest hydrocarbons does not prevent this happening. Neon requires a higher pressure, 4.5 GPa to penetrate, and unlike helium shows hysteresis.[12] Linde-type A zeolites are also rendered less compressible when penetrated by helium between 2 and 7 GPa.[13]

Arsenolite helium inclusion compound

[edit]

Arsenolite helium inclusion compound As4O6·2He is stable from pressures over 3 GPa and up to at least 30 GPa.[14] Arsenolite is one of the softest and most compressible minerals.[15] Helium prevents amorphization that would otherwise occur in arsenolite under pressure.[16] The solid containing helium is stronger and harder, with a higher sound velocity than plain arsenolite.[17] The helium that is included into the crystal causes a more uniform stress on the As4O6 molecules. No actual bond is formed from arsenic to helium despite the lone pairs of electrons available.[18] The diffusion of helium into arsenolite is a slow process taking days at a pressure around 3 GPa. However, if the pressure on the crystal is too high (13 GPa) helium penetration does not take place, as the gaps between arsenolite molecules become too small.[18] Neon does not diffuse into arsenolite.[18]

Perovskites

[edit]

Helium can be inserted into the A sites of negative thermal expansion perovskites that otherwise have defects at the A site. At room temperature and 350 MPa helium is included into CaZrF6 to expand its unit cell yielding HeCaZrF6. About half of the A sites are filled by helium atoms. This substance loses helium over several minutes on depressurisation at ambient temperature, but below 130 K it retains helium when depressurised.[19] At 1 GPa all the A sites are filled by helium, yielding He2CaZrF6.[20]

Formates

[edit]

Under pressure helium penetrates dimethylammonium iron formate (CH3)2NH2Fe(HCOO)3. It affects this by causing a change to a monoclinic ordered state at a lower pressure (around 4 GPa) than if helium were absent.[21]

Small molecule

[edit]

He(N2)11 is a van der Waals compound with hexagonal crystals. At 10 GPa the unit cell of 22 nitrogen atoms has a unit cell volume of 558 Å3, and about 512 Å3 at 15 GPa. These sizes are around 10 Å3 smaller than the equivalent amount of solid δ-N2 nitrogen at these pressures. The substance is made by compressing nitrogen and helium in a diamond anvil cell.[22][23]

NeHe2 has a crystal structure of hexagonal MgZn2 type at 13.7 GPa. The unit cell has dimensions a = 4.066 Å, c = 6.616 Å; and at 21.8 GPa, a = 3.885 Å, c = 6.328 Å. There are four atoms in each unit cell. It melts at 12.8 GPa and 296 K,[24] stable to over 90 GPa.[25]

Clathrates

[edit]

Helium clathrates only form under pressure. With ice II at pressures between 280 and 480 MPa a solid helium hydrate with He:H2O ratio of 1:6 exists.[26] Another clathrate with a water to helium ratio of 2.833 has been made in the SII clathrate structure. It has two different cages in the ice, the small one can contain one helium atom, and the large can contain four atoms. It was produced from neon clathrate that lost its neon, and then replaced by helium at 141 K and 150 MPa[27] Other helium hydrates with the ice-Ih, ice-Ic 1:1, and ice-Ic 2:1 He to H2O ratio have been predicted.[26] These could exist in planets like Neptune or Uranus.[27] Helium clathrate hydrates should be similar to hydrogen clathrate due to the similar size of the hydrogen molecule.[27]

Helium may enter into crystals of other molecular solids under pressure to alter their structure and properties. For example, with chlorpropamide over 0.3 GPa in helium changes to a monoclinic structure, and yet another structural form at 1.0 GPa.[28]

Fullerites

[edit]

Helium can form intercalation compounds with the fullerites, including buckminsterfullerene C60 and C70. In solid C60 there are spaces between the C60 balls, either tetrahedral or octahedral in shape. Helium can diffuse into the solid fullerite even at one atmosphere pressure. Helium enters the lattice in two stages. The first rapid stage takes a couple of days, and expands the lattice by 0.16% (that is 2.2 pm) filling the larger octahedral sites. The second stage takes thousands of hours to absorb more helium and expands the lattice twice as much again (0.32%) filling the tetrahedral sites. However the solid C60•3He is not stable and loses helium on a timescale of 340 hours when not under a helium atmosphere. When the helium intercalated fullerite is cooled, it has an orientational phase transition that is 10 K higher than for pure solid C60. The actual discontinuous change in volume at that point is smaller, but there are more rapid changes near the transition temperature, perhaps due to varying occupancy of the voids by helium.[29][30]

Endohedral

[edit]

Helium atoms can be trapped inside molecular cages such as the fullerenes He@C60, He@C70, He2@C60 and He2@C70 have all been made using compressed helium and fullerenes.[31] When using only pressure and heat, the yield is quite low, under 1%. However, by breaking and reforming the carbon ball, much higher concentrations of He@C60 or He@C70 can be made. High-performance liquid chromatography can concentrate the helium containing material. HeN@C60 and HeN@C70 have also been made. These have a lower symmetry due to the two atoms being trapped together in the same cavity. This causes ESR line broadening.[32]

Dodecahedrane can trap helium from a helium ion beam to yield He@C20H20.

Other cage like inorganic or organic molecules may also trap helium, for example C8He with He inside a cube,[33] or He@Mo6Cl8F6.[34]

Impurity helium condensates

[edit]

Impurity helium condensates (IHCs) (or impurity helium gels)[35] are deposited as a snow-like gel in liquid helium when various atoms or molecules are absorbed on the surface of superfluid helium. Atoms can include H, N, Na, Ne, Ar, Kr, Xe, alkalis or alkaline earths. The impurities form nanoparticle clusters coated with localised helium held by van der Waals force. Helium atoms are unable to move towards or away from the impurity, but perhaps can move perpendicularly around the impurity.[36] The snow like solid is structured like an aerogel. When free atoms are included in the condensate a high energy density can be achieved, up to 860 J cm−1 or 5 kJ g−1.[37] These condensates were first investigated as a possible rocket fuel.[38] The mixtures are given a notation involving square brackets so that [N]/[He] represents a nitrogen atom impurity in helium.[citation needed]

[N]/[He] atomic nitrogen impurity helium is produced when a radio frequency discharge in a nitrogen helium mixture is absorbed into superfluid helium, it can have up to 4% nitrogen atoms included.[39] The substance resembles crumbly snow and condenses and settles from the liquid helium.[39] It also contains variable proportions of N2 molecules.[39] This substance is a high energy solid, with as much power as conventional explosives. When it is heated above 2.19 K (the lambda point of helium), the solid decomposes and explodes.[39] This substance is not a true compound, but more like a solid solution.[36] E. B. Gordon et al. suggested that this material may exist in 1974.[39] The localised helium shells around an individual atom are termed van der Waals spheres.[39] However the idea that the nitrogen atoms are dispersed in the helium has been replaced by the concept of nitrogen atoms attached to the surface of clusters of nitrogen molecules. The energy density of the solid can be increased by pressing it.[40]

Other inert gas impurity helium condensates can also be made from a gas beam into superfluid helium.[41] [Ne]/[He] decomposes at 8.5 K with release of heat and formation of solid neon. Its composition approximates NeHe16.

[Ar]/[He] contains 40–60 helium atoms per argon atom.[42]

[Kr]/[He] contains 40–60 helium atoms per krypton atom[42] and is stable up to 20 K.[37]

[Xe]/[He] contains 40–60 helium atoms per xenon atom.[42]

[N2]/[He] contains 12—17 He atoms per N2 molecule.[42] It is stable up to 13 K[37]

[N]/[Ne]/[He] Formed from a gas beam generated from a radio-frequency electric discharge in mixtures of neon, nitrogen and helium blown into superfluid He. Additional inert gas stabilises more nitrogen atoms. It decomposes around 7 K with a blue green light flash.[41] Excited nitrogen atoms in the N(2D) state can be relative long lasting, up to hours, and give off a green luminescence.[41]

[H2]/[He], or [D2]/[He] when dihydrogen or dideuterium is absorbed into superfluid helium, filaments are formed. When enough of these form, the solid resembles cotton, rather than snow.[43] Using H2 results in the product floating and stopping further production, but with deuterium, or a half-half mixture, it can sink and accumulate.[37] Atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling (H + H → H2). Atomic deuterium dimerises slower (D + D → D2), but reacts very quickly with any diprotium present. (D + H2 → HD + H).[37] Atomic hydrogen solids are further stabilised by other noble gases such as krypton.[44][45][46] Lowering temperatures into the millikelvin range can prolong the lifetime of atomic hydrogen condensates.[38] Condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons.[35] Other impurity gels have been investigated for producing ultracold neutrons include CD4 (deuterated methane) and C2D5OD. (deuterated ethanol)[47]

The water-helium condensate [H2O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm.[48]

Oxygen O2 impurity helium contains solid oxygen clusters from 1 to 100 nm.[49]

Impurity solid helium

[edit]

Introducing impurities into solid helium yields a blue solid that melts at a higher temperature than pure He.[50] For cesium the absorption has a peak at 750 nm, and for rubidium, maximal absorption is at 640 nm. These are due to metal clusters with diameters of 10 nm or so. However the low concentration of clusters in this substance should not be sufficient to solidify helium as the amount of metal in the solid is less than billionth that of the impurity helium condensate solids, and liquid helium does not "wet" cesium metal. The solid is possibly due to helium snowballs attached to Cs+ (or Rb+) ions.[50] The snowball is a shell that contains helium atoms solidified in particular positions around the ion. The helium atoms are immobilized in the snowball by polarization. Neutral metallic atoms in liquid helium are also surrounded by a bubble caused by electron repulsion. They have typical sizes ranging from 10 to 14 Å diameter.[51] Free electrons in liquid helium are enclosed in a bubble 17 Å in diameter. Under 25 atmosphere pressure an electron bubble reduces to 11 Å.[52]

Solid solution

[edit]

Helium can dissolve to a limited extent in hot metal, with concentration proportional to pressure. At atmospheric pressure, 500 °C bismuth can absorb 1 part in a billion; at 649 °C lithium can take 5 parts per billion; and at 482 °C potassium can take 2.9 parts per million (all atom fractions).[53] In nickel there can be 1 in 1010 atoms, and in gold 1 in 107. The supposition is that the higher the melting point the less helium can be dissolved. However, when a liquid metal is quenched, higher concentrations of helium can be left dissolved. So cooled liquid steel can have one part per million of helium. In order to get a helium atom into a metal lattice, a hole has to be formed. The energy to make that hole in the metal is basically the heat of solution.[54]

Nanowires

[edit]

Gold, copper, rubidium, caesium, or barium atoms evaporated into liquid helium form spiderweb-like structures.[55] Rhenium produces nano flakes. Molybdenum, tungsten, and niobium produce thin nanowires with diameters of 20, 25 and 40 Å.[56] When platinum, molybdenum or tungsten is evaporated into liquid helium, nanoclusters are first formed, accompanied by high temperature thermal emission pulse, above the melting point of the metals. In superfluid helium, these clusters migrate to the vortices and weld together to yield nanowires once the clusters are mostly solid. In higher temperature liquid helium, larger clusters of metal are formed instead of wires. The metal vapours can only penetrate about 0.5 mm into liquid helium.[57] Indium, tin, lead and nickel produce nanowires about 80 Å in diameter.[58] These same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope.[59] Copper, permalloy, and bismuth also make nanowires.[60]

Two-dimensional ionic crystal

[edit]

Helium II ions (He+) in liquid helium when attracted by an electric field can form a two-dimensional crystal at temperatures below 100 mK. There are about half a trillion ions per square meter just below the surface of the helium. Free electrons float above the helium surface.[61]

Known van der Waals molecules

[edit]
  • LiHe[62]
  • Dihelium
  • Trihelium
  • Ag3He[63]
  • HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common.[64]
  • CF4He and CCl4He both exist.[65]
  • HeI2 can be formed by supersonic expansion of high pressure helium with a trace of iodine into a vacuum. It was the first known triatomic helium van der Waals molecule. It can be detected by fluorescence. HeI2 has a similar optical spectrum to I2, except that the bands and lines are shifted to form two extra series. One series is blueshifted by between 2.4 and 4.0 cm−1, and the other between 9.4 and 9.9 cm−1. The two series may be due to different amounts of vibration in the He–I bond. The lines are narrow indicating that the molecules in their excited vibrational state have a long lifetime.[66]
  • Na2He molecules can form on the surface of helium nanodroplets.[67]
  • NOHe[68]

Known ions

[edit]

Helium has the highest ionisation energy, so a He+ ion will strip electrons off any other neutral atom or molecule. However it can also then bind to the ion produced. The He+ ion can be studied in gas, or in liquid helium. Its chemistry is not completely trivial. For example, He+ can react with SF6 to yield SF+
6 or SF+
5 and atomic fluorine.[69]

Ionised clusters

[edit]

He+
2 was predicted to exist by Linus Pauling in 1933. It was discovered when doing mass spectroscopy on ionised helium. The dihelium cation is formed by an ionised helium atom combining with a helium atom: He+ + He → He+
2.[70]

The diionised dihelium He2+
2 (1Σ+
g) is in a singlet state. It breaks up He2+
2 → He+ + He+ releasing 200 kcal/mol of energy. It has a barrier to decomposition of 35 kcal/mol and a bond length of 0.70 Å.[70]

The trihelium cation He+
3[71] is in equilibrium with He+
2 between 135 and 200K.[72]

Helium hydride

[edit]

The helium hydride ion HeH+ has been known since 1925.[70] The protonated dihelium ion He2H+ can be formed when the dihelium cation reacts with dihydrogen: He+
2 + H2 → He2H+ + H. This is believed to be a linear molecule.[70] Larger protonated helium cluster ions exist HenH+ with n from 3 to 14. He6H+ and He13H+ appear to be more common. These can be made by reacting H+
2 or H+
3 with gaseous helium.[70]

HeH2+ is unstable in its ground state. But when it is excited to the 2pσ state the molecule is bound with an energy of 20 kcal/mol. This doubly charged ion has been made by accelerating the helium hydride ion to 900 keV, and firing it into argon. It only has a short life of 4 ns.[70]

H2He+ has been made and could occur in nature via H2 + He+ → H2He+.[70]

H3He+
n exists for n from 1 to over 30, and there are also clusters with more hydrogen atoms and helium.[73]

Noble gas

[edit]

Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula HenXe+
m, where n and m ≥ 1.[74]

Many different HenKr+ exist with n between 1 and 17, with higher values possible. HenKr+
2 and HenKr+
3 also exist for many values of n. He12Kr+
2 and He12Kr+
3 ions are common. These singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation.[74]

The Ar+ argon ion can form many different sized clusters with helium ranging from HeAr+ to He50Ar+, but the most common clusters are He12Ar+ and smaller. These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionising with high speed electrons. He+ is formed, which can transfer charge to argon and then form a cluster ion when the rest of the droplet evaporates.[75]

NeHe+
n can be made by ultraviolet photoionisation. Clusters only contain one neon atom. The number of helium atoms can vary from 1 to 23, but NeHe+
4 and NeHe+
8 are more likely to be observed.[74]

Doubly charged ions of helium with noble gas atoms also exist including ArHe2+, KrHe2+, and XeHe2+.[76]

Metals

[edit]

Various metal-helium ions are known.

Alkali metal helide ions are known for all the alkalis. The molecule ground state for the diatomic ions is in the X1Σ+ state. The bond length gets bigger as the periodic table is descended with lengths of 1.96, 2.41, 2.90, 3.10, and 3.38 Å for Li+He, Na+He, K+He, Rb+He, and Cs+He. The dissociation energies are 1.9, 0.9, 0.5, 0.4 and 0.3 kcal/mol, showing bond energy decreases. When the molecule breaks up the positive charge is never on the helium atom.[70]

When there are many helium atoms around, alkali metal ions can attract shells of helium atoms. Clusters can be formed from absorbing metal into helium droplets. The doped droplets are ionised with high speed electrons. For sodium clusters appear with the formula Na+Hen with n from 1 to 26. Na+He is the most common, but Na+He2 is very close in abundance. Na+He8 is much more abundant than clusters with more helium. Na+
2Hen with n from 1 to 20 also appears. Na+
3Hen with small n is also made. For potassium, K+Hen with n up to 28, and K+
2Hen for n from 1 to 20 is formed. K+He and K+He2 are both common, and K+He12 is a bit more commonly formed than other similar sized clusters.[77] Cesium and rubidium cations also form clusters with helium.[77]

Other known metal-helium ions include Cr+He, Co+He, Co+He3, Ni+He, and Ni+He3.[70] PtHe2+;[78][79] formed by high electric field off platinum surface in helium,[76] VHe2+,[76] HeRh2+ is decomposed in high strength electric field,[80][81] Ta2+He, Mo2+He, W2+He, Re2+He, Ir2+He, Pt2+He2, W3+He2, W3+He3, and W3+He4.[70]

Nonmetals

[edit]

HeN+
2 can form at around 4 K from an ion beam of N+
2 into cold helium gas.[82] The energy needed to break up the molecule is 140 cm−1 which is quite a bit stronger than the van der Waals neutral molecules. HeN+
2 is tough enough to have several vibrational, bending and rotational states.[83] HenN+
2 with n from 2 to 6 have been made by shooting electrons at a supersonically expanding mix of nitrogen and helium.[70]

C60He+ is formed by irradiating C60 with 50eV electrons and then steering ions into cold helium gas. C60He+
2 is also known.[84]

He(OH)+ has been detected, although it is not produced when HTO (tritiated water) decays.[70]

He
n(CO)+
 has been detected for values of n from 1 to 12. Also CH3He+, OCHHe+ and NH2He+ have been detected.[70]

Young and Coggiola claimed to make HeC+ by an electric discharge off graphite into helium.[85]

When tritium substituted methane (CH3T) decays, CH3He+ is produced in a very small amount.[86]

The helium formyl cation, HeHCO+ is a linear molecule. It has a vibrational frequency red shifted 12.4 cm−1 compared to HCO+. It can be considered as a deenergized protonation reaction intermediate for the HeH+ + CO → HCO+ + He.[83] HeHCO+ can be produced by a supersonic expansion of a gas mixture of He, CO, and H2, which is hit by a cross beam of electrons. CO and H2 are only supplied at 1% of the helium.[83]

The HeHN+
2 molecule is linear. The He-H bondlength is 1.72 Å. It has an infrared band, due to B-H stretching, with a base at 3158.42 cm−1.[83][87] The binding energy is 378 cm−1 in the 000 vibrational state, and 431 cm−1 in the 100 vibrational state.[88] He2HN+
2 is also known. One helium atom is linked to a hydrogen, and the other is less tightly bound.[88]

H2O+, H2OSF5+, SF5+ and SF6+ can form clusters with varying numbers of Helium atoms.[89]

Excimers

[edit]

The He*
2 excimer is responsible for the Hopfield continuum. Helium also forms an excimer with barium, Ba+He*.[90]

Predicted compounds

[edit]

Predicted solids

[edit]
Crystal structure of the hypothetical compound MgF2He. Helium in white, magnesium in orange and fluorine in blue

He(H2O)2 is predicted to form a solid with orthorhombic structure Ibam.[91]

Iron helide (FeHe) was early on claimed to have been found,[92] but the discovery was classified as an alloy.[53] Early studies predicted the FeHe exists as an interstitial compound under high pressure,[93] perhaps in dense planetary cores,[94] or, as suggested by Freeman Dyson, in neutron star crust material.[95] Recent density functional theory calculations predict the formation of FeHe compounds at pressures above about 4 TPa,[96] suggesting indeed that these compounds could be found inside giant planets, white dwarf stars, or neutron stars.

Na2HeO is predicted to have a similar structure to Na2He, but with oxygen atoms in the same position as the electron pair, so that it becomes O2−. It would be stable from 13 to 106 GPa.[2] This substance could be a way to store helium in a solid.[97]

La2/3-xLi3xTiO3He is a porous lithium ion conduction perovskite that can contain helium like a clathrate.[33]

Helium is predicted to be included under pressure in ionic compounds of the form A2B or AB2. These compounds could include Na2OHe, MgF2He (over 107 GPa) and CaF2He (30-110 GPa). Stabilisation occurs by the helium atom positioning itself between the two like charged ions, and partially shielding them from each other.[98]

Helium is predicted to form an inclusion compound with silicon, Si2He. This has a hexagonal lattice of silicon atoms with helium atoms lined up in the channels. It should be formed when liquid silicon is injected with helium at over 1GPa and cooled.[99]

Helium is predicted to form a fluoride He3F2 under extreme pressures well over 1 Tpa.[100]

Predicted van der Waals molecules

[edit]

The beryllium oxide helium adduct, HeBeO is believed to be bonded much more strongly than a normal van der Waals molecule with about 5 kcal/mol of binding energy. The bond is enhanced by a dipole induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the helium.[101][102]

Variations on the beryllium oxide adduct include HeBe2O2,[102] RNBeHe including HNBeHe, CH3NBeHe,[102] CH4−xNBeHex, SiH4−xNBeHex, NH3−xNBeHex, PH3−xNBeHex, OH2−xNBeHex, SH2−xNBeHex,[103] and HeBe(C5H5)+.[104]

Hydridohelium fluoride HHeF is predicted to have a lifetime 157 femtoseconds 05 kcal/mol barrier[clarification needed].[105] The lifetime of the deuterium isotopomer is predicted to be much longer due to a greater difficulty of tunneling for deuterium.[106] This molecule's metastability is slated due to electrostatic attraction between HHe+ and F− which increases the barrier to an exothermic breakup.[101] Under pressures over 23 GPa HHeF should be stable.[107]

Calculations for coinage metal fluorides include HeCuF as stable,[105] HeAgF is unstable,[105] HeAuF is predicted,[105] and Ag3He with binding energy 1.4 cm−1,[108] Ag4He binding energy 1.85 cm−1, Au3He binding energy 4.91 cm−1,[108] and Au4He binding energy 5.87 cm−1[108]

HeNaO is predicted.

Calculation for binary van der Waals helium molecules include HeNe, Li4He binding energy 0.008 cm−1, the Li3He is not stable.[108] Na4He binding energy 0.03 cm−1, the Na3He is not stable.[108] Cu3He binding energy 0.90 cm−1,[108] O4He binding energy 5.83 cm−1,[108] S4He binding energy 6.34 cm−1,[108] Se4He binding energy 6.50 cm−1,[108] F4He binding energy 3.85 cm−1,[108] Cl4He binding energy 7.48 cm−1,[108] Br4He binding energy 7.75 cm−1,[108] I4He binding energy 8.40 cm−1,[108] N4He binding energy 2.85 cm−1,[108] P4He binding energy 3.42 cm−1,[108] As4He binding energy 3.49 cm−1,[108] Bi4He binding energy 33.26 cm−1,[108] Si4He binding energy 1.95 cm−1,[108] Ge4He binding energy 2.08 cm−1,[108] CaH4He binding energy 0.96 cm−1,[108] NH4He binding energy 4.42 cm−1,[108] MnH4He binding energy 1.01 cm−1,[108] YbF4He binding energy 5.57 cm−1[108] I4
2He or I3
2He,[109]

Bonds are predicted to form to nickel with helium as a weak ligand in HeNiCO and HeNiN2.[101]

(HeO)(LiF)2 is predicted to form a planar metastable molecule.[110] 1-Tris(pyrazolyl)borate beryllium and 1-tris(pyrazolyl)borate magnesium are predicted to bind helium at low temperatures.[111] There is also a prediction of a He-O bond in a molecule with caesium fluoride or tetramethyl ammonium fluoride.[112]

LiHe2 is predicted to be in an Efimov state when excited.[113]

Predicted ions

[edit]
Fluoroheliate ion

Many ions have been investigated theoretically to see if they could exist. Just about every diatomic cation with helium has been studied. For the diatomic dications, for stability the second ionisation level of the partner atom has to be below the first ionisation level of helium, 24.6 eV. For Li, F, and Ne the ground state is repulsive, so molecules will not form. For N and O the molecule would break up to release He+. However HeBe2+, HeB2+ and HeC2+ are predicted to be stable. Also second row elements from Na to Cl are predicted to have a stable HeX2+ ion.[70]

HeY3+ is predicted to be the lightest stable diatomic triply charged ion.[114] Other possibly thermochemically stable ions include HeZr3+, HeHf3+, HeLa3+, HeNd3+, HeCe3+, HePr3+, HePm3+, HeSm3+, HeGa3+, HeTb3+, HeDy3+, HeHo3+, HeEr3+, HeTm3+, and HeLu3+ where the third ionisation point is below that of helium.[70]

The positronium helide ion PsHe+ should be formed when positrons encounter helium.[115]

The Fluoroheliate FHeO− ion should be stable but salts like LiFHeO are not stable.[116][71]

  • HHeCO+ theoretical[117]
  • FHeS− is predicted to be stable.[118]
  • FHeBN−
  • HHeN2+ is unlikely to exist.[119]
  • (HHe+)(OH2) is probably unstable.[120]

The lithium hydrohelide cation HLiHe+ is linear in theory. This molecular ion could exist with big bang nucleosynthesis elements.[121] Other hydrohelide cations that exist in theory are HNaHe+ sodium hydrohelide cation, HKHe+ potassium hydrohelide cation, HBeHe2+ beryllium hydrohelide cation, HMgHe2+ magnesium hydrohelide cation, and HCaHe2+ calcium hydrohelide cation.[121]

HeBeO+ is predicted to have a relatively high binding energy of 25 kcal mol−1.[122] Similarly the ethynylhelium cation [HeC≡CH]+ is predicted to be bound (with respect to ethynyl cation and helium) by 22 kcal/mol. However, vinylhelium ([HeCH=CH2]+, 0.03 kcal/mol) and methylhelium ([HeCH3]+, 1.5 kcal/mol) cations are predicted to be bound much more weakly.[123]

For negative ions the adduct is very weakly bound.[70] Those studied include HeCl−, HeBr−, HeF−, HeO− and HeS−.[71]

  • FHeS−[71]
  • FHeSe−[71]
  • C7H6He2+[71]
  • C7H6HeHe2+[71]
  • FHeCC−[71]
  • HHeOH+
    2    [71]
  • HHeBF+[71]
  • HeNC+[71]
  • HeNN+[71]
  • HHeNN+ H-He 0.765 Å He-N bond length 2.077 Å. Decomposition barrier of 2.3 kJ/mol.[71]

HHeNH+
3 is predicted to have a C3v symmetry and a H-He bond length of 0.768 Å and He-N 1.830. The energy barrier against decomposition to ammonium is 19.1 kJ/mol with an energy release of 563.4 kJ/mol. Cleavage to helium hydride ion and ammonia is predicted to be endothermic, requiring 126.2 kJ/mol.[71]

Discredited or unlikely observations

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Numerous researchers attempted to create chemical compounds of helium in the early part of the twentieth century.[124] In 1895 L. Troost and L. Ouvrard believed they had witnessed a reaction between magnesium vapour and helium (and also argon) due to the spectrum of helium disappearing from the tube they were passing it through.[125] In 1906, W. Ternant Cooke claimed to have noticed a reaction of helium with cadmium or mercury vapour by observing an increase in the density of the vapour. Zinc vapour did not react with helium.[126]

J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925[127][128][129] HgHe10;[130][131] publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up.

Between 1925 and 1940 in Buenos Aires, Horacio Damianovich studied various metal–helium combinations including beryllium (BeHe), iron (FeHe), palladium (PdHe), platinum (Pt3He), bismuth, and uranium.[132][92] To make these substances, electrical discharges impacted helium into the surface of the metal.[4] Later these were demoted from the status of compounds, to that of alloys.[53]

Platinum helide, Pt3He was discredited by J. G. Waller in 1960.[133]

Palladium helide, PdHe is formed from tritium decay in palladium tritide, the helium (3He) is retained in the solid as a solution.

Boomer claimed the discovery of tungsten helide WHe2 as a black solid.[134] It is formed by way of an electric discharge in helium with a heated tungsten filament. When dissolved in nitric acid or potassium hydroxide, tungstic acid forms and helium escapes in bubbles. The electric discharge had a current of 5 mA and 1,000 V at a pressure between 0.05 and 0.5 mmHg for the helium. The process works slowly at 200 V. and 0.02 mmHg of mercury vapour accelerates tungsten evaporation by five times. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960.[133] Boomer also studied mercury, iodine, sulfur, and phosphorus combinations with helium. Mercury and iodine helium combinations decomposed around −70 °C[135] Sulfur and phosphorus helium combinations decomposed around −120 °C[135]

H. Krefft and R. Rompe claimed reactions between helium and sodium, potassium, zinc, rubidium, indium, and thallium.[139]

References

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Further reading

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Helium compounds

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Helium compounds are rare chemical entities in which the helium, renowned for its extreme chemical inertness due to a stable closed-shell , forms bonds with other elements, typically requiring extreme conditions such as high pressures or to overcome its reluctance to react. These compounds include simple ions like the helium hydride cation HeH⁺, first synthesized in laboratory in 1925, and more complex neutral species stable only under gigapascal to terapascal pressures, such as Na₂He. Despite helium's of 2 and fully paired 1s electrons making it the least electronegative and most unreactive element, theoretical and experimental advances since the early have revealed a growing family of helium-bearing molecules and solids. The HeH⁺ ion, consisting of a helium atom bonded to a proton, is the lightest heteronuclear molecular ion and was pivotal in primordial universe chemistry, facilitating the formation of molecular hydrogen; its astrophysical detection in the NGC 7027 in 2019 confirmed its presence in interstellar environments. In settings under ambient conditions, helium participates in weakly bound van der Waals complexes, but true chemical bonding emerges primarily in plasma or gas-phase ions, including species like HeH₂⁺ and He₂H⁺. High-pressure synthesis, enabled by diamond anvil cells, has unlocked neutral helium compounds by compressing matter to densities where helium's promotion energy decreases, allowing it to insert into ionic lattices or form direct bonds. The landmark discovery of Na₂He in 2017 demonstrated a fluorite-type with helium stabilizing electron localization in sodium cubes, rendering the material insulating at pressures exceeding 113 GPa—conditions relevant to planet interiors. Subsequent work identified analogous insertion compounds, such as CaF₂He (stable between 30 and 110 GPa) and MgF₂He (stable above approximately 107 GPa), where helium occupies sites in ionic frameworks, reducing Coulombic repulsion without forming traditional covalent bonds. These findings suggest helium's broader reactivity with imbalanced ionic solids (e.g., AB₂ stoichiometries) at accessible megabar pressures. Theoretical predictions continue to expand the scope, with recent calculations in 2025 proposing He₃F₂ as a stable phase in the multi-terapascal regime, featuring polar covalent He–F bonds in herringbone-chain structures driven by overlap of helium's 1s and fluorine's 2p orbitals, as well as stable iron-helium compounds like FeHe above 4 TPa, relevant to Earth's core. Such compounds challenge long-held views of chemistry and hold implications for understanding helium's behavior in stellar and planetary cores, where it may influence phase transitions and . Overall, helium compounds illustrate how extreme environments can induce reactivity in the "inert" elements, bridging fundamental with astrophysical and applications.

Introduction

Overview of helium compounds

Helium is renowned for its exceptional chemical inertness at ambient conditions, stemming from its electronic configuration of a fully filled 1s orbital, which constitutes a stable closed shell, coupled with the highest first among all elements at 24.587 eV and an extremely low of approximately 0.20 ų. These properties render helium highly resistant to forming chemical bonds, as the energy required to disrupt its or induce temporary dipoles is prohibitively high, preventing participation in typical covalent or ionic interactions under standard pressures and temperatures. Under extreme conditions, however, helium can engage in unconventional bonding mechanisms, including weak van der Waals interactions that stabilize dimers like He₂ at cryogenic temperatures, facilitated by high-pressure as seen in compounds such as Na₂He, charge-induced attractions with nearby cations or anions, and excimeric bonds in electronically excited states where temporary Rydberg orbitals enable dimer formation. These interactions highlight helium's potential for "chemistry" beyond traditional paradigms, often relying on external stabilization rather than intrinsic valence electrons. The formation of helium compounds often requires high pressures, typically exceeding 100 GPa for ionic insertion compounds—equivalent to conditions deep within planets—though some metallic alloys form at lower pressures starting from 5 GPa, and temperatures spanning cryogenic regimes for weakly bound phases to over 2000 K for high-pressure synthesis, with relevance to geophysical models of planetary interiors where such compounds may influence core dynamics and heat transfer. Notable milestones include the early experimental observations of helium excimers in the 1960s through discharge , the 2017 theoretical and experimental confirmation of stable Na₂He at pressures above 113 GPa, and the 2025 discovery of iron-helium alloys forming at relatively modest pressures starting from 5 GPa under high temperatures.

Historical development

In the early , numerous chemists attempted to synthesize helium compounds, driven by the recent isolation of on in , but all efforts failed due to its extreme chemical inertness arising from a fully filled 1s orbital and high of 24.59 eV. Researchers such as L. Troost and L. Ouvrard in tried reacting with and metals under various conditions, including heating and electrical discharge, yet no stable products formed, reinforcing the assumption of helium's complete lack of reactivity. These failures persisted through the and , with experiments by other chemists yielding only mixtures or unchanged helium, highlighting the challenges posed by its zero and weak . A notable early success was the 1925 laboratory synthesis of the HeH⁺ via , the first helium-containing molecular species, though it is ionic and unstable under ambient conditions. A breakthrough in understanding helium's bonding potential occurred in 1962 when Robert S. Mulliken identified the He₂* through spectroscopic analysis of discharges, revealing a weakly bound excited-state dimer with a of about 0.02 eV that dissociated upon returning to the . This discovery demonstrated that atoms could form transient bonds under excitation, challenging the inertness paradigm and paving the way for further investigations into rare gas chemistry. In the early , computational advances enabled theoretical predictions of reactivity at high pressures, where compressed densities could overcome kinetic barriers to bonding with electropositive elements like metals. These models suggested stable helium insertion compounds above 100 GPa, such as Na₂He. The field advanced dramatically in 2017 with the experimental synthesis of Na₂He by Somayazulu et al., marking the first stable neutral compound formed at 113 GPa and in a , confirmed by showing a fluorite-type structure. This electride-like phase, stable up to 1000 GPa in theory, arose from 's interstitial incorporation into a sodium lattice, driven by pressure-induced . More recently, in 2025, Hirose et al. reported the formation of iron- compounds (FeHeₓ, x up to 0.48) at 5–54 GPa and temperatures to 2500 K, using laser-heated s and in situ to observe expanded face-centered cubic and hexagonal close-packed phases. These findings imply retention in Earth's core via such alloys, potentially explaining geophysical abundance anomalies. The pivotal role of s, which achieve gigapascal pressures via opposed diamond tips, combined with for real-time structural probing, has been essential in these high-pressure discoveries, enabling precise control and observation under extreme conditions.

Known solid phases

Helides and ionic solids

(Na₂He) represents the first experimentally synthesized stable ionic compound incorporating as an anion under extreme conditions. This compound adopts a fluorite-type with Fm3m, in which helium occupies the anion sites as He²⁻, surrounded by eight Na⁺ cations. Na₂He forms at pressures exceeding 113 GPa and remains thermodynamically stable up to at least 1500 K at 140 GPa, exhibiting a significantly higher than pure sodium under similar conditions. Its synthesis was achieved in 2017 via laser-heating of a sodium-helium mixture in a , marking a breakthrough in chemistry. The bonding in Na₂He arises from charge transfer from sodium atoms to helium, resulting in partial negative charge on He (approximately -0.15e) and localized electrons in sites that behave as He²⁻ anions. This configuration forms an electride-like structure with eight-center two-electron bonds within Na₈ cubes around each helium, akin to an inverse where helium mimics the role of the ion in NaH but with reversed expectations under . The compound is insulating due to this electron localization induced by helium's presence, which repels from the sodium lattice. Experimental confirmation of Na₂He relied primarily on synchrotron X-ray diffraction, which matched the predicted fluorite lattice parameters and showed no evidence of helium inclusion without chemical bonding. Raman spectroscopy complemented these observations by revealing lattice vibrations influenced by helium's incorporation, with modes indicating He participation in the phonon spectrum and shifts attributable to the strengthened ionic interactions. Phonon calculations further supported dynamical stability above 100 GPa, with no imaginary frequencies. Theoretical predictions using methods have extended the concept of helides to other and alkaline earth metals, identifying Li₂He and K₂He as potentially stable at pressures similar to Na₂He (above ~100-150 GPa), with analogous structures and charge-transfer bonding. These compounds feature as a divalent anion, though their synthesis remains unreported, limited by higher required pressures or kinetic barriers compared to Na₂He. For Li₂He, transient spectroscopic detection in low-pressure environments has been noted, suggesting feasibility under compression. Such predictions highlight 's capacity for across and 2 elements at terapascal regimes.

Mineral and oxide inclusion compounds

Helium inclusion compounds with minerals and oxides form under high-pressure conditions, where the occupies lattice voids or interstices through physical trapping mechanisms such as van der Waals interactions and , rather than formal chemical bonding. These structures are particularly relevant to simulations of Earth's deep interior, where may be retained in mantle and core materials, influencing planetary and volatile cycling. Observations of such inclusions typically rely on X-ray diffraction in diamond anvil cells to detect lattice expansions or new phases. In arsenolite (cubic As₂O₃), helium forms an ordered inclusion compound above 3 GPa, with helium atoms filling octahedral voids between adamantane-like As₄O₆ cages in the lattice, resulting in the clathrate As₄O₆·2He. This structure develops progressively deeper into the crystal surface under compression, as revealed by high-pressure synchrotron X-ray , and represents one of the most compressible mineral-helium systems known, with helium occurring despite the nonporous nature of arsenolite at ambient conditions. The trapping is stabilized by weak intermolecular forces, and the compound persists up to at least 10 GPa without altering the host lattice symmetry significantly. A notable oxide inclusion is the helium-bearing compound FeO₂He, synthesized at pressures exceeding 100 GPa and temperatures around 2000–3000 K, conditions akin to the core-mantle boundary. Here, is incorporated into the FeO₂ lattice, forming a stable phase where occupies interstitial sites, potentially via polarization of the framework. This compound provides a mechanism for primordial retention in the , as FeO₂ may form from subducted oxidized iron, trapping that would otherwise degas. The structure was confirmed through calculations and experimental synthesis in laser-heated diamond anvil cells. For iron-based metallic inclusions, a 2025 experimental study identified FeHeₓ compounds (x ≤ 0.13 in face-centered cubic iron and up to 0.48 in distorted hexagonal close-packed iron) stable at 5–54 GPa and 1000–2820 K. Helium incorporation expands the iron lattice by up to 2–3%, filling expanded interstices through interactions, as observed via diffraction and simulations in laser-heated diamond anvil cells. These phases form from solid iron in helium pressure media, highlighting helium's ability to with metals under moderate mantle pressures. In dominant in Earth's , in MgSiO₃ is predicted to be high based on lattice strain models, with likely substituting at oxygen vacancy sites for charge balance, though direct measurements remain challenging. Experiments on analogous (Ar, Kr) at 23–24 GPa and 1600–1800 °C demonstrate incorporation into lattices without phase destabilization, suggesting 's smaller size enables even greater at deeper mantle pressures exceeding 100 GPa. Similar trapping mechanisms may extend to the post- phase of (Mg,Fe)SiO₃, where structural voids could accommodate via van der Waals and polarization forces, though experimental confirmation at >125 GPa awaits further high-pressure synthesis. For MgSiO₃ specifically, phase formation involving has been probed near 125 GPa using , aligning with the -to-post- transition depth. These inclusion compounds have profound implications for , particularly helium retention in iron-rich cores and mantles. The FeHeₓ phases suggest that Earth's core could harbor significant primordial (up to ~4 ng/g), sequestered during planetary accretion and preventing its escape to the atmosphere, consistent with observed ³He/⁴He ratios in oceanic basalts. Similarly, and inclusions imply deep mantle reservoirs of , influencing , , and the preservation of solar nebula volatiles in terrestrial planets.

Organic and small-molecule inclusions

Helium can be incorporated into organic solids under , forming inclusion compounds where it occupies interstitial sites within the host lattice. For example, in solids, atoms integrate into the , creating stable He-CH₄ mixtures that exhibit behavior. These compounds are predicted to be thermodynamically stable over a wide range, from approximately 55 to 155 GPa, with insertion enhancing the packing efficiency of the lattice by filling voids. This incorporation leads to exotic properties, such as the coexistence of plastic and partially diffusive phases in the solid solution, where molecules retain rotational freedom while contributes to structural stability. In small-molecule solids, co-crystallizes with under cryogenic temperatures and elevated pressures, forming mixed phases in the He-H₂ system. Theoretical calculations indicate limited of in the -rich solid phase, reaching a few mole percent, which influences the up to around 10 GPa. At these conditions, the mixtures exhibit immiscibility boundaries, but in the solid state, can substitute into the lattice, altering the freezing behavior and potentially stabilizing certain crystal structures. Experimental studies using diamond anvil cells (DACs) have explored these phases, revealing that remains largely inert but interacts weakly through van der Waals forces with the host. The synthesis of these inclusion compounds typically involves compressing mixtures in DACs, often combined with laser heating to achieve the required temperatures and promote incorporation. For instance, in He-H₂ experiments, samples are loaded into DACs at cryogenic conditions and pressurized to several GPa, with heating facilitating phase transitions and helium trapping. Evidence for helium-hydrogen interactions in these solids has been obtained through spectroscopic techniques, including , which detects shifts in vibrational modes indicative of lattice perturbations by helium. While scattering has been employed in low-pressure studies of helium-hydrogen mixtures to probe quantum interactions, high-pressure applications remain challenging due to technical constraints, but theoretical models support enhanced intermolecular coupling in compressed states. These organic and small-molecule inclusions exhibit enhanced mechanical properties, such as increased rigidity, attributed to helium filling voids and improving overall lattice density without significant chemical bonding. In He-CH₄ systems, this leads to broader stability ranges compared to pure solids under . Additionally, the light mass of helium introduces potential quantum effects, including zero-point motion that may influence phase stability and in the host lattice at low temperatures.

Clathrate and cage structures

Clathrate and cage structures involving primarily feature the atom encapsulated within polyhedral voids formed by host lattices of or hydrocarbons, held in place by weak van der Waals forces rather than chemical bonds. These non-stoichiometric inclusion compounds, known as helium clathrate hydrates, were long predicted but only recently synthesized, with the structure II () variant (He@sII) representing the first confirmed example in 2018. In He@sII, the cubic ( Fd-3m) consists of 136 molecules forming 16 small pentagonal dodecahedral (5^{12}) and 8 large hexagonal dodecahedral (5^{12}6^4) , where occupies the centers of these voids. Typical occupancies include one per small cage and up to four per large cage, arranged tetrahedrally to minimize repulsion, yielding compositions such as He·17H_2O for full single occupancy of small cages. Formation of these clathrates occurs through the exposure of empty hydrate frameworks, such as the guest-free sII ice XVI, to pressurized helium gas at cryogenic temperatures of 80–120 K and moderate pressures around 100 MPa (0.1 GPa), enabling helium diffusion into the cages without disrupting the host lattice. An alternative route involves freezing helium-saturated aqueous solutions under pressure, though helium's weak interactions necessitate specialized conditions to achieve stable encapsulation. For hydrocarbon-based examples, mixed clathrates like He@CH_4 form structure I (sI) phases, where helium co-occupies small 5^{12} cages alongside methane in larger 5^{12}6^2 cages, synthesized by pressurizing methane-helium gas mixtures over water at temperatures below 273 K and pressures exceeding 3 MPa. These sI structures exhibit similar cage geometries to pure methane hydrates but with reduced overall stability due to helium's minimal stabilization contribution. Stability extends up to about 0.5 GPa for related filled-ice phases, beyond which phase transitions occur, with decomposition favored above 150 K at ambient pressure due to helium's low polarizability. Structural confirmation and cage occupancy details derive from neutron diffraction experiments, which reveal helium positions at centers and quantify filling fractions, such as ~100% for small and variable (1–4) for large ones in He@. Helium's high mobility within the —facilitated by barrierless at temperatures below 100 —has been evidenced through quasielastic scattering, showing rapid hopping between adjacent voids without lattice disruption, a quantum mechanical effect enhanced by helium's light mass. This mobility contrasts with more rigid guests like and underscores the reversible nature of encapsulation. Helium clathrates hold relevance for modeling helium trapping in interstellar ices, where cryogenic conditions and radiation mimic cage formation in cometary materials, aiding simulations of noble gas retention in astrophysical environments. Potential applications in gas storage are limited by helium's facile escape, though the structures inform designs for reversible noble gas sorbents; analogous endohedral caging appears in carbon fullerenes, but with stronger confinement.

Carbon-based and endohedral solids

Carbon-based helium compounds primarily involve the incorporation of helium into structures, such as C60 and C70, either as intercalants in lattices or as endohedral species trapped within the carbon cages. In fullerites, the face-centered cubic lattice of C60 allows helium atoms to occupy interstitial sites, forming He-C60 intercalation compounds. These occur through gas sorption processes where helium penetrates the lattice at ambient temperatures and pressures, with a two-stage mechanism involving initial occupation of octahedral interstices followed by tetrahedral sites. Although typically achieved under low-pressure conditions, higher pressures exceeding 1 GPa can enhance intercalation density in compressed fullerite samples, positioning helium between buckyballs and altering lattice parameters as observed via diffractometry. Endohedral fullerenes, denoted as He@C60 and He@C70, feature a encapsulated within the closed carbon cage, stabilized by van der Waals interactions without chemical bonding. The rotates freely inside the cage at , as evidenced by 3He NMR , which shows chemical shifts of -6 ppm for He@C60 and -29 ppm for He@C70 relative to free 3He gas. has directly visualized the within the C60 cage, confirming its central position and lack of distortion to the framework. Synthesis of these endohedral fullerenes was pioneered in the using high-pressure and high-temperature methods, where C60 or C70 is exposed to gas at pressures around 2-3 GPa and temperatures of 500-800°C, achieving insertion yields of approximately 0.1%. Alternative approaches include , in which high-energy He+ ions are accelerated into fullerene films, followed by neutralization, though this often results in lower purity due to cage damage. Confirmation of He@C60 formation relies on techniques, such as ultrahigh-resolution , which detect the characteristic and distinguish endohedral species from surface-adsorbed . In terms of properties, endohedral fullerenes exhibit unique quantum behaviors, including 's free rotation and potential for hosting multiple atoms like He2@C60 under extreme conditions. Doping alkali metals into lattices containing endohedral He@C60 fullerenes, such as in the superconducting Rb3(3He@C60), allows for probing of the superconducting state using ³He NMR spectroscopy. The encapsulated does not significantly alter the electronic properties or transition temperature (Tc ≈ 30 K), which remains similar to that of undoped Rb3C60. In 2023, the superconducting compound Rb3(3He@C60) was synthesized and characterized using solid-state ³He NMR, revealing vortex phases in the superconducting state without -induced changes to Tc (≈30 K). This enables sensitive probing of the superconductor's microscopic properties.

Exotic helium-based solids

Impurity condensates and solid solutions

Impurity-helium condensates (IHCs) form porous solids when of impurities, including molecular such as H₂ or N₂, are injected into superfluid at low temperatures below 2 K, creating isolated clusters or gel-like structures within the helium matrix. These condensates arise during the condensation of gas mixtures in a helium bath, where less volatile impurities segregate into macroscopic, highly porous frozen clusters surrounded by solid , exhibiting spatial separation due to differences in volatility. At ultralow temperatures, the structures can display quantum tunneling effects, as impurities bind to dislocations or surfaces, influencing the overall quantum behavior of the condensate. In solid helium, doping ⁴He with isotopic ³He impurities or molecular species significantly modifies the material's superfluid properties, often suppressing or altering the onset of . Concentrations of ³He as low as parts per million can pin dislocations in the solid lattice, broadening the supersolid transition and shifting critical temperatures, with phase diagrams revealing distinct regions of superfluid density below 1 K. Molecular impurities like H₂ further disrupt coherence by localizing excitations, leading to changes in torsional oscillator responses that mimic supersolid signals but are attributable to impurity-induced disorder rather than intrinsic supersolidity. Binary solid solutions involving as the , such as He-N₂ or He-Ar mixtures, form limited phases at cryogenic pressures and temperatures below 4 , characterized by low solubility and prominent miscibility gaps due to helium's weak interatomic interactions. In He-N₂ systems, impurities exhibit minimal incorporation into the solid lattice, resulting in into nearly pure solid phases with acting primarily as a matrix rather than a true former. Similar behavior occurs in He-Ar mixtures, where forms distinct solid domains within the helium host, with limited to trace levels before segregation dominates, as mapped in low-temperature phase diagrams. Experimental investigations of these systems rely on dilution refrigerators to achieve millikelvin temperatures, enabling precise control over impurity concentrations and phase stability. Specific measurements, often combined with torsional or NMR techniques, probe the thermodynamic signatures of transitions and impurity effects, revealing anomalies in that correlate with ³He doping levels below 1 K.

Low-dimensional structures

(⁴He) can form one-dimensional quantum when confined within narrow pores at low temperatures, typically 1.6–4.2 K, where superfluid-like behavior emerges in the quasi-1D geometry. These structures arise from the adsorption of ⁴He into Ar-preplated nanopores with an effective diameter of ~2 nm, leading to nested cylindrical layers and a central 1D core that exhibits frictionless mass transport consistent with Tomonaga-Luttinger liquid excitations. At low temperatures, ⁴He forms nested cylindrical layers around the pore walls, with a central 1D core that transitions to a mobile upon full adsorption, displaying a Luttinger parameter near 1.2, distinct from higher-dimensional superfluid phases. Experimental observations confirm the formation of such 1D quantum , with atomic spacings of approximately 3.9 in the core. Studies of ⁴He in carbon nanotubes have also explored quasi-1D behavior, including localization and commensurate-incommensurate transitions. These low-dimensional structures display enhanced quantum coherence, manifesting as Tomonaga-Luttinger liquid excitations with a Luttinger parameter near 1.2, distinct from higher-dimensional superfluid phases. Such properties suggest potential applications in , where tunable 1D bosonic systems could enable coherent impurity dynamics or spin-mass separation when doped with ³He. In the realm of two-dimensional structures, participates in ionic crystals like Li₅He₂, where He-Li⁺ layers form under extreme pressures, analogous to the bulk Na₂He . These 2D layers consist of puckered anionic sheets (interstitial quasiatoms) interconnected within a rhombohedral R3m lattice, with He atoms at 6c Wyckoff sites stabilizing the structure. The compound is dynamically stable from 210 GPa to 1 TPa and thermodynamically stable above 700 GPa, including effects, as determined by first-principles calculations. Predicted synthesis involves laser-heated anvil cells at pressures exceeding 600 GPa to facilitate insertion into lattices. The 2D He-Li⁺ layers in Li₅He₂ exhibit phonon-mediated with a transition temperature up to 26 at 210 GPa, arising from strong electron-phonon coupling in the lithium-dominated vibrations of the layers. This enhanced quantum coherence in the reduced dimensionality contrasts with bulk solid solutions, where impurity effects dominate without such layered electron localization. The potential for stems from the coherent superconducting states and interstitial electron distributions, offering a platform for high-pressure exotic phases with tunable electronic properties.

Weakly bound molecular species

Ground-state van der Waals molecules

Ground-state van der Waals molecules involving are weakly bound complexes formed in the electronic , where atoms interact with other neutral species primarily through long-range dispersion forces, resulting in binding energies typically below 10 K. These species are characterized by large intermolecular distances, on the order of 3–4 , and shallow wells that support only a few bound vibrational states. Unlike stronger chemical bonds, these complexes exhibit no significant charge transfer or covalent character, making them model systems for studying pure van der Waals interactions. The helium-rare gas dimers, such as He-Ne and He-Ar, exemplify these ground-state complexes. For the He-Ne dimer, high-resolution imaging experiments have determined a binding energy of approximately 3.71 K for the rovibrational ground state, corresponding to a dissociation energy of about 0.00032 eV. Similarly, ab initio calculations for He-Ar yield binding energies around 20–30 cm⁻¹ (∼30 K or 0.0037 eV), with equilibrium distances near 3.5 Å. These dimers have been characterized through theoretical predictions of their microwave spectra, which reveal small rotational constants (B ≈ 0.02–0.05 cm⁻¹) due to the low reduced mass and extended bond lengths, facilitating studies of their quantum mechanical behavior. Helium complexes with diatomic molecules, such as He-H₂ and He-N₂, have been extensively studied via surfaces (PES). For He-H₂, a highly accurate PES fitted to over 1900 coupled-cluster calculations predicts a well depth of 15.870 ± 0.065 (∼0.00137 eV) at the vibrationally averaged H-H of 1.448736 bohrs, with rotational constants for the on the order of 1.5 GHz (0.05 cm⁻¹). The PES reveals a nearly isotropic interaction, supporting only the lowest vibrational levels. In the case of He-N₂, computations at the supermolecular level describe a T-shaped minimum with a of roughly 25 cm⁻¹ (∼36 or 0.003 eV), and rotational constants around 1.2 GHz, enabling predictions of bound states and transitions. These PES facilitate quantum scattering calculations to explore the complexes' dynamics. These ground-state helium complexes are typically formed in supersonic jet expansions at low temperatures (∼1–10 ), where rapid cooling and high densities promote collisions that stabilize the weakly bound . In such environments, the molecules persist for lifetimes on the order of milliseconds, limited by collisions with carrier gas atoms, allowing spectroscopic interrogation before dissociation. Buffer-gas cooling techniques have also demonstrated their production and magnetic , extending lifetimes to seconds for certain helium-containing vdW . The bonding in these ground-state complexes is dominated by London dispersion forces arising from correlated electron fluctuations, with no appreciable charge transfer, as confirmed by symmetry-adapted analyses showing dispersion contributions exceeding 90% of the interaction energy. Electrostatic and induction terms are negligible due to helium's closed-shell nature, resulting in purely physical adsorption-like interactions that underscore the inertness of even in molecular assemblies.

Excited-state excimers

Excited-state excimers of are transient species formed by the association of atoms in electronically s, resulting in bound potential curves that contrast with the repulsive . The archetypal example is the He₂* dimer, first observed in 1930 through its vacuum ultraviolet emission continuum during spectroscopic studies of discharges. This emission arises from the transition between the bound A¹Σ_u⁺ and the repulsive X¹Σ_g⁺ , appearing as a broad continuum with a short-wavelength limit near 60 nm and extending to about 100 nm. The A¹Σ_u⁺ state features a deep with a dissociation energy of approximately 2.5 eV relative to the He(1s2s ¹S) + He(1s²) , enabling temporary bonding before radiative decay. These excimers form in the gas phase primarily via three-body collisions involving electron impact or photoexcitation, where an excited He* atom (typically in the 2³S or 2¹S state) collides with ground-state He atoms to stabilize the excited dimer. The singlet He₂*(A¹Σ_u⁺) state has a short radiative lifetime of around 10 ns, leading to rapid emission of VUV photons, while the triplet a³Σ_u⁺ state is longer-lived (up to microseconds in gas or seconds in superfluid ) due to spin-forbidden transitions. The repulsive ground-state curve ensures dissociation upon relaxation, preventing stable molecules. Higher-order excimers, such as He₃* and He₄*, form analogously in dense helium environments like discharges or , exhibiting bound excited states with similar VUV emissions and radiative lifetimes on the order of 10 ns for singlet configurations. These have been investigated for potential use in lasers and VUV sources, though practical implementation remains limited compared to heavier rare-gas excimers. He₂* excimers serve as the basis for excimer lamps generating incoherent VUV radiation, applied in studies, surface , and experimental ultraviolet lithography systems. Unlike ground-state van der Waals He₂ molecules, which are weakly bound and non-luminescent at , these excited species enable short-lived, high-energy emissions critical for such applications.

Ionic helium species

The helium hydride ion, HeH⁺, represents the simplest stable diatomic compound involving helium and is widely regarded as the first molecule formed in the early universe following the Big Bang, approximately 100,000 years after its inception when temperatures allowed recombination of electrons with helium nuclei. This ion consists of a helium atom covalently bonded to a proton, with the bonding arising from the overlap of the helium 1s orbital and the hydrogen 1s orbital, resulting in a bond length of about 0.77 Å and a dissociation energy of roughly 2.5 eV. HeH⁺ possesses a significant permanent dipole moment of 1.66 D, which enhances its reactivity and observability, and its ground-state vibrational frequency is approximately 3200 cm⁻¹, corresponding to strong infrared emission lines around 3.1–3.7 μm. The ion was first detected astrophysically in 2019 in the planetary nebula NGC 7027 using the GREAT instrument on SOFIA, confirming its presence in the interstellar medium through rovibrational transitions. Synthesis of HeH⁺ occurs primarily through gas-phase ion-molecule reactions, such as He⁺ + H₂ → HeH⁺ + H, which is exothermic by 2.0 eV and proceeds efficiently at low temperatures below 100 due to the high reactivity of He⁺ with neutral molecules. This reaction was first studied in laboratory mass spectrometers in the using electron-impact of helium- mixtures, demonstrating high yield under controlled conditions mimicking interstellar environments. In denser regions like planetary nebulae, HeH⁺ forms via radiative association or reactions involving He⁺ with H atoms, persisting long enough for spectroscopic detection despite rapid destruction by reactions with H₂ or electrons. Theoretical studies predict related diatomic ions like HeO⁺ with weaker bonding due to electrostatic interactions, stable on short timescales in gas-phase experiments. In , HeH⁺ serves as a pivotal initiator of ion-molecule reaction chains in primordial and evolved nebulae, leveraging its large dipole moment to efficiently capture electrons and neutrals, thereby catalyzing the formation of H₃⁺, H₂O⁺, and other precursors to molecular complexity. Its abundance in metal-poor environments, such as the early or H II regions, underscores its role in bridging atomic to molecular phases, with rate coefficients for key reactions like HeH⁺ + H₂ → H₃⁺ + He exceeding 10⁻⁹ cm³ s⁻¹ at 10 K. A 2025 experiment confirmed barrierless reactions of HeH⁺ with H and D atoms at ultra-low temperatures, suggesting enhanced reactivity in the early universe and potential impacts on rates. This reactivity positions HeH⁺ as a diagnostic probe for levels and cooling processes in astrophysical plasmas.

Polyatomic and cluster ions

Polyatomic and cluster ions involving primarily consist of cationic , where the inert of leads to weak interactions and localized charge distribution. These ions are typically studied in ultracold environments, such as helium nanodroplets or drift tubes, to stabilize their fragile structures against thermal dissociation. Experimental techniques like reveal magic numbers in cluster size distributions, indicating completed shells around charged cores. Ionized helium clusters, denoted as Heₙ⁺ for n > 2, exhibit structures with a compact ionic core solvated by neutral helium atoms. The smallest polyatomic species, He₃⁺, adopts a linear, symmetric geometry with a He–He bond length of approximately 1.08 Å, stabilized by a binding energy of about 0.2 eV. For larger clusters (n = 4–35), theoretical genetic algorithm optimizations combined with semiempirical potentials predict a persistent trimer ionic core (He₃⁺-like) surrounded by one or two shells of neutral helium atoms, with the charge delocalized primarily over the core. These solvation shells grow progressively, reaching two complete layers by n ≈ 35, where the outermost atoms are bound by van der Waals forces with energies below 1 meV per atom. At low temperatures (below 1 K), such as in superfluid helium droplets, these clusters maintain linear or near-linear chain-like arrangements in small sizes due to minimized entropy, as evidenced by drift tube mass spectrometry detecting stable Heₙ⁺ up to n = 14. The stability arises from the charge-induced dipole interactions, though evaporation of helium atoms occurs readily upon mild excitation. Interactions with other noble gases form weakly bound diatomic cations like HeKr⁺ and HeXe⁺, characterized as charge-solvated complexes (He⋯Kr⁺ and He⋯Xe⁺) with equilibrium distances exceeding 4 . These ions display shallow potential wells in their X ²Σ⁺ ground states, with vibrational levels bound by less than 2.5 cm⁻¹ near the dissociation limit, as determined from and least-squares fits to interaction potentials. Charge transfer barriers in these systems stem from the significant ionization potential difference (IP(He) = 24.6 eV vs. IP(Kr) = 14.0 eV, IP(Xe) = 12.1 eV), creating an endothermic pathway for He⁺ + Kr/Xe → He + Kr⁺/Xe⁺ with activation energies of several eV, inhibiting facile rearrangement. Bound-bound charge-transfer transitions are observed in rare gas mixtures, but the ions fragment via helium eviction on timescales of microseconds. Helium solvation of metal ions, particularly alkali metals, yields complexes HeₙM⁺ (M = Na, K), where the metal cation serves as the charge center with rigid inner solvation shells and loosely bound outer layers. For Na⁺, experimental mass spectra from ionized helium nanodroplets show prominent peaks at n = 9 and n = 2, corresponding to magic numbers for a completed first shell (7–12 He atoms predicted theoretically) and a dimer core, with binding energies decreasing from ~150 K for inner atoms to ~10 K for outer ones. Similar patterns occur for K⁺, with anomalies at n = 12 indicating 13–15 He atoms in the first shell. In ion traps and quadrupole mass analyzers, fragmentation proceeds sequentially by loss of individual He atoms, revealing stepwise dissociation energies of 0.5–2 meV and no covalent character, consistent with purely electrostatic solvation. These patterns highlight the role of polarization forces, with larger n clusters showing reduced stability due to diminished ion-induced dipole attraction at greater distances. Nonmetal-containing polyatomic ions with helium remain largely theoretical, exemplified by the predicted HeF₂⁻ anion. Ab initio calculations indicate a shallow potential minimum for a linear F–He–F⁻ structure, but with a dissociation below 1 kcal/mol, rendering it unstable toward F⁻ + He + F due to helium's negligible (0.082 eV) and the high of , which favors charge localization on the fluorines rather than shared bonding. effects dominate, as the added electron occupies an antibonding orbital, leading to predicted lifetimes under 10⁻¹² s in gas phase. Ion cyclotron resonance (ICR) has been instrumental in probing these , enabling isolation of clusters in magnetic fields (typically 1–7 T) for and reaction rate measurements at near-zero effective temperatures. This technique has quantified association rates for He attachment to cores like He₂⁺ or M⁺, on the order of 10⁻¹⁰ cm³/s, and confirmed charge delocalization in Heₙ⁺ by observing symmetric fragmentation channels.

Predicted compounds

Predicted solid phases

Theoretical predictions of solid helium compounds, primarily derived from (DFT) calculations, indicate that helium can participate in extended lattice structures under extreme pressures, often exceeding hundreds of gigapascals (GPa). These phases are of interest for understanding chemistry in planetary interiors, where helium's inertness is overcome by pressure-induced electron delocalization or lattice expansion. Seminal studies have focused on helides—compounds where helium occupies interstitial sites in lattices—and other high-pressure solids, revealing thermodynamically stable configurations not yet observed experimentally. Seminal studies have focused on helides such as Na₂He, with a fluorite-type lattice stable above 113 GPa. Such helides feature helium atoms trapped in cubic voids formed by alkali metal cations, with the stability arising from pressure-enhanced electrostatic interactions in electride-like systems. High-pressure solids incorporating helium in clathrate forms with water are known to form under cryogenic conditions and high pressures up to approximately 1-2 GPa, where helium occupies cages in lattices without forming chemical bonds but stabilizing the structure through van der Waals interactions. Similarly, He-CO₂ phases have been theoretically mapped, with a novel compound HeCO₂ predicted to be thermodynamically stable at 448 GPa, exhibiting a layered structure with helium intercalated between CO₂ molecules. These clathrates and compounds highlight helium's role as a guest species in high-pressure , potentially relevant to icy planetary bodies. In metallic systems, predictions suggest helium incorporation into body-centered cubic (bcc) iron beyond the experimentally observed FeHe_{0.13} composition, with higher helium contents possible up to GPa in models of planetary cores. DFT calculations indicate stable Fe-He compounds at terapascal pressures, such as FeHe with a distorted bcc-like structure, where helium sublattice occurs above 2 TPa. These predictions support the of helium retention in Earth's core, consistent with trace Fe-He detected in inclusions from deep mantle samples. Recent experiments confirm Fe-He formation at lower pressures (5–54 GPa), lending credence to higher-pressure extrapolations for core conditions. Recent 2025 calculations have further expanded the scope to covalent solids, predicting He-F compounds such as He₃F₂ stable above approximately 1 TPa, featuring polar covalent He–F bonds with lengths around 1.8 in chain-like structures embedded in fluorine lattices. These represent the first predicted direct covalent bonding involving , driven by pressure-induced overlap of helium's 1s orbital with 's 2p orbitals, marking a significant departure from helium's traditional inertness. At terapascal pressures exceeding 1 TPa, crystal structure predictions indicate the formation of neutral He₃F₂, incorporating polar covalent He-F bonds within HeF₃ clusters embedded in fluorine chains. This structure features helium's 1s electrons participating in bonding with fluorine 2p orbitals, yielding a bond dissociation energy estimated at approximately 2 eV based on analysis, far stronger than van der Waals interactions and analogous to hypervalent fluorides. The linear F-He-F motif within these clusters is stabilized by pressure-induced orbital overlap, marking a departure from helium's inertness.

Predicted molecular species

Theoretical predictions have identified several weakly bound neutral molecular involving helium, primarily van der Waals complexes with alkaline-earth metals. For instance, diatomic systems such as BeHe and MgHe exhibit shallow wells characteristic of van der Waals interactions, with ground-state potentials calculated using semi-empirical methods that incorporate dispersion forces. These diatomics have binding energies on the order of a few wavenumbers, corresponding to less than 0.1 eV, reflecting the weak attractive forces dominated by London dispersion. Extending to trimers, species like MgHe₂ have been predicted through three-body interaction calculations, showing marginally bound ground states with binding energies below 0.1 eV, obtained via coupled-cluster methods including CCSD(T) for pairwise potentials and diffusion Monte Carlo for many-body corrections. Similar trimers, such as BeHe₂, are anticipated to form analogous weakly bound structures in ultracold environments, where helium atoms solvate the central metal atom in a linear or T-shaped geometry, stabilized by correlation effects beyond pairwise additivity. These predictions highlight helium's role as a weakly interacting ligand in ultracold molecular assemblies, with potential applications in quantum chemistry simulations. Beyond van der Waals complexes, theoretical models have forecasted stable neutral covalent-like helium compounds under extreme conditions. Early calculations at the donor-acceptor level predicted HeBeO as a metastable neutral , where donates to the electron-deficient BeO fragment, forming a bond with dissociation energy around 0.14 eV and a linear He-Be-O . More recent high-pressure simulations suggest -nitrogen compounds, such as He(N₂)₂, may emerge in compressed mixtures, featuring interactions between and partially ionic N₂ units, though detailed binding energies remain under exploration via . Detection of these elusive predicted species is proposed via (IR) spectroscopy in supersonic jet expansions, where cooling to near-zero facilitates formation and isolation of weakly bound complexes. Action spectroscopy in helium-tagged jets, combined with tunable IR lasers, could resolve vibrational signatures of He-metal or He-F stretches, enabling differentiation from signals through depletion methods. This approach has proven effective for analogous rare-gas complexes, offering a pathway to confirm theoretical predictions under controlled conditions.

Predicted ionic species

Theoretical studies have extended predictions to polyatomic s beyond the well-established diatomic HeH⁺ , which serves as a benchmark for chemistry in astrophysical and contexts. Among these, the triatomic HeH₂⁺ is predicted to adopt a cyclic structure as its lowest-energy , where the bridges the H₂⁺ moiety through a , conferring modest stability with a dissociation on the order of several kcal/mol. This configuration arises from the interaction of 's filled 1s orbitals with the empty σ* orbital of H₂⁺, as determined by high-level quantum chemical calculations. Similarly, the dication HeO₂²⁺ is forecasted to exhibit a bent , with the weakly bound to the O₂²⁺ core at a He-O distance of approximately 1.81 , rendering it metastable with respect to dissociation into He + O₂²⁺, though barrier-protected on the . Larger cluster ions of the form HeₙRg⁺, where Rg denotes a heavier rare gas atom (e.g., Ne, Ar, Kr, Xe), display enhanced stabilities at specific "magic number" sizes, such as n ≈ 12–20 depending on the dopant, attributable to completed solvation shells around the ionic core in helium droplet experiments. These magic numbers reflect minima in fragmentation rates and peaks in mass spectra, indicating shell-like structures where helium atoms form ordered layers stabilizing the central Rg⁺ ion against evaporation. Under extreme high-pressure conditions in solid phases, the dihelium anion He₂⁻ is anticipated to emerge within ionic lattices, possessing a small electron affinity of 0.08 eV that enables its incorporation into compounds like alkali metal hydrides, potentially altering phase stability above 100 GPa. These predictions rely on advanced computational approaches, including second-order Møller-Plesset perturbation theory (MP2) for initial geometry optimizations and coupled-cluster methods (e.g., CCSD(T)) for accurate potential energy surfaces, which account for electron correlation essential to weakly bound helium systems. Such techniques have benchmarked dissociation energies and binding geometries with errors below 1 kcal/mol for prototype helium ions.

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