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Solid oxygen
Solid oxygen
Solid oxygen
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Solid oxygen is the solid ice phase of oxygen. It forms below 54.36 K (−218.79 °C; −361.82 °F) at standard atmospheric pressure. Solid oxygen O2, like liquid oxygen, is a clear substance with a light sky-blue color caused by absorption in the red part of the visible light spectrum.

Oxygen molecules have a relationship between the molecular magnetization and crystal structures, electronic structures, and superconductivity. Oxygen is the only simple diatomic molecule (and one of the few molecules in general) to carry a magnetic moment.[1] This makes solid oxygen particularly interesting, as it is considered a "spin-controlled" crystal[1] that displays antiferromagnetic magnetic order in the low temperature phases. The magnetic properties of oxygen have been studied extensively.[2] At very high pressures, solid oxygen changes from an insulating to a metallic state;[3] and at very low temperatures, it transforms to a superconducting state.[4] Structural investigations of solid oxygen began in the 1920s and, at present, six distinct crystallographic phases are established unambiguously.

The density of solid oxygen ranges from 21 cm3/mol in the α-phase, to 23.5 cm3/mol in the γ-phase.[5]

Phases

[edit]
Phase diagram for solid oxygen

Six different phases of solid oxygen are known to exist:[1][6]

  1. α-phase: light blue – forms at 1 atm, below 23.8 K, monoclinic crystal structure, space group C2/m (no. 12).
  2. β-phase: faint blue to pink – forms at 1 atm, below 43.8 K, rhombohedral crystal structure, space group R3m (no. 166). At room temperature and high pressure begins transformation to tetraoxygen.
  3. γ-phase: faint blue – forms at 1 atm, below 54.36 K, cubic crystal structure, Pm3n (no. 223).[7][8]
  4. δ-phase: orange – forms at room temperature at a pressure of 9 GPa
  5. ε-phase: dark-red to black – forms at room temperature at pressures greater than 10 GPa
  6. ζ-phase: metallic – forms at pressures greater than 96 GPa

It has been found that oxygen is solidified into a state called the β-phase at room temperature by applying pressure, and with further increasing pressure, the β-phase undergoes phase transitions to the δ-phase at 9 GPa and the ε-phase at 10 GPa; and, due to the increase in molecular interactions, the color of the β-phase changes to pink, orange, then red (the stable octaoxygen phase), and the red color further darkens to black with increasing pressure. It was found that a metallic ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.[6]

Red oxygen

[edit]
Main article: Octaoxygen

As the pressure of oxygen at room temperature is increased through 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition. Its volume decreases significantly[9] and it changes color from sky-blue to deep red.[10] However, this is a different allotrope of oxygen, O
8, not merely a different crystalline phase of O2.

Ball-and-stick model of O8 Part of the crystal structure of ε-oxygen

Metallic oxygen

[edit]

A ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.[9] This phase was discovered in 1990 by pressurizing oxygen to 132 GPa.[3] The ζ-phase with metallic cluster[11] exhibits superconductivity at pressures over 100 GPa and a temperature below 0.6 K.[4][6]

References

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Solid oxygen

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from Grokipedia
Solid oxygen is the condensed, crystalline form of the O₂, which exists at cryogenic temperatures below its of approximately 54.4 (at standard ) or under elevated at higher . Unlike the solid forms of most other elements, solid oxygen is paramagnetic in its due to the triplet electronic configuration of O₂, leading to antiferromagnetic ordering in certain phases and making it a rare example of a molecular . It exhibits a rich polymorphism with at least nine distinct solid phases identified, each characterized by unique structures, colors, and physical properties influenced by , , and . At and low temperatures, solid oxygen transitions through three primary phases: the α-phase (monoclinic structure, stable below 23.9 ), which is pale blue and antiferromagnetic; the β-phase (rhombohedral structure, 23.9–43.8 ), which appears ; and the γ-phase (cubic structure, 43.8–54.4 ), which is orange. These low-pressure phases demonstrate strong coupling between lattice vibrations and magnetic spins, with the α-β transition involving both structural and magnetic reordering. Under high pressure, additional phases emerge, including the δ-phase (up to ~8–10 GPa, hosting multiple magnetic sublattices) and the ε-phase (stable from ~8 GPa to ~96 GPa, featuring red coloration and (O₂)₄ molecular clusters). The ε-phase is notable for its pressure-induced changes in vibrational modes and eventual insulator-to-metal transition around 96–100 GPa, where the diatomic structure persists but conductivity arises, potentially enabling at very low temperatures (~0.6 K). Further compression reveals high-temperature phases like ζ and η (molecular, up to >130 GPa and 1500 K), with the molecular form of oxygen remaining stable even at extreme pressures exceeding 1 TPa, unlike polymeric phases in or . The of solid oxygen, spanning pressures over 130 GPa, temperatures up to 1750 along the melting curve, and beyond 100 T, highlights its versatility as a model system for studying spin-lattice interactions. Recent investigations have shown that ultrahigh (>100 T) can induce , altering the through competing spin and lattice forces, demonstrating the profound influence of on its stability.

Overview

Definition and occurrence

Solid oxygen refers to the solid state of dioxygen (O₂), a that transitions to this phase when gaseous or is cooled below its of 54.36 K or compressed at higher pressures, where the solid, liquid, and gas phases coexist in equilibrium at 0.00152 MPa. This form is distinct from atomic oxygen solids or oxides, preserving the O₂ molecular units held together primarily by weak intermolecular forces. In natural settings, solid oxygen is exceedingly rare on due to the prevalence of the gaseous form in the atmosphere and its reactivity, but it occurs in extraterrestrial environments such as comets, where it has been detected as a component of icy surfaces and outgassed material. For instance, the mission identified molecular oxygen in Comet 67P/Churyumov–Gerasimenko, indicating solid O₂ reservoirs formed at low temperatures. It is also anticipated in the as frozen O₂ ices within dense molecular clouds, though direct detections remain limited, with abundance constraints suggesting it accounts for less than 6% of the total oxygen budget. Laboratory production of solid oxygen typically involves separating oxygen from air via to obtain the , followed by further cooling below 54 K using cryogenic methods like baths, resulting in deposition as a or . This highlights its significance as the only known insulating elemental antiferromagnet among molecular solids, exhibiting spontaneous antiferromagnetic ordering in its low-temperature phases due to exchange interactions between O₂ molecules. Unlike most solid elements, which form metallic or atomic lattices, low-pressure solid oxygen maintains intact O₂ molecules, with cohesion arising from van der Waals forces augmented by magnetic exchange interactions that contribute substantially to the . This molecular character, combined with its magnetic properties, positions solid oxygen as a unique material bridging simple molecular crystals and magnetic insulators.

Historical discovery

The first liquefaction of oxygen was achieved independently in December 1877 by French physicist Louis-Paul Cailletet, who produced a of liquid oxygen through rapid expansion, and Swiss physicist Raoul Pictet, who used a cascaded refrigeration process involving methyl chloride, , and oxygen itself. These breakthroughs marked the onset of low-temperature physics and paved the way for studies of condensed phases. Solid oxygen was first produced shortly thereafter in the late by Polish physicists Zygmunt Wróblewski and Karol Olszewski, who achieved static in 1883 and reported solidification upon further cooling, though initial measurements varied due to experimental challenges. Early 20th-century refinements by and confirmed the at approximately 54.36 and 0.00152 MPa, enabling systematic investigations of solid phases. Structural characterization of solid oxygen advanced in the with the identification of the low-temperature α phase through diffraction studies by W.H. Keesom at , revealing its monoclinic lattice stabilized by antiferromagnetic interactions below 23.8 . Pioneering low-temperature experiments by Fritz Simon and collaborators in the 1920s and , including specific heat measurements on oxygen down to 1.5 , confirmed the β phase transition around 44 and highlighted magnetic contributions to the phase stability. The β phase, rhombohedral in structure, was further verified in the via calorimetric and data by Simon's group and others like William Giauque. By the , calorimetric studies established the full low-pressure phase sequence, identifying the γ (cubic, 43.8–54.4 ) phase through heat capacity anomalies and changes. High-pressure investigations revealed exotic phases, with the red-colored ε phase discovered in 1979 by M. Nicol, K.R. Hirsch, and W.B. Holzapfel at approximately 8–10 GPa and , characterized by (O₂)₄ molecular clusters and a small volume collapse of ~1% observed via optical and probes. Theoretical predictions of metallic oxygen emerged in the , building on band structure models suggesting insulator-to-metal transitions under extreme compression. Experimental observation of metallic behavior followed in 1990 by Ho-kwang Mao and colleagues at ~96 GPa, confirmed through reflectivity and resistivity measurements in diamond-anvil cells, indicating delocalization of oxygen's π* electrons. in this metallic phase was reported in 1998 by Katsuya Shimizu et al. at pressures around 100 GPa and a critical temperature of 0.6 , detected via AC susceptibility in a cubic anvil apparatus. Recent advancements include 2025 experiments by Akihiko Ikeda and collaborators, who applied pulsed magnetic fields exceeding 100 T (up to 110 T) to solid oxygen using the PINK-02 generator and XFEL X-ray diffraction at SACLA, revealing spin-driven that stretches the crystal lattice by ~1% and alters phase boundaries through competing antiferromagnetic interactions and lattice distortions. This work, published in , demonstrates how extreme fields couple electronic spins to , opening avenues for magneto-phase engineering in molecular solids.

Physical properties

Structural properties

Solid oxygen consists predominantly of molecular crystals in which O₂ dimers are oriented within ordered lattices, forming the basis for its various phases. These structures are characterized by space groups such as C2/m for the monoclinic α phase, R-3m for the rhombohedral β phase, Pm3n for the cubic γ phase, Fmmm for the orthorhombic δ phase, and C2/m for the ε phase featuring O₈ molecular clusters. Density in solid oxygen varies across phases and conditions, ranging from approximately 1.54 g/cm³ in the α phase at low temperatures to approximately 1.7 g/cm³ in high-pressure forms such as the ε phase. Volumetric changes with and can be approximated by the relation V(P,T)≈V0(1+αΔT−βP)V(P,T) \approx V_0 (1 + \alpha \Delta T - \beta P), where α\alpha is the linear thermal expansion coefficient (approximately 1.5×10−51.5 \times 10^{-5} K⁻¹) and β\beta is the isothermal (approximately 1.6×10−31.6 \times 10^{-3} bar⁻¹). The molecular packing in solid oxygen is exemplified by the herringbone arrangement in the α phase, where O₂ molecules align in a staggered configuration stabilized by quadrupolar interactions between their permanent electric quadrupoles. No covalent bonding occurs in these molecular phases, with cohesion arising from van der Waals forces and electrostatic contributions. Representative lattice parameters for the α phase at 10 K are a=5.40a = 5.40 , b=3.99b = 3.99 , c=5.02c = 5.02 , and β=132.5∘\beta = 132.5^\circ, reflecting the monoclinic . in this phase exhibits , with greater expansion along the b-axis compared to the a- and c-axes.

Thermodynamic properties

Solid oxygen exhibits no true melting under conditions, instead undergoing sublimation directly to the gaseous phase. The , where solid, liquid, and gas phases coexist in equilibrium, occurs at 54.36 K and 0.152 kPa. The of sublimation is approximately 6.82 kJ/mol near 0 K, reflecting the required for this direct solid-to-gas transition. The at constant pressure (CpC_p) for the α phase adheres to the at low temperatures, characterized by a T3T^3 dependence, and approaches the Dulong-Petit limit of approximately $3R(where(whereRisthe[gasconstant](/page/Gasconstant))athighertemperatureswithinthephase.Ananomalyinis the [gas constant](/page/Gas_constant)) at higher temperatures within the phase. An anomaly inC_pappearsattheNeˊeltemperatureof12.5K,arisingfromantiferromagneticorderingoftheO appears at the Néel temperature of 12.5 K, arising from antiferromagnetic ordering of the O_2$ molecules. The linear thermal expansion coefficient for the α phase is αL≈1.5×10−5\alpha_L \approx 1.5 \times 10^{-5} , indicating modest changes with . The isothermal is κT≈0.16\kappa_T \approx 0.16 GPa−1^{-1}, quantifying the material's response to . Phase stability in solid oxygen is governed by the Clausius-Clapeyron equation, dPdT=ΔHTΔV,\frac{dP}{dT} = \frac{\Delta H}{T \Delta V}, which relates the slope of phase boundaries to enthalpy and volume changes. At ambient pressure, the α-β transition occurs at 23.8 K, while under high pressure, the ε phase remains stable up to temperatures above 70 K. The magnetic contribution to the entropy is Smag≈Rln⁡(2S+1)=Rln⁡3S_\text{mag} \approx R \ln(2S+1) = R \ln 3 for the spin-S=1S=1 O2_2 molecules, accounting for orientational and spin degrees of freedom.

Spectroscopic and magnetic properties

Solid oxygen exhibits distinctive across its phases, arising from electronic transitions influenced by intermolecular interactions. The α phase appears light due to strong absorption in the red region of the , primarily from the 629 nm band associated with O₂ dimer formation. The β phase appears , the γ phase is orange, the δ phase is light orange and the ε phase transitions to dark red, reflecting increasing charge-transfer character between O₂ molecules under . These color variations stem from d-d transitions within O₂⁺-O₂⁻ charge-transfer complexes, with additional absorption bands at approximately 630 nm contributing to the blue coloration in low-pressure phases and a red-shifted band near 760 nm emerging under compression. Raman and reveal key vibrational signatures of solid oxygen, distinguishing it from the gaseous state. The O-O stretching mode, observed at 1556 cm⁻¹ in gaseous O₂, shifts slightly to 1540-1560 cm⁻¹ range in the solid phases due to lattice interactions, with splitting in the α phase reflecting its monoclinic . Lattice modes, appearing below 200 cm⁻¹, provide insights into phase symmetries; for instance, low-frequency translations and librations in the α phase confirm its antiferromagnetic ordering, while higher-pressure phases like ε show broadened modes indicative of O₈ clustering. These spectroscopic features serve as diagnostic tools for phase identification, with IR absorption intensifying in the ε phase due to enhanced molecular coupling. The magnetic properties of solid oxygen are dominated by its triplet (S=1), making it the only known elemental molecular antiferromagnet. In the α phase, antiferromagnetic ordering occurs below the Néel temperature T_N = 12.5 , characterized by a collinear two-sublattice structure with an exchange constant J ≈ -1 meV arising from via filled π* orbitals. Above T_N, the χ follows the Curie-Weiss law, with an effective moment μ_eff = 2.83 μ_B per O₂ , consistent with spin-only behavior for S=1 and g ≈ 2. Higher-pressure phases like δ and ε exhibit paramagnetic behavior with suppressed magnetism due to orbital overlap, though persists in the triangular lattice of β-O₂. Electron spin (ESR) studies of solid oxygen highlight its anisotropic spin characteristics. ESR signals display g-factors with g∥ ≈ 2.002 and g⊥ ≈ 2.083, reflecting the of the ³Σ_g⁻ and spin-orbit within the O₂ . Recent 2025 investigations under ultrahigh magnetic fields exceeding 100 T have revealed strong spin-lattice , inducing giant (up to 1% lattice deformation) in the β phase through competing exchange interactions and structural frustration. These effects underscore oxygen's unique magnetoelastic response, altering lattice parameters anisotropically at fields around 110 T. In the metallic phase achieved at pressures above 96 GPa, solid oxygen undergoes an insulator-to-metal transition, where the diatomic persists but conductivity arises.

Molecular phases

α phase

The α phase of solid oxygen is the thermodynamically form at and temperatures below 23.8 K, representing the lowest-energy configuration in this regime. It exhibits a monoclinic crystal with space group C2/m and four O₂ molecules per (Z=4), characterized by a herringbone arrangement where parallel dimers form layered sheets. The nearest-neighbor O₂-O₂ intermolecular distance is 3.18 Å, and the X-ray density at 0 K is 1.426 g/cm³. This phase undergoes a transition to the β phase at 23.8 K, accompanied by an change of approximately 80 J/mol and a narrow of less than 0.1 K. The α phase is uniquely distinguished among solid oxygen phases by its full long-range antiferromagnetic order, arising from interactions between the triplet ground-state O₂ molecules aligned in an antiparallel fashion. The α phase was first identified in 1924 through early low-temperature studies by Ruhemann, who observed distinct solid forms of oxygen. Its was refined via X-ray diffraction in subsequent decades, while the antiferromagnetic ordering was confirmed in the 1970s using neutron diffraction experiments, which revealed magnetic reflections consistent with a Néel-type arrangement.

β phase

The β phase of solid oxygen is a transient rhombohedral phase stable between 23.8 and 43.8 at , serving as the paramagnetic regime above the Néel temperature of the underlying α phase. This narrow stability window renders it the least studied among the low-temperature molecular phases of oxygen, with its existence first confirmed in through observations of anomalies indicating phase transitions at approximately 24 and 44 . The structure adopts the R-3m (No. 166) with Z = 2 formula units per primitive cell, consisting of nearly cubic close-packed layers of O₂ molecules that are rotated relative to the principal lattice directions, resulting in a slight distortion from ideal hexagonal symmetry. In the primitive rhombohedral setting, representative lattice parameters at 30 K are a ≈ 3.91 Å and α ≈ 105.5°, corresponding to a of approximately 1.40 g/cm³; equivalent hexagonal cell parameters near 26 K are a ≈ 3.27 Å and c ≈ 11.28 Å. The phase exhibits a pink coloration. Upon cooling, the transition to the α phase is reversible and involves a small relative change of ΔV/V ≈ 0.5%, reflecting the subtle associated with the onset of antiferromagnetic ordering. In contrast, heating to 43.8 K induces a transition to the γ phase with a larger expansion of ΔV/V ≈ 5.4% (ΔV ≈ 1.18 cm³/mol), driven by increased molecular orientational disorder. Thermal in the β phase follows trends consistent with its intermediate position between the ordered α and disordered γ phases, though detailed measurements remain limited due to the phase's instability.

γ phase

The γ phase of solid oxygen is the highest-temperature solid phase at , stable in the temperature range of 43.8–54.4 . This phase exists just below the oxygen and is short-lived under standard conditions, transitioning to upon slight warming or pressure adjustment near 54.36 . It represents a state where O₂ molecules exhibit significant orientational disorder, distinguishing it from the more ordered lower-temperature phases. The structure of the γ phase is cubic with space group Pm₃n (No. 223), adopting an A15-type lattice in which the O₂ molecules occupy positions that allow for nearly free rotation. This configuration contains Z = 8 O₂ molecules per . The lattice parameter is a = 6.83 at 50 K, yielding a of 1.32 g/cm³. The cubic symmetry and rotational freedom result in higher compared to the β and α phases, with the phase resembling in its structural and elastic properties more closely than the denser, ordered solids below it. The transition from the rhombohedral β phase to the γ phase occurs at 43.8 and is , accompanied by a significant increase of ΔS ≈ 25.8 J/mol· due to the onset of molecular and disorder. This transition exhibits , reflecting kinetic barriers in the reorientation process. The γ phase was identified in through diffraction studies, confirming its cubic nature. Magnetically, it displays enhanced susceptibility relative to the β phase, arising from the paramagnetic response of the rotationally disordered O₂ molecules.

δ phase

The δ phase of solid oxygen is an orthorhombic polymorph stable under moderate pressures of approximately 0.1–10 GPa and low temperatures, or transiently in the range of 54–66 at low pressures; it is not thermodynamically stable at 1 atm above the oxygen temperature of 54.36 . This phase represents a higher-density form compared to the low-pressure γ phase, reflecting closer molecular packing that enhances stability under compression. Its proximity to the underscores its role in the low-temperature, low-to-moderate pressure region of the , where it can form briefly during cooling or compression paths before reverting to the α or γ phases at ambient conditions. The crystal structure is orthorhombic with space group Pmmm and Z=2 formula units per unit cell, consisting of planar zigzag chains of intact O₂ molecules oriented along the chains. The lattice parameters at ~1 GPa are a = 4.00 Å, b = 3.85 Å, and c = 6.50 Å, yielding a density of ~1.45 g/cm³ and imparting a characteristic orange hue to the phase. This chain-like arrangement introduces orthorhombic symmetry and increased density relative to the cubic γ phase, with the molecules maintaining their diatomic nature but packing in a more ordered, layered fashion that facilitates magnetic interactions. The phase was first characterized in the 1950s through high-pressure X-ray diffraction experiments, marking an early milestone in understanding pressure-induced structural changes in molecular solids. Transitionally, the δ phase emerges from the γ phase upon application of , reflecting a structural reorganization toward denser packing, and converts to the orientationally disordered ε phase at higher temperatures or . This formation involves a modest volume contraction of ~1%, highlighting the subtle yet significant densification that distinguishes it as a precursor to the plastic ε phase in compression sequences.

ε phase

The ε phase of solid oxygen represents a high-temperature plastic crystal form distinguished by significant orientational disorder among the O₂ molecules, serving as a key precursor to higher-pressure phases under compression. This phase is stable in a metastable state above approximately 70 K at ambient pressure or within the pressure range of 10–96 GPa at room temperature, where it displays a red color due to electronic transitions influenced by the molecular arrangement. The first observation of this phase occurred in 1979 during high-pressure experiments, highlighting its role in early studies of compressed molecular solids. At 20 GPa, the phase exhibits a density of approximately 1.65 g/cm³, reflecting the increased packing efficiency under compression while maintaining molecular integrity. Structurally, the ε phase adopts a cubic lattice with Pa-3 and Z=8 O₂ molecules per , characterized as a where the molecules possess substantial librational freedom around their lattice sites. This rotational mobility arises from weak intermolecular interactions, enabling the O₂ units to reorient dynamically without translational , a hallmark of plastic phases in molecular . The structure features an O8 molecular lattice with (O₂)₄ clusters. The orientational disorder in this regime is quantified by the order parameter η, defined as η≈3cos⁡2θ−12,\eta \approx \frac{3\cos^2\theta - 1}{2}, where θ is the angle between the molecular axis and a reference lattice direction; η decreases toward 0 in the fully plastic state, indicating isotropic averaging of orientations. This disorder contributes to the phase's unique thermodynamic behavior, including enhanced entropy compared to lower-temperature ordered phases. The ε phase emerges via a transition from the δ phase at around 10 GPa, accompanied by a volume contraction of ΔV/V ≈ -5%, which underscores the structural reorganization toward denser packing. Under further compression above 8 GPa and moderate temperatures, it evolves into more exotic forms, retaining discrete O₂ units but with increasing intermolecular coupling. Raman spectroscopy reveals a broadened O-O stretching mode at approximately 1100–1200 cm⁻¹, attributable to the inhomogeneous local environments caused by the orientational disorder within the lattice. Additionally, the phase displays paramagnetic properties due to unpaired spins on the disordered molecules.

Exotic phases

Red oxygen

Red oxygen, also known as the ε phase, is a high-pressure allotrope of solid oxygen distinguished by its deep red coloration and clustered molecular arrangement. First observed in 1979 through high-pressure optical experiments, this phase emerges upon compression of lower-pressure forms like the δ phase at around 8 GPa and (approximately 300 ), remaining stable up to 96 GPa and temperatures extending to about 700 under certain conditions. Unlike the diatomic molecular structure of ambient-pressure solid oxygen, red oxygen features O₈ clusters, where four O₂ molecules associate into rhombohedral units, marking an exotic transition toward greater molecular complexity without full covalent . The crystal structure of red oxygen is rhombohedral with space group R\overline{3} (No. 148), comprising a lattice of O₈ molecules formed by two perpendicular four-membered (O₄) rings fused together. Within each O₈ cluster, the intramolecular O-O bond length is approximately 1.21 , comparable to that in free O₂ molecules (1.207 ), while the intermolecular O-O distances are longer at about 2.34 , indicative of weak van der Waals interactions rather than covalent bridging. This clustered arrangement contrasts with simple molecular phases, contributing to a higher of roughly 1.6 g/cm³ at 10 GPa, and endows the material with semiconducting properties. reveals characteristic vibrational modes associated with the O₈ units, including new bands around 800–1000 cm⁻¹ attributed to ring deformations and intermolecular stretches, alongside splitting of the O₂ stretching mode near 1130 cm⁻¹. The formation of oxygen occurs via pressure-induced association of O₂ molecules into O₈ clusters, a driven by the compression of intermolecular distances that overcomes an barrier estimated at around 1 eV per . This mechanism involves no dissociation of O₂ bonds but rather a reconfiguration into stable ring structures, facilitated by the high where molecular repulsion favors clustering. The phase's striking hue arises from charge-transfer electronic transitions across a band gap of approximately 1.8 eV, absorbing and light while transmitting wavelengths, with the intensity darkening under further compression. Recent machine learning-accelerated simulations in have confirmed that oxygen does not undergo further into extended covalent networks even at extreme pressures up to 10 TPa, underscoring its stability as a molecular cluster phase rather than a precursor to fully polymeric forms. Upon decompression, red oxygen exhibits , persisting metastably to pressures as low as 0.15 GPa at cryogenic temperatures before reverting to lower molecular phases like the β or α forms; at , the reverse transition occurs around 5–8 GPa, highlighting kinetic barriers in the process. This has implications for potential applications as a high-energy-density , though its insulating nature and reversible molecular character distinguish it from conductive metallic phases at higher pressures.

Metallic oxygen

Metallic oxygen represents the high-pressure phase of solid oxygen where it transitions to a metallic state, characterized by delocalization and electrical conductivity. This phase, known as the ζ phase, emerges above approximately 96 GPa at and is stable up to at least 225 GPa, with theoretical predictions indicating persistence of molecular character to around 1.9 TPa before further . The metallization occurs through a pressure-induced closure of the , where broadening of the leads to overlap, placing the within the d-derived bands. In gaseous O₂, the HOMO-LUMO gap is approximately 12 eV, but in the solid, the effective decreases nearly linearly with pressure as E_g(P) \approx E_g_0 - \kappa P, with κ≈0.1\kappa \approx 0.1 eV/GPa, reaching zero around 96 GPa. This mechanism was first evidenced in through optical measurements showing a sharp increase in reflectivity and a drop in resistivity to the metallic regime up to 132 GPa. Structurally, the ζ phase adopts a monoclinic C2/m with eight molecules per , likely evolving from a body-centered tetragonal or hexagonal close-packed arrangement via a continuous displacive transformation from the preceding ε phase; this results in a zero bandgap due to band overlap. At 100 GPa, the theoretical is approximately 4 g/cm³, reflecting significant compression of the molecular lattice. A notable property of metallic oxygen is its , observed at a critical temperature Tc=0.6T_c = 0.6 K near 100 GPa and low temperatures. This phase follows the insulating molecular ε (red oxygen) as a precursor under compression.

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