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Bismuth(III) oxide
Bismuth(III) oxide
Bismuth(III) oxide
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Bismuth(III) oxide
Bismuth trioxide
Bismuth trioxide
Names
IUPAC names
Bismuth trioxide
Bismuth(III) oxide
Bismite (mineral)
Other names
Bismuth oxide, bismuth sesquioxide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.759 Edit this at Wikidata
EC Number
  • 215-134-7
UNII
  • InChI=1S/2Bi.3O checkY
    Key: WMWLMWRWZQELOS-UHFFFAOYSA-N checkY
  • InChI=1/2Bi.3O/rBi2O3/c3-1-5-2-4
    Key: WMWLMWRWZQELOS-JOBWJGIYAA
  • O=[Bi]O[Bi]=O
Properties
Bi2O3
Molar mass 465.958 g·mol−1
Appearance yellow crystals or powder
Odor odorless
Density 8.90 g/cm3, solid
Melting point 817 °C (1,503 °F; 1,090 K)[1]
Boiling point 1,890 °C (3,430 °F; 2,160 K)
insoluble
Solubility soluble in acids
−83.0·10−6 cm3/mol
Structure
monoclinic, mP20,
Space group P21/c (No 14)
pseudo-octahedral
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
Flash point Non-flammable
Safety data sheet (SDS) ThermoFisher SDS
Related compounds
Other anions
 Bismuth trisulfide
Bismuth selenide
Bismuth telluride
Other cations
 Dinitrogen trioxide
Phosphorus trioxide
Arsenic trioxide
Antimony trioxide
Supplementary data page
Bismuth(III) oxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Bismuth(III) oxide is a compound of bismuth, with the chemical formula Bi2O3. It is found naturally as the mineral bismite (monoclinic) and sphaerobismoite (tetragonal, much more rare), but is usually obtained as a by-product of the smelting of copper and lead ores. Dibismuth trioxide is commonly used to produce the "Dragon's eggs" effect in fireworks, as a replacement of red lead.[1]

Structure

[edit]
Existence domains of the four polymorphs of Bi2O3 as a function of temperature. (a) The α-phase transforms to the δ-phase when heated above 727 °C, which remains the structure until the melting point, 824 °C, is reached. When cooled, the δ-phase transforms into either the β-phase at 650 °C, shown in (b), or the γ-phase at 639 °C, shown in (c). The β-phase transforms to the α-phase at 303 °C. The γ-phase may persist to room temperature when the cooling rate is very slow, otherwise it transforms to the α-phase at 500 °C.[2]

Bismuth trioxide has five crystallographic polymorphs. The room temperature phase, α-Bi2O3 has a monoclinic crystal structure. There are three high temperature phases, a tetragonal β-phase, a body-centred cubic γ-phase, a cubic δ-Bi2O3 phase and an ε-phase. The room temperature α-phase has a complex structure with layers of oxygen atoms with layers of bismuth atoms between them. The bismuth atoms are in two different environments which can be described as distorted 6 and 5 coordinate respectively.[3]

β-Bi2O3 has a structure related to fluorite.[2]

γ-Bi2O3 has a structure related to that of sillenite (Bi12SiO20), but in which a small fraction of the bismuth atoms occupy positions occupied by silicon atoms in sillenite, so the formula may be written as Bi12Bi0.8O19.2. The crystals are chiral (space group I23, or no. 197) with two Bi12Bi0.8O19.2 formulas per unit cell.[4]

δ-Bi2O3 has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant.[5] ε-Bi2O3 has a structure related to the α- and β- phases but as the structure is fully ordered it is an ionic insulator. It can be prepared by hydrothermal means and transforms to the α- phase at 400 °C.[4]

The monoclinic α-phase transforms to the cubic δ-Bi2O3 when heated above 729 °C, which remains the structure until the melting point, 824 °C, is reached. The behaviour of Bi2O3 on cooling from the δ-phase is more complex, with the possible formation of two intermediate metastable phases; the tetragonal β-phase or the body-centred cubic γ-phase. The γ-phase can exist at room temperature with very slow cooling rates, but α-Bi2O3 always forms on cooling the β-phase. Even though when formed by heat, it reverts to α-Bi2O3 when the temperature drops back below 727 °C, δ-Bi2O3 can be formed directly through electrodeposition and remain relatively stable at room temperature, in an electrolyte of bismuth compounds that is also rich in sodium or potassium hydroxide so as to have a pH near 14.

Conductivity

[edit]

The α-phase exhibits p-type electronic conductivity (the charge is carried by positive holes) at room temperature which transforms to n-type conductivity (charge is carried by electrons) between 550 °C and 650 °C, depending on the oxygen partial pressure. The conductivity in the β, γ and δ-phases is predominantly ionic with oxide ions being the main charge carrier. Of these δ-Bi2O3 has the highest reported conductivity. At 750 °C the conductivity of δ-Bi2O3 is typically about 1 S cm−1, about three orders of magnitude greater than the intermediate phases and four orders greater than the monoclinic phase. δ-Bi2O3 has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant. These intrinsic vacancies are highly mobile due to the high polarisability of the cation sub-lattice with the 6s2 lone pair electrons of Bi3+. The Bi–O bonds have covalent bond character and are therefore weaker than purely ionic bonds, so the oxygen ions can jump into vacancies more freely.

The arrangement of oxygen atoms within the unit cell of δ-Bi2O3 has been the subject of much debate in the past. Three different models have been proposed. Sillén (1937) used powder X-ray diffraction on quenched samples and reported the structure of Bi2O3 was a simple cubic phase with oxygen vacancies ordered along <111>, the cube body diagonal.[6] Gattow and Schroder (1962) rejected this model, preferring to describe each oxygen site (8c site) in the unit cell as having 75% occupancy. In other words, the six oxygen atoms are randomly distributed over the eight possible oxygen sites in the unit cell. Currently, most experts seem to favour the latter description as a completely disordered oxygen sub-lattice accounts for the high conductivity in a better way.[7]

Willis (1965) used neutron diffraction to study the fluorite (CaF2) system. He determined that it could not be described by the ideal fluorite crystal structure, rather, the fluorine atoms were displaced from regular 8c positions towards the centres of the interstitial positions.[8] Shuk et al. (1996)[9] and Sammes et al. (1999)[10] suggest that because of the high degree of disorder in δ-Bi2O3, the Willis model could also be used to describe its structure.

Use in solid-oxide fuel cells (SOFCs)

[edit]

Interest has centred on δ-Bi2O3 as it is principally an ionic conductor. In addition to electrical properties, thermal expansion properties are very important when considering possible applications for solid electrolytes. High thermal expansion coefficients represent large dimensional variations under heating and cooling, which would limit the performance of an electrolyte. The transition from the high-temperature δ-Bi2O3 to the intermediate β-Bi2O3 is accompanied by a large volume change and consequently, a deterioration of the mechanical properties of the material. This, combined with the very narrow stability range of the δ-phase (727–824 °C), has led to studies on its stabilization to room temperature.

Bi2O3 easily forms solid solutions with many other metal oxides. These doped systems exhibit a complex array of structures and properties dependent on the type of dopant, the dopant concentration and the thermal history of the sample. The most widely studied systems are those involving rare earth metal oxides, Ln2O3, including yttria, Y2O3. Rare earth metal cations are generally very stable, have similar chemical properties to one another and are similar in size to Bi3+, which has a radius of 1.03 Å, making them all excellent dopants. Furthermore, their ionic radii decrease fairly uniformly from La3+ (1.032 Å), through Nd3+ (0.983 Å), Gd3+ (0.938 Å), Dy3+ (0.912 Å) and Er3+ (0.89 Å), to Lu3+ (0.861 Å) (known as the "lanthanide contraction"), making them useful to study the effect of dopant size on the stability of the Bi2O3 phases.

Bi2O3 has also been used as sintering additive in the Sc2O3-doped zirconia system for intermediate temperature SOFC.[11]

Preparation

[edit]

The trioxide can be prepared by ignition of bismuth hydroxide.[1] Bismuth trioxide can be also obtained by heating bismuth subcarbonate at approximately 400 °C.[12]

Reactions

[edit]

Atmospheric carbon dioxide or CO2 dissolved in water readily reacts with Bi2O3 to generate bismuth subcarbonate.[12] Bismuth oxide is considered a basic oxide, which explains the high reactivity with CO2. However, when acidic cations such as Si(IV) are introduced within the structure of the bismuth oxide, the reaction with CO2 do not occur.[12]

Bismuth(III) oxide reacts with a mixture of concentrated aqueous sodium hydroxide and bromine or aqueous potassium hydroxide and bromine to form sodium bismuthate or potassium bismuthate, respectively.[13]

Dissolution of bismuth(III) oxide in aqueous acids gives [Bi6O4(OH)4]6+ and [Bi(OH2)9]3+.[14][15]

Usage

[edit]

Medical devices

[edit]

Bismuth oxide is occasionally used in dental materials to make them more opaque to X-rays than the surrounding tooth structure. In particular, bismuth (III) oxide has been used in hydraulic silicate cements (HSC), originally in "MTA" (a trade name, standing for the chemically-meaningless "mineral trioxide aggregate") from 10 to 20% by mass with a mixture of mainly di- and tri-calcium silicate powders. Such HSC is used for dental treatments such as: apicoectomy, apexification, pulp capping, pulpotomy, pulp regeneration, internal repair of iatrogenic perforations, repair of resorption perforations, root canal sealing and obturation. MTA sets into a hard filling material when mixed with water. Some resin-based materials also include an HSC with bismuth oxide. Problems have allegedly arisen with bismuth oxide because it is claimed not to be inert at high pH, specifically that it slows the setting of the HSC, but also over time can lose color[16] by exposure to light or reaction with other materials that may have been used in the tooth treatment, such as sodium hypochlorite.[17]

Radiative cooling

[edit]

Bismuth oxide was used to develop a scalable colored surface high in solar reflectance and heat emissivity for passive radiative cooling. The paint was non-toxic and demonstrated a reflectance of 99% and emittance of 97%. In field tests the coating exhibited significant cooling power and reflected potential for the further development of colored surfaces practical for large-scale radiative cooling applications.[18]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bismuth(III) oxide

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Bismuth(III) oxide is an with the Bi₂O₃ and CAS number 1304-76-3. It occurs naturally as the bismite. It appears as a bright , odorless powder that is insoluble in water. It exhibits a high density of 8.9 g/cm³, melts at 825 °C, and has a of 1890 °C, making it thermally stable for various high-temperature applications. As the most stable of , it displays amphoteric and exists in multiple polymorphs, including the monoclinic α-phase (stable at ), tetragonal β-phase, body-centered cubic γ-phase, and face-centered cubic δ-phase, each with unique structural and electronic properties influencing its reactivity and performance in . This compound is widely utilized in industrial and technological contexts due to its low , , and chemical versatility. Key applications include its role as a component in ceramics and for varistors and sensors, a catalyst in and (such as for pollutant degradation), an in and enamel production, and an additive in rubber and fireproofing agents for and polymers. In biomedical fields, it serves in dental cements to provide radiopacity and in the synthesis of bismuth-based for antibacterial and anticancer therapies, leveraging its and low . Recent research highlights its potential in nanostructured forms for advanced and sensors, driven by its bandgap tunability across polymorphs.

Overview

Chemical identity and natural occurrence

Bismuth(III) oxide has the Bi₂O₃ and is systematically named bismuth(III) oxide, indicating the +3 of the cation. Alternative names include dibismuth trioxide and the mineral name bismite. Its is 465.958 g/mol. In nature, bismuth(III) oxide occurs as the mineral bismite, which corresponds to the monoclinic α-phase and forms as an oxidation product of minerals in secondary deposits. Bismite is typically found in earthy or powdery masses and is associated with other compounds like bismutite and . Another natural form is sphaerobismoite, the tetragonal β-phase, which is rarer and also arises from the oxidation of sulfides such as wittichenite and emplectite. These minerals are primarily encountered in the oxidized zones of deposits, which form in hydrothermal vein systems linked to high- to medium-temperature intrusive igneous rocks. was first isolated from ores in the , following the recognition of as a distinct element, establishing it as a foundational compound in chemistry.

Physical and thermal properties

Bismuth(III) oxide appears as a yellow crystalline powder or as yellow monoclinic crystals and is odorless. The compound has a density of 8.90 g/cm³ for the solid α-phase at standard conditions. It exhibits a of 825 °C and a of 1,890 °C, at which it vaporizes. Bismuth(III) oxide is insoluble in but soluble in concentrated acids such as (HCl) and (HNO₃). Regarding thermal properties, bismuth(III) oxide demonstrates good thermal stability and remains unchanged in air and up to its . The of the compound is approximately 113.5 J/mol· at 298 .
PropertyValueConditions/Source
8.90 g/cm³Solid, α-phase
825 °CStandard pressure
1,890 °C
Solubility in Insoluble
113.5 J/mol·298

Crystal structure and conductivity

Polymorphs and phase transitions

Bismuth(III) oxide, Bi₂O₃, is known to exist in five polymorphs, each with distinct crystal structures influenced by temperature or pressure conditions: the monoclinic α-phase, stable at room temperature; the tetragonal β-phase; the body-centered cubic γ-phase; the face-centered cubic δ-phase, which is fluorite-like and predominant at high temperatures; and the hexagonal ε-phase, formed under high pressure. The α-phase adopts the space group P2₁/c, characterized by layered arrangements of bismuth and oxygen atoms, where bismuth cations are coordinated in distorted BiO₈ polyhedra due to the stereochemical activity of the Bi³⁺ lone pair, leading to asymmetric bonding and structural distortion. Its unit cell parameters are a = 5.53 Å, b = 5.64 Å, c = 10.15 Å, and β ≈ 93.7°. The β-phase, with space group P-4̄21c, features a distorted fluorite-related structure where oxygen vacancies are more ordered compared to the δ-phase, exhibiting tetragonal symmetry. The γ-phase, space group I2̅3, displays a body-centered cubic arrangement with bismuth atoms in a more symmetric environment, though still influenced by lone-pair effects. In contrast, the high-temperature δ-phase (space group Fm-3m) possesses a defect fluorite lattice with approximately 25% oxygen vacancies, resulting in high disorder in the oxygen sublattice and enabling dynamic behavior. Its unit cell parameter is a ≈ 5.66 Å at 750 °C. The ε-phase, synthesized at 6 GPa and 880 °C, adopts a hexagonal structure isotypic to A-type rare-earth sesquioxides (space group P-3m1), with unit cell parameters a = 3.878 Å and c = 6.303 Å, representing a denser packing under pressure. Phase transitions among these polymorphs are temperature-dependent and often reversible. The α-phase undergoes a first-order transition to the δ-phase at 729 °C, accompanied by a 6.9% volume expansion, and reverts to α upon cooling below this temperature. The β-phase forms metastably upon cooling the δ-phase around 645 °C and remains stable up to approximately 650 °C before transforming to α between 450–550 °C. Similarly, the γ-phase emerges metastably near 640 °C during cooling from δ and can be quenched to . These transitions are characterized by diffraction patterns showing shifts in peak positions and intensities, reflecting changes in and vacancy ordering. The δ-phase's oxygen vacancies provide the structural foundation for its ionic conductivity properties.

Ionic conductivity mechanisms

Bismuth(III) oxide displays varying conductivity behaviors across its polymorphs, primarily electronic in lower-temperature phases and ionic in the high-temperature δ-phase. The α-phase exhibits p-type electronic conductivity at , with charge carriers consisting of positive holes. Between 550 and 650 °C, this transitions to n-type electronic conductivity dominated by . In the δ-phase, however, the material functions predominantly as an oxygen conductor, achieving ionic conductivity values of approximately 1 S/cm at 750 °C. The exceptional oxide ion mobility in the δ-phase stems from its defective , which incorporates roughly 25% intrinsic oxygen vacancies in the anion sublattice, facilitating rapid of oxygen ions through the lattice. These vacancies enable a vacancy-mediated conduction mechanism, enhanced by the high of cations that lowers migration barriers. Additionally, Frenkel defects—pairs of oxygen vacancies and interstitials—contribute to the overall defect population, further promoting ionic transport in this phase. Compared to other solid electrolytes, the δ-phase of bismuth(III) oxide demonstrates superior ionic conductivity at intermediate temperatures; for instance, it outperforms (YSZ), which requires higher operating temperatures (above 800 °C) to achieve comparable values around 0.1 S/cm. Ionic conductivity in bismuth(III) oxide is typically characterized using electrochemical impedance spectroscopy (EIS), which separates bulk, , and electrode contributions to yield total conductivity and activation energies. For the δ-phase, EIS measurements reveal activation energies of approximately 1.0 eV, reflecting the relatively low energy barrier for oxygen ion hopping in the vacancy-rich structure.

Synthesis and preparation

Laboratory synthesis methods

Bismuth(III) oxide, Bi₂O₃, is commonly prepared in laboratory settings through of suitable precursors such as nitrate or bismuth carbonate. This method involves heating the precursor in a , typically at temperatures between 400 and 800 °C, to yield the stable α-Bi₂O₃ polymorph. For instance, bismuth carbonate decomposes according to the general reaction (BiO)₂CO₃ → Bi₂O₃ + CO₂, with the process monitored to ensure complete conversion and minimal impurities. The temperature range allows for phase-pure α-Bi₂O₃ formation, though higher ends (around 700–800 °C) promote better crystallinity. Another widely used laboratory technique is precipitation followed by calcination. Bismuth salts, such as bismuth nitrate (Bi(NO₃)₃), are reacted with sodium hydroxide (NaOH) in aqueous solution to precipitate bismuth hydroxide (Bi(OH)₃), as described by the balanced equation: 2Bi(NO3)3+6NaOH→2Bi(OH)3+6NaNO32\text{Bi(NO}_3)_3 + 6\text{NaOH} \rightarrow 2\text{Bi(OH)}_3 + 6\text{NaNO}_3 This step occurs under stirring at room temperature, producing a white precipitate that is filtered, washed, and dried. The hydroxide is then calcined at 300–500 °C to decompose into Bi₂O₃ via: Bi(OH)3→12Bi2O3+32H2O\text{Bi(OH)}_3 \rightarrow \frac{1}{2}\text{Bi}_2\text{O}_3 + \frac{3}{2}\text{H}_2\text{O} yielding primarily α-Bi₂O₃ with high purity if excess alkali is avoided during precipitation. Hydrothermal synthesis is employed to access the high-temperature δ-phase of Bi₂O₃ under controlled conditions. This involves sealing bismuth precursors, such as Bi(NO₃)₃ or Bi(OH)₃, in an aqueous medium (often with alkali like KOH) within a Teflon-lined autoclave and heating to 200–300 °C under autogenous pressure for several hours. The process favors the cubic δ-Bi₂O₃ structure due to the elevated temperature and pressure, which stabilize this polymorph at ambient conditions post-synthesis. In all these methods, yield and purity depend on factors like precursor quality, control, and thermal ramp rates, often exceeding 90% for optimized runs while minimizing contaminants such as residual nitrates or hydroxides. Polymorph selection, such as α versus δ, is influenced by synthesis conditions, with details on phase transitions covered elsewhere.

Industrial production processes

Bismuth(III) oxide is primarily produced on an industrial scale as a byproduct of bismuth metal refining, where the metal is recovered from processes of and lead ores containing bismuth minerals such as bismite (Bi₂O₃). The metallic bismuth, often obtained as a during the pyrometallurgical treatment of these ores, is subsequently oxidized in air or oxygen at elevated temperatures (typically 700–900°C) using furnaces to yield the oxide via the reaction 4Bi + 3O₂ → 2Bi₂O₃. This method leverages the abundance of bismuth as a minor metal in global operations, with annual bismuth metal production around 16,000 tons as of 2024, a portion of which is converted to oxide. Another key industrial route involves the of precipitated bismuth compounds derived from aqueous solutions. (BiCl₃) solutions, obtained from dissolving concentrates in , undergo controlled by adding bases like , forming bismuth hydroxide precipitates (Bi(OH)₃). These precipitates are then filtered, dried, and roasted in air at 600–900°C to decompose into pure Bi₂O₃, with as the primary byproduct. This wet chemical process allows for better impurity control compared to direct oxidation and is scaled for continuous production in chemical plants. Industrial bismuth(III) oxide is manufactured in various purity grades to meet diverse applications, ranging from technical grade (typically >99%) used in pigments and ceramics to high-purity grades of 99.999% (5N) for and optoelectronic components. In the , annual production is registered at 1,000–10,000 tons, with global output higher and primarily concentrated in regions with bismuth mining activities like . Economically, the process benefits from bismuth's status as a low-cost metal, though energy-intensive steps like contribute to operational costs. Environmentally, bismuth(III) oxide exhibits low , classifying as a "green metal" with minimal ecological impact compared to other like lead, though of fine dust during handling poses respiratory hazards requiring standard industrial controls such as ventilation and . Recycling efforts increasingly recover bismuth compounds, including oxides, from streams, where they are present in solders and capacitors, supporting practices through hydrometallurgical extraction methods.

Chemical reactivity

Reactions with acids and bases

Bismuth(III) oxide exhibits amphoteric behavior, dissolving in strong acids to form bismuth(III) salts while showing limited reactivity with dilute bases unless an oxidant is present. In acidic media, it readily reacts with hydrochloric acid according to the equation: \ceBi2O3+6HCl−>2BiCl3+3H2O\ce{Bi2O3 + 6HCl -> 2BiCl3 + 3H2O} This dissolution produces bismuth(III) chloride, which in aqueous solution exists primarily as the hydrated ion [Bi(H₂O)₉]³⁺. Similarly, reaction with nitric acid yields bismuth(III) nitrate via Bi₂O₃ + 6HNO₃ → 2Bi(NO₃)₃ + 3H₂O, facilitating the formation of soluble bismuth species suitable for further processing. The Bi³⁺ ion in solution undergoes rapid even in mildly acidic conditions ( ≈ 1), forming polynuclear oxo-clusters such as [Bi₆O₄(OH)₄]⁶⁺ due to the stereochemically active 6s , which promotes asymmetric coordination and basic salt precipitation. This hydrolysis tendency limits the stability of simple aquo ions and favors the assembly of hydroxo-bridged structures, influencing and across pH ranges. In basic environments, bismuth(III) oxide displays low solubility in dilute or solutions, remaining largely as the solid phase without significant dissolution. However, upon fusion with concentrated NaOH or KOH in the presence of as an oxidant, it undergoes oxidation to form sodium or potassium bismuthate(V), NaBiO₃ or KBiO₃, respectively, via processes like Bi₂O₃ + 6NaOH + 2Br₂ → 2NaBiO₃ + 4NaBr + 3H₂O. This reaction highlights the oxide's potential for higher chemistry under alkaline oxidative conditions. The pH-dependent speciation of Bi(III) is depicted in stability diagrams, which illustrate the transition from dominant [Bi₆O₄(OH)₄]⁶⁺ clusters in acidic media (pH < 2) to hydrolyzed species like Bi(OH)₃ and eventual Bi₂O₃ precipitation near neutral pH, with bismuthate formation requiring basic and oxidative environments. These diagrams underscore the role of hydrolysis in controlling oxo-cluster assembly and solubility.

Reactions with carbon dioxide and halogens

Bismuth(III) oxide reacts with carbon dioxide to form bismuth subcarbonate, Bi₂O₂CO₃, particularly when exposed to moist CO₂ or under conditions that facilitate CO₂ absorption, such as in neutral or basic aqueous environments. The reaction proceeds as follows: \ceBi2O3+CO2−>Bi2O2CO3\ce{Bi2O3 + CO2 -> Bi2O2CO3} This transformation is reversible and often observed in electrocatalytic systems, where the subcarbonate phase forms from Bi₂O₃ precursors during CO₂ exposure, contributing to enhanced selectivity for production in CO₂ reduction. The kinetics of this reaction are influenced by , with elevated temperatures accelerating the absorption and formation process, and by , as nanoscale Bi₂O₃ exhibits faster conversion due to greater surface area exposure to CO₂. With , undergoes chlorination at high temperatures (typically above 500°C) to produce , BiCl₃, a volatile compound useful in . In the presence of carbon, the carbochlorination reaction is represented by: \ceBi2O3+3C+3Cl2−>2BiCl3+3CO\ce{Bi2O3 + 3C + 3Cl2 -> 2BiCl3 + 3CO} This method facilitates the conversion of the stable oxide to the , aiding in purification by allowing selective volatilization of bismuth species from oxide mixtures. Similar reactions occur with other like and iodine, though chlorination is the most commonly studied due to the lower of BiCl₃. The reaction rates depend on and particle morphology, with finer particles promoting more rapid chlorination owing to improved gas-solid contact. Regarding redox behavior, remains stable in the +3 under neutral or oxidative conditions but can be oxidized to bismuth(V) in basic media using such as . For instance, reaction with Br₂ in concentrated NaOH yields , NaBiO₃, demonstrating the potential for higher-valent species under alkaline .

Applications and uses

Electronics and energy applications

Bismuth(III) oxide, particularly in its δ-phase (δ-Bi₂O₃), serves as a promising material in solid-oxide fuel cells (SOFCs) due to its exceptionally high oxygen-ion conductivity, enabling operation at intermediate temperatures of 600–800 °C. This conductivity arises from the material's fluorite-like structure, which facilitates rapid oxide-ion transport, outperforming traditional electrolytes at lower temperatures and reducing operational costs associated with high-temperature requirements. In practical applications, δ-Bi₂O₃-based electrolytes have demonstrated power densities up to 1.0 W/cm² at 650 °C in single-cell configurations, highlighting their potential for efficient energy conversion. To enhance stability and address the inherent phase instability of pure δ-Bi₂O₃, which tends to revert to lower-conductivity polymorphs at reduced temperatures, doping with elements such as vanadium and copper is employed. For instance, the variant Bi₂V₀.₉Cu₀.₁O₅.₅₋δ (BICUVOX.10) exhibits improved oxygen nonstoichiometry and sustained conductivity, with values around 0.1 S/cm at 600 °C, making it suitable for durable SOFC electrolytes. Rare-earth dopants like erbium (Er) and yttrium (Y) further stabilize the cubic fluorite phase, as seen in compositions such as (Bi₂O₃)₀.₇₀₅(Er₂O₃)₀.₂₄₅(WO₃)₀.₀₅, which maintain high ionic conductivity while mitigating degradation over extended operation. Beyond fuel cells, bismuth(III) oxide leverages its sensitivity to oxygen changes for use in solid-state oxygen gas sensors. These sensors operate on potentiometric principles, where conductivity variations in doped Bi₂O₃, such as Bi₂O₃-Y₂O₃ mixtures, generate measurable voltage differences proportional to oxygen concentration, enabling accurate detection at temperatures as low as 400 °C. Such devices are particularly valuable in industrial monitoring, offering response times under 10 seconds and stability in harsh environments. Recent advancements since 2020 have focused on integrating bismuth(III) oxide into low-temperature SOFCs, with innovations like lanthanum-doped Bi₂O₃ nanocomposite thin film cathodes achieving area-specific resistances of 8.3 Ω cm² at 625 °C, representing a three-order-of-magnitude improvement over planar LSM cathodes. These developments, including co-doped systems like Er/Ce in bismuth oxide, have improved long-term stability, with cells showing no significant performance decline after 100 hours at 550 °C, thus addressing key barriers to commercialization. In 2025, researchers developed a bismuth oxide electrolyte exhibiting the highest oxygen-ion conductivity at ≥600 °C in its cubic phase and stability at 500 °C for 100 hours in its rhombohedral phase, with no detectable conductivity decay.

Materials and ceramics applications

Bismuth(III) oxide serves as a key additive in glazes, acting as a to lower melting temperatures and facilitate low-firing processes. Its incorporation into boron-based frits enables the production of lead-free glazes with comparable fluidity and to traditional lead oxide formulations, reducing toxicity while maintaining glaze stability during firing at temperatures below °C. In ceramics, Bi₂O₃ enhances structural and optical performance, particularly in and tellurite systems, where it promotes network modification for improved . In optical glass production, bismuth(III) oxide increases the due to the high of Bi³⁺ ions, making it ideal for applications requiring high light transmission and low dispersion, such as lenses and fibers. Concentrations of 5-20 mol% Bi₂O₃ can elevate the to values exceeding 1.8, surpassing many conventional glasses without compromising transparency in the visible and near-infrared spectra. This property positions Bi₂O₃ as a preferred lead substitute in eco-friendly optical materials, leveraging its stability to withstand processing temperatures up to 700°C. As a pigment, bismuth(III) oxide provides a vibrant yellow hue in paints and cosmetics, serving as a non-toxic alternative to lead chromate with excellent opacity and lightfastness. The color saturation is influenced by particle size, where nanoparticles (80-200 nm) yield brighter, more uniform yellow tones compared to micron-sized particles, which appear duller due to light scattering effects. Formulations incorporating 10-30 wt% Bi₂O₃ achieve high tinting strength while remaining stable under UV exposure, suitable for decorative and industrial coatings. Bismuth(III) oxide functions as a in polymer composites, particularly in and matrices, where it synergizes with brominated compounds to promote char formation and suppress smoke evolution. By replacing at loadings of 5-15 wt%, Bi₂O₃ enhances limiting oxygen index values to over 30% and reduces peak heat release rates by up to 40% during , attributed to its high stability and barrier properties. This makes it valuable for wire insulation and structural plastics requiring UL-94 V-0 ratings. Recent advancements include Bi₂O₃-based paints for , developed in 2022, which scatter sunlight effectively while emitting in the mid-infrared . These formulations, using Bi₂O₃ particles in an acrylic binder, achieve 99% solar reflectance and 97% thermal emittance, enabling sub-ambient cooling by a temperature drop of 2.31 °C under 885 W/m² without energy input. The non-toxic, scalable nature of these paints supports applications in building envelopes for energy-efficient thermal management.

Medical and pharmaceutical applications

Bismuth(III) oxide serves as a key radiopacifying agent in dental materials, particularly in (MTA) used for sealants and . Incorporated at concentrations of 10-20% by weight, it enhances the material's visibility on radiographs by providing radiopacity equivalent to at least 3 mm of aluminum, as required by ANSI/ADA Specification #57, allowing clinicians to distinguish the sealant from surrounding tooth structures. This addition ensures effective endodontic treatment outcomes, though higher levels can influence the material's color stability and mechanical properties. In diagnostic imaging, bismuth(III) oxide-based nanoparticles have emerged as promising radiocontrast agents for and computed (CT) scans of the . Ultra-small dextran-coated bismuth oxide nanoparticles (approximately 3.4 nm in size) exhibit high attenuation surpassing that of clinical , enabling clear visualization of intestinal loops and targeted accumulation at inflammatory sites in conditions like following . These variants offer advantages over traditional iodine-based agents due to bismuth's higher and , with potential for non-invasive detection of gastrointestinal pathologies. Pharmaceutically, derivatives of bismuth(III) oxide, such as , are utilized in formulations to neutralize and provide protective coatings for the stomach lining in treatments for and ulcers. , another related compound found in products like Pepto-Bismol, demonstrates antimicrobial activity against enteric pathogens including Clostridium difficile, O157:H7, , and , reducing bacterial growth by 3-9 logs and viral infectivity by up to 2.7 logs at concentrations of 2-35 mg/mL through membrane binding and intracellular accumulation. This contributes to its efficacy in managing traveler's diarrhea, infections, and nonspecific gastrointestinal upset, with eradication rates up to 90% in combination therapies. Bismuth(III) oxide and its compounds exhibit low , with an oral LD50 exceeding 5 g/kg in models, supporting in medical applications. However, chronic exposure may lead to bismuth accumulation in tissues, potentially causing reversible effects like or nephropathy at blood levels above 150 µg/L. Regulatory bodies, including the FDA, recognize certain bismuth compounds like subsalicylate and subcarbonate as (GRAS) for over-the-counter gastrointestinal use, with approvals for protectant drug products under 21 CFR 310.545.

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