Bismuth trioxide
Encyclopedia
Bismuth oxide is perhaps the most industrially important compound of bismuth
. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral bismite
(monoclinic) and sphaerobismoite (tetragonal, much more rare), but it is usually obtained as a by-product of the smelting of copper
and lead
ores. Bismuth trioxide is commonly used to produce the "Dragon's eggs
" effect in fireworks
, as a replacement of red lead
.
. It has a monoclinic crystal structure, designated α- Bi2O3, at room temperature. This transforms to the cubic
fluorite-type crystal structure, δ-Bi2O3, when heated above 727°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.
δ- Bi2O3 has the highest reported conductivity. At 750°C the conductivity of δ- Bi2O3 is typically about 1 Scm− 1, about three orders of magnitude greater than the intermediate phases and four orders greater than the monoclinic phase. The conductivity in the β, γ and δ-phases is predominantly ionic with oxide ions being the main charge carrier. 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. It is therefore unsuitable for electrolyte applications. 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. Sillen (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>, i.e. along the cube body diagonal (Figure 2a). 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.
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 (Figure 2c). Shuk et al. (1996) and Sammes et al. (1999) suggest that because of the high degree of disorder in δ- Bi2O3, the Willis model could also be used to describe its structure.
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 (727-824oC), 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.
. Addition of excess sodium hydroxide followed by continuous heating of the mixture precipitates bismuth(III) oxide as a heavy yellow powder. Also, the trioxide can be prepared by ignition of bismuth hydroxide.
and dilute caustic soda gives bismuth tetroxide. The same product can be obtained by using other oxidizing agents such as potassium ferricyanide
and concentrated caustic potash solution.
Electrolysis of bismuth(III) oxide in hot concentrated alkali solution gives a scarlet red precipitate of bismuth(V) oxide. Bismuth(III) oxide reacts with mineral acids to give the corresponding bismuth(III) salts.
Reaction with acetic anhydride
and oleic acid
gives bismuth trioleate.
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral bismite
Bismite
Bismite is a bismuth oxide mineral, bismuth trioxide or Bi2O3. It is a monoclinic mineral, but the typical form of occurrence is massive and clay-like with no macroscopic crystals. The color varies from green to yellow. It has a Mohs hardness of 4 to 5 and a specific gravity of 8.5 to 9.5, quite...
(monoclinic) and sphaerobismoite (tetragonal, much more rare), but it is usually obtained as a by-product of the smelting of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
ores. Bismuth trioxide is commonly used to produce the "Dragon's eggs
Dragon's eggs
Dragon's eggs are fireworks pyrotechnic stars which first burn for a period for a visual effect then explode with a loud report. Manufacture of this effect became controversial because of the heavy metals once used, particularly lead tetroxide...
" effect in fireworks
Fireworks
Fireworks are a class of explosive pyrotechnic devices used for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display. A fireworks event is a display of the effects produced by firework devices...
, as a replacement of red lead
Red lead
Lead tetroxide, also called minium, red lead or triplumbic tetroxide, is a bright red or orange crystalline or amorphous pigment. Chemically, red lead is lead tetroxide, Pb3O4, or 2PbO·PbO2....
.
As a material for fuel cell electrolytes
Bismuth oxide has seen interest as a material for solid oxide fuel cells or SOFCs since it is an ionic conductor, i.e. oxygen atoms readily move through it. Pure bismuth oxide, Bi2O3 has four crystallographic polymorphsPolymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
. It has a monoclinic crystal structure, designated α- Bi2O3, at room temperature. This transforms to the cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
fluorite-type crystal structure, δ-Bi2O3, when heated above 727°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.
δ- Bi2O3 has the highest reported conductivity. At 750°C the conductivity of δ- Bi2O3 is typically about 1 Scm
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
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. Sillen (1937) used powder X-ray diffraction on quenched samples and reported the structure of Bi2O3 was a simple cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
phase with oxygen vacancies ordered along<111>, i.e. along the cube body diagonal (Figure 2a). 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.
Willis (1965) used neutron diffraction
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...
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 (Figure 2c). Shuk et al. (1996) and Sammes et al. (1999) suggest that because of the high degree of disorder in δ- Bi2O3, the Willis model could also be used to describe its structure.
In addition to electrical properties, thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
properties are very important when considering possible applications for solid electrolytes. High thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
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 (727-824oC), 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
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
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
Lanthanide contraction
Lanthanide contraction is a term used in chemistry to describe the decrease in ionic radii of the elements in the lanthanide series from atomic number 58, Cerium to 71, Lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, Hafnium...
’), making them useful to study the effect of dopant size on the stability of the Bi2O3 phases.
Preparation
Bismuth trioxide is commercially made from bismuth subnitrate. The latter is produced by dissolving bismuth in hot nitric acidNitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
. Addition of excess sodium hydroxide followed by continuous heating of the mixture precipitates bismuth(III) oxide as a heavy yellow powder. Also, the trioxide can be prepared by ignition of bismuth hydroxide.
Reactions
Oxidation with ammonium persulfateAmmonium persulfate
Ammonium persulfate 2S2O8 is a strong oxidizing agent. It is very soluble in water; the dissolution of the salt in water is endothermic. It is a radical initiator. It is used to etch copper on printed circuit boards as an alternative to ferric chloride solution...
and dilute caustic soda gives bismuth tetroxide. The same product can be obtained by using other oxidizing agents such as potassium ferricyanide
Potassium ferricyanide
Potassium ferricyanide is the chemical compound with the formula K3[Fe6]. This bright red salt contains the octahedrally coordinated [Fe6]3− ion. It is soluble in water and its solution shows some green-yellow fluorescence.-Preparation:...
and concentrated caustic potash solution.
Electrolysis of bismuth(III) oxide in hot concentrated alkali solution gives a scarlet red precipitate of bismuth(V) oxide. Bismuth(III) oxide reacts with mineral acids to give the corresponding bismuth(III) salts.
Reaction with acetic anhydride
Acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula 2O. Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis...
and oleic acid
Oleic acid
Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...
gives bismuth trioleate.