Bismuth(III) oxide is perhaps the most industrially important compound of
bismuthBismuth is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all...
. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral
bismiteBismite 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...
(monoclinic) and sphaerobismoite (tetragonal, much more rare), but it is usually obtained as a by-product of the smelting of
copperCopper 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 rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
and
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
ores. Bismuth trioxide is commonly used to produce the "
Dragon's eggsDragon's eggs are 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
fireworksA firework is a low explosive pyrotechnic device used primarily 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 leadRed lead, also called minium, lead tetroxide or triplumbic tetroxide, is a bright red or orange crystalline or amorphous pigment. Its Latin name minium originates from the Minius River in northwest Spain where it was first mined. Natural minium is uncommon, forming only in extreme oxidizing...
.
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, Bi
2O
3 has four crystallographic
polymorphsPolymorphism 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 elemental solids...
. It has a monoclinic crystal structure, designated α- Bi
2O
3, at room temperature. This transforms to the
cubic-General:*cubicle, a small area set off by walls for special use, such as a place to work, to shower, or with a toilet.-Science and mathematics:*cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex...
fluorite-type crystal structure, δ-Bi
2O
3, when heated above 727°C, which remains the structure until the melting point, 824°C, is reached. The behaviour of Bi
2O
3 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- Bi
2O
3 always forms on cooling the β-phase.
δ- Bi
2O
3 has the highest reported conductivity. At 750°C the conductivity of δ- Bi
2O
3 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- Bi
2O
3 has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant. These intrinsic
vacanciesIn crystallography, a vacancy is a type of point defect in a crystal.Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom, such as silicon, is missing from one of the lattice sites is known as a 'vacancy' defect.Vacancies occur naturally in...
are highly mobile due to the high polarisability of the cation sub-lattice with the 6
s2 lone pairA lone pair is a 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 Bi
3+. The Bi-O bonds have covalent bond character and are therefore weaker than purely ionic bonds, so the oxygen ions can jump into
vacanciesIn crystallography, a vacancy is a type of point defect in a crystal.Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom, such as silicon, is missing from one of the lattice sites is known as a 'vacancy' defect.Vacancies occur naturally in...
more freely.
The arrangement of oxygen atoms within the unit cell of δ- Bi
2O
3 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 Bi
2O
3 was a simple
cubic-General:*cubicle, a small area set off by walls for special use, such as a place to work, to shower, or with a toilet.-Science and mathematics:*cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex...
phase with oxygen
vacanciesIn crystallography, a vacancy is a type of point defect in a crystal.Crystals inherently possess imperfections, often referred to as 'crystalline defects'. A defect wherein an atom, such as silicon, is missing from one of the lattice sites is known as a 'vacancy' defect.Vacancies occur naturally in...
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 diffractionNeutron diffraction is a method for the determination of the atomic and/or magnetic structure of a material. It can be equally well applied to study crystalline solids , gasses, liquids or amorphous materials. Neutron diffraction is a form of elastic scattering where the neutrons exiting the...
to study the fluorite (CaF
2) 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- Bi
2O
3, the Willis model could also be used to describe its structure.
In addition to electrical properties,
thermal expansionThermal expansion is the tendency of a matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this...
properties are very important when considering possible applications for solid electrolytes. High
thermal expansionThermal expansion is the tendency of a matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this...
coefficients represent large dimensional variations under heating and cooling which would limit the performance of an electrolyte. The transition from the high-temperature δ- Bi
2O
3 to the intermediate β- Bi
2O
3 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
oC), has led to studies on its stabilization to room temperature.
Bi
2O
3 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 earthAs defined by IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids...
metal oxides, Ln
2O
3, including yttria, Y
2O
3. Rare earth metal cations are generally very stable, have similar chemical properties to one another and are similar in size to Bi
3+, which has a radius of 1.03 Å, making them all excellent dopants. Furthermore, their ionic radii decrease fairly uniformly from La
3+ (1.032 Å), through Nd
3+, (0.983 Å), Gd
3+, (0.938 Å), Dy
3+, (0.912 Å) and Er
3+, (0.89 Å), to Lu
3+, (0.861 Å) (known as the ‘
lanthanide contractionLanthanide contraction is a term used in chemistry to describe different but closely related concepts associated with smaller than expected ionic radii of the elements in the lanthanide series .-Cause:...
’), making them useful to study the effect of dopant size on the stability of the Bi
2O
3 phases.
Preparation
Bismuth trioxide is commercially made from bismuth subnitrate. The latter is produced by dissolving bismuth in hot
nitric acidNitric 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 stronger yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as...
. Addition of excess
sodium hydroxideSodium hydroxide , also known as lye and caustic soda, is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide production in 1998 was around...
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 2S2O8 is a strong oxidizing agent. It is very soluble in cold water, a large fall of temperature accompanying solution. 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 ferricyanidePotassium ferricyanide is the chemical compound with the formula K
3[Fe
6]. This bright red salt consists of the coordination compound [Fe
6]
3−...
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
hydrofluoric acidHydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and difficult to handle, it is technically a weak acid...
forming
bismuth trifluorideBismuth trifluoride is a chemical compound of bismuth and fluorine. The chemical formula is BiF3. It is a grey-white powder melting at 649°C.-Synthesis:Bismuth trifluoride can be prepared by reacting bismuth oxide with hydrofluoric acid:...
.
Reaction with acetic anhydride and
oleic acidOleic acid is a mono-unsaturated omega-9 fatty acid found in various animal and vegetable sources. It has the formula CH37CH=CH7COOH)...
gives bismuth trioleate.