Bismuth(III) oxide
Bismuth oxide is a compound of bismuth, with the chemical formula Bi2O3. It is found naturally as the mineral bismite and sphaerobismoite, 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.
Structure
Bismuth trioxide has five crystallographic polymorphs. The room temperature phase, α- has a monoclinic crystal structure. There are three high temperature phases, a tetragonal β-phase, a body-centred cubic γ-phase, a cubic δ- 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.
β- has a structure related to fluorite.
γ- has a structure related to that of sillenite, 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. The crystals are chiral with two formulas per unit cell.
δ- has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant.
ε- 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.
The monoclinic α-phase transforms to the cubic δ- when heated above 729 °C, which remains the structure until the melting point, 824 °C, is reached. The behaviour of 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- always forms on cooling the β-phase. Even though when formed by heat, it reverts to α- when the temperature drops back below 727 °C, δ- 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
The α-phase exhibits p-type electronic conductivity at room temperature which transforms to n-type conductivity 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- has the highest reported conductivity. At 750 °C the conductivity of δ- 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- 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. 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- has been the subject of much debate in the past. Three different models have been proposed. Sillén used powder X-ray diffraction on quenched samples and reported the structure of was a simple cubic phase with oxygen vacancies ordered along <111>, the cube body diagonal. Gattow and Schroder rejected this model, preferring to describe each oxygen 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 used neutron diffraction to study the fluorite 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. Shuk et al. and Sammes et al. suggest that because of the high degree of disorder in δ-, the Willis model could also be used to describe its structure.
Use in solid-oxide fuel cells (SOFCs)
Interest has centred on δ- 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- to the intermediate β- 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, has led to studies on its stabilization to room temperature.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,, including yttria,. Rare earth metal cations are generally very stable, have similar chemical properties to one another and are similar in size to, which has a radius of 1.03 Å, making them all excellent dopants. Furthermore, their ionic radii decrease fairly uniformly from, through,, and, to, making them useful to study the effect of dopant size on the stability of the phases.
has also been used as sintering additive in the -doped zirconia system for intermediate temperature SOFC.
Preparation
The trioxide can be prepared by ignition of bismuth hydroxide. Bismuth trioxide can be also obtained by heating bismuth subcarbonate at approximately 400 °C.Reactions
Atmospheric carbon dioxide or dissolved in water readily reacts with to generate bismuth subcarbonate. Bismuth oxide is considered a basic oxide, which explains the high reactivity with. However, when acidic cations such as Si are introduced within the structure of the bismuth oxide, the reaction with do not occur.Bismuth 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.
Dissolution of bismuth oxide in aqueous acids gives 6+ and 3+.