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
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. Also, it can be obtained by heating
bismuth subcarbonate at approximately 400 °C.
α phase The α phase is found naturally as the mineral
bismite.
β phase β- has a structure related to
fluorite.
δ phase δ- has a defective
fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant. 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 (1937) used powder X-ray diffraction on quenched samples and reported the structure of was a simple
cubic phase with oxygen
vacancies ordered along , the cube body diagonal. • 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 () 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. (1996) and Sammes et al. (1999) suggest that because of the high degree of disorder in δ-, the Willis model could also be used to describe its structure. 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.
ε phase 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. ==Conductivity==