The relative stabilities of Bi2MoO6 polymorphs

The relative stability of Bi₂Mo₆ polymorphs was studied by isothermal heating at 250–635°C, and 100–1500 kg/cm². The results obtained follow: (1) A reversible transition was observed between two stable phases at low (L) and high (H) temperatures, γ(L)-Bi₂MoO₆ with the koechlinite structure and γ(H)-Bi₂MoO₆ (=γ′ labeled by Elman). (2) A pressure-temperature phase diagram of Bi₂MoO₆ was drawn and it showed that the γ(L)-form was more stable than the γ(H)-form in the low-temperature and high-pressure region. (3) The transition temperature of γ(L) ? γ(H) under atmospheric pressure was estimated to be about 570°C by extrapolation of the phase boundary. (4) A third modification, γ″-Bi₂MoO₆ (a metastable phase), was not detected in the experiments. A free-energy-temperature diagram for the three modifications, γ(L), γ(H), and γ″, is proposed on the basis of the present experimental results and previously published data.