Phase transitions among four BiB3O6 polymorphs: a detailed investigation

Abstract

The stability and phase transformations of the four BiB₃O₆ polymorphs, i.e., α-, β-, γ- and δ-BiB₃O₆, were investigated under different temperatures and pressures. β-BiB₃O₆ is a metastable phase which can only be obtained by boric acid flux method in a sealed system at low temperature. The other polymorphs, α-, γ- and δ-BiB₃O₆, are thermodynamic stable phases. Owing to the large kinetic effect, the phase transformations of the BiB₃O₆ polymorphs are generally slow, thus an extensive annealing process was used in this study. The study shows that α-BiB₃O₆ is the high-temperature phase and its stable area is narrow (710–715 °C) in atmospheric conditions, while γ-BiB₃O₆ is a stable phase in the intermediate temperature range (∼680–710 °C). The results also confirm that δ-BiB₃O₆ is a high-pressure phase and all other polymorphs may transfer to δ-phase under high pressure. However, the δ-phase can be obtained by annealing α- or β-BiB₃O₆ at relatively low temperature under ambient pressure. Therefore, δ-BiB₃O₆ can also be regarded as the low-temperature phase in this system. Based on the experimental results, as well as the thermodynamic principles, a pressure–temperature (P–T) phase diagram for the BiB₃O₆ system is proposed.