Metastable δ-Bi12SiO20 and its effect on the quality of grown single crystals of γ-Bi12SiO20

Abstract

Metastable δ-Bi12SiO20 may crystallize from the overheated Bi12SiO20 melt and transform into stable γ-Bi12SiO20 at about 569.5 °C during the subsequent slow cooling process. The transition δ-Bi12SiO20 → γ-Bi12SiO20 is irreversible and the γ-Bi12SiO20 is stable up to the melting temperature. By quenching the Bi12SiO20 melt, pure δ-Bi12SiO20 can be obtained at room temperature. The quenched δ-Bi12SiO20 crystal is nontransparent and has a space group of Fm3m (225) and a lattice constant of 55.417 Å at 20 °C. The quenched metastable δ-Bi12SiO20 can transform into pure γ-Bi12SiO20 at 382.5–386.1 °C with an exothermic heat of 31.68–32.38 J/g. The transition-produced δ-Bi12SiO20 crystal is still nontransparent and has a large lattice distortion. The transition δ-Bi12SiO20 → γ-Bi12SiO20 causes about 6% volume contraction, which may result in cracking of the grown crystal. By controlling the growth parameters, this transition can be effectively avoided.