METASTABLE DELTA-BI12SIO20 AND ITS EFFECT ON THE QUALITY OF GROWN SINGLE-CRYSTALS OF GAMMA-BI12SIO20

Metastable delta-Bi12SiO20 may crystallize from the overheated Bi12SiO 20 melt and transform into stable gamma-Bi12SiO20 at about 569.5 degre es C during the subsequent slow cooling process. The transition delta- Bi12SiO20 --> gamma-Bi12SiO20 is irreversible and the gamma-Bi12SiO20 is Stable up to the melting temperature. By quenching the Bi12SiO20 me lt, pure delta-Bi12SiO20 can be obtained at room temperature. The quen ched delta-Bi12SiO20 crystal is nontransparent and has a space group o f Fm3m (225) and a lattice constant of 55.417 Angstrom at 20 degrees C . The quenched metastable delta-Bi12SiO20 can transform into pure gamm a-Bi12SiO20 at 382.5-386.1 degrees C with an exothermic heat of 31.68- 32.38 J/g. The transition-produced gamma-Bi12SiO20 crystal is still no ntransparent and has a large lattice distortion. The transition delta- Bi12SiO20 --> gamma-Bi12SiO20 causes about 6% volume contraction, whic h may result in cracking of the grown crystal. By controlling the grow th parameters, this transition can be effectively avoided.