Solid-phase reaction in synthesis of Bi12SiO20 source-rods for single-crystal growth in a floating zone

Abstract

Abstract The solid-phase reaction and the concomitant phase transitions in the synthesis of the source rod for Bi 12 SiO 20 single-crystal growth in a floating zone were studied by X-ray diffraction, differential scanning calorimeter (DSC), differential thermal analysis (DTA), and thermogravimetry (TG). The following experimental phenomena were observed. (1) The solid phase reaction (SRI), i.e., 6α-Bi 2 O 3 (monoclinic) + SiO 2 (hexagonal) → γ-Bi 12 SiO 20 (b.c.c.), appears to be a diffusion-controlled process, and the reaction is affected by both temperature and time. (2) A metastable phase, Bi 2 SiO 5 (tetragonal), may be formed in the synthesis of γ-Bi 12 SiO 20 source rod if the reaction temperature is over 500 °C. The Bi 2 SiO 5 can be caused to react with the remaining Bi 2 O 3 to form pure γ-Bi 12 SiO 20 by increasing the aging temperature or through crystal growth of Bi 12 SiO 20 by a floating zone method. However, the Bi 2 SiO 5 intermediate greatly accelerates the solid-phase reaction SR1. Thus, different reaction mechanisms are involved depending on whether Bi 2 SiO 5 is formed. At below 500 °C (without Bi 2 SiO 5 ), the reaction order of the solid-phase reaction SR1 is 3.5; and at above 500 °C (with Bi 2 SiO 5 ) it is 3.0. (3) The quality of the sintered source rod is directly dependent on the sintering method used, i.e., the reaction rate. The suitable multi-sintering process proposed in this paper proved to be useful for controlling the reaction rate and avoiding cracking of the sintered source rod.