SOLID-PHASE REACTION IN SYNTHESIS OF BI12SIO20 SOURCE-RODS FOR SINGLE-CRYSTAL GROWTH IN A FLOATING-ZONE

The solid-phase reaction and the concomitant phase transitions in the synthesis of the source rod for Bi12SiO20 single-crystal growth in a f loating zone were studied by X-ray diffraction, differential scanning calorimeter (DSC), differential thermal analysis (DTA), and thermograv imetry (TG). The following experimental phenomena were observed. (1) T he solid phase reaction (SR1), i.e., 6 alpha-Bi2O3(monoclinic) + SiO2 (hexagonal) --> gamma-Bi12SiO20 (b.c.c.), appears to be a diffusion-co ntrolled process, and the reaction is affected by both temperature and time. (2) A metastable phase, Bi2SiO5(tetragonal), may be formed in t he synthesis of gamma-Bi12SiO20 source rod if the reaction temperature is over 500 degrees C. The Bi2SiO5 can be caused to react with the re maining Bi2O3 to form pure gamma-Bi12SiO20 by increasing the aging tem perature or through crystal growth of Bi12SiO20 by a floating zone met hod. However, the Bi2SiO5 intermediate greatly accelerates the solid-p hase reaction SR1. Thus, different reaction mechanisms are involved de pending on whether Bi2SiO5 is formed. At below 500 degrees C (without Bi2SiO5), the reaction order of the solid-phase reaction SR1 is 3.5; a nd at above 500 degrees C (with Bi2SiO5) 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 p roposed in this paper proved to be useful for controlling the reaction rate and avoiding cracking of the sintered source rod.