Crystallization and Decomposition of Compounds with the Aurivillius Crystal Structure in the Bi2GeO5–Bi2SiO5 Pseudobinary Metastable System

Abstract

The possibility of partial substitution of Si for Ge in the structure of Bi2GeO5 during synthesis from the melt and the effect of the substitution on the structure of the synthesized material have been studied. The microstructure of the synthesized compounds after their complete decomposition has been investigated. Using the phase powder X-ray diffraction- and optical microscopy methods, it has been found that metasta-ble phases of bismuth silicate and germanate with structures of the Aurivillius type form a continuous seriesof solid solutions. It is shown that a mixed structure composed of large Bi4Si(Ge)3O12 crystals and the Bi12Si(Ge)O20 + Bi4Si(Ge)3O12 point eutectic is obtained after the decomposition of compounds of the Bi2SiO5–Bi2GeO5 quasi-binary system, regardless of the percentage of substitution of Si for Ge. Upon slow heating up to annealing temperatures (at a rate of 13.5°C/min), a structure of decomposition that is more finely dispersed and uniform than the structure obtained upon rapid heating (when charg the material into an already preheated furnace), which is more coarsely grained and inhomogeneous. Moreover, regions with a large structure similar to a dendritic one can appear in the material when the contents of silicon and germa-nium oxides in the alloy are close to each other (20/30–30/20 mol %). Such regions differ slightly in chemical composition from the surrounding material and appear during both slow and rapid heating of the material to annealing temperatures.