REACTION PATHWAYS IN THE SYNTHESIS OF PHOTOREFRACTIVE GAMMA-BI12MO20 (M = SI, GE, OR TI)

Reaction pathways in the synthesis of three photorefractive silicates- gamma-Bi12SiO20 (BSO), gamma-Bi12GeO20 (EGO), and gamma-Bi12TiO20 (BTO )-were systematically investigated, The main results were as follows: (i) all the reactions of the form 6Bi(2)O(3) + MO2 --> gamma-Bi12MO20 (SR1 for M = Si, SR2 for M = Ge, SR3 for M = Ti) in the solid state se emed to be diffusion-controlled processes and were affected by both te mperature and time, where the reaction temperature increases in the or der SR1 < SR2 < SR3; (ii) the metastable phases Bi2SiO5 (tetragonal) i n reaction SR1, Bi2GeO5 (orthorhombic) in reaction SR2, Bi4Ti3O12 (ort horhombic) in reaction SR3 may be formed and seemed to greatly acceler ate the above-mentioned solid-state reaction processes; and (iii) for a continuous heating process, pure gamma-Bi12SiO20 and gamma-Bi12GeO20 could be produced before melting, whereas pure gamma-Bi12TiO20 could not be produced, even if all the mixed phases had melted.