Evaluation of solid solution formation between ThO2 and δ-Bi2O3 by molecular precursor route

The high chelating ability of nitrilotriacetic acid has been advantageously utilized to prepare molecular precursors containing thorium and bismuth. This aided to determine the extent of solid solution formation between thoria and delta-Bi2O3 in fluorite structure by calcining the precursors in air. Up to 60 atomic % of Bi3+ could be substituted for Th4+ in fluorite structure beyond which phase separation occurred. Slight increase in cubic lattice constant was observed with increasing bismuth content along with indication of increased tensile strain of lattice. Porous morphology and presence of bismuth in calcined samples were confirmed from SEM and EDX measurements. Defect bands emerged in the Raman spectra near 590 cm(-1) in addition to a fluorite band near 460 cm(-1) for the bismuth substituted samples. Intermediate energy states were formed by the substitution of Bi3+ for Th4+ arising possibly from oxygen vacancies and it resulted in narrowing of direct band gap.