Oxygen ion conductivity studies of bismuth and bismuth-calcium co-doped ThO2

A series of trivalent (Bi3+) doped and divalent (Ca2+) co-doped ThO2 samples i.e., Th0.50-xCaxBi0.50O2-delta (x = 0.00, 0.05, 0.10, 0.15 and 0.20) have been synthesized by citrate-nitrate solution combustion route and investigated in the context of oxygen ion conductivity and dielectric relaxation phenomena. The Rietveld refinement of the Powder X-ray diffraction data confirmed monophasic fluorite structures (S.G. Fm3m) for calcium concentrations up to 20 mol %. The optical band gap decreased with the increase in Ca2+ concentration up to 10 mol %. In contrast, the defect band's intensity in the Raman spectra increased due to oxygen vacancies on divalent addition. The quantitative aspect of oxygen vacancy and defect concentration was derived from Raman spectra. The crystallographic index application was further employed to interpret the optimum doping concentration to maximize oxide ion conductivity. Remarkably high oxide ion conductivity (-10-3 S/cm) was observed for Bi3+ doped (50 mol %), and Bi3+ (50 mol %)-Ca2+ (10 mol %) co-doped ThO2 samples at 773 K. The Nyquist plot exhibited grain contribution for low dopant levels. Both grain and grain boundary contribution were present in the higher dopant concentrations. Conductivity, dielectric, and modulus properties of doped and co-doped samples have been compared, from which 10 mol % of Ca2+-doping was identified to be the optimum concentration.

Automated summary

Trivalent (Bi3+) doped and divalent (Ca2+) co-doped ThO2 samples were synthesized and studied for oxygen ion conductivity and dielectric relaxation phenomena. The optical band gap decreased with increasing Ca2+ concentration, while the Raman spectra showed increased intensity of defect bands due to oxygen vacancies from divalent addition. The highest oxide ion conductivity was observed for Bi3+ doped (50 mol %) and Bi3+ (50 mol %)-Ca2+ (10 mol %) co-doped ThO2 samples.