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., Th_0.50-xCa_xBi_0.50O_2-δ (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. Fm$\stackrel{￣}{3}$m) 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.