Proton conductivity and transport number of complex perovskite barium strontium tantalate

Abstract

Previously, complex perovskites barium strontium tantalate, i.e., Ba3Sr1.4T1.6O8.4 (BST40), has exhibited excellent proton conductivity due to its high basicity and oxygen vacancy concentration. Herein, we report proton conduction mechanism of BST40 using brick-layer model and defect equilibria model. Briefly, BST40 is prepared by solid-state synthesis method, resulting in hexagonal perovskite structure with space group of P63/m. Interestingly, grain interior and grain boundary thickness of BST40 are almost equal and the grain boundary conduction mainly dominates the conductivity at T > 500 °C. Total standard molar hydration enthalpy of BST40 and grain interior is found to be −79.6 kJ/mol and −101.3 kJ/mol, respectively. Activation energy for BST40, protons, oxygen-ions, and holes in oxygen-containing humid atmosphere is found to be 0.63 eV, 0.51 eV, 0.92 eV, and 1.08 eV, respectively. These values are higher than corresponding values for the grain interior. At 700 °C, proton transport number of BST40 and grain interior is found to be 0.7 and 0.72, respectively, which is significantly higher than other congeneric complex perovskite-type proton conductors. Overall, these results confirm that BST40 possesses excellent proton conductivity.