Proton conductors based on alkaline-earth substituted La28−xW4+xO54+3x/2

Abstract

Lanthanum tungstates, “La₆WO₁₂”, are mixed ion proton–electronic conductors with very interesting properties for technological applications and better phase stability compared to alkaline earth perovskites. A new series of compounds La_27.04−xM_xW_4.96O_55.44−x/2□_8.56+x/2 (M = Ca²⁺, Sr²⁺ and Ba²⁺) are investigated with the aim of increasing the concentration of oxygen vacancies and studying their effects on the structure and transport properties. The materials have been studied by high-resolution laboratory X-ray powder diffraction and scanning electron microscopy combined with energy dispersive spectroscopy (EDS). High temperature X-ray powder diffraction and thermal analysis in wet and dry N₂ gas did not show any evidence of phase transition up to 800 °C. The total conductivity was studied by impedance spectroscopy under dry and wet atmospheres and as a function of the oxygen partial pressure. The electronic contribution to the conductivity was determined by the Hebb–Wagner polarization method. The generation of extrinsic vacancies in the lattice with alkaline earth doping leads to a decrease of the ionic conductivity for high doping level, suggesting a proton trapping mechanism.