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

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 La27.04-xMxW4.96O55.44-x/2 square(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 N-2 gas did not show any evidence of phase transition up to 800 degrees 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.