Stability against reduction of fluorite-like rhombohedral La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 fluorite: Conductivity and neutron diffraction study

Zr-substituted rare-earth molybdates Ho5.4Zr0.6MoO12.3 and unsubstituted La5.5MoO11.25 demonstrate appreciable mixed electron-proton conductivity in the 200–470 °C and 145–730 °C temperature range, respectively, under wet oxidizing and mild reducing conditions (air, Ar, Ar-5% H2). Rhombohedral fluorite-like La5.5MoO11.25 showed the highest bulk conductivity of about 1 × 10−5 S/cm at 400 °C in wet air and Ar. Its impedance spectra did not provide any evidence of the grain boundary contribution in wet atmosphere. Total conductivity of Ho5.4Zr0.6MoO12.3 fluorite is much lower and is 3 × 10−7 S/cm at 400 °C in wet air. Besides, it should be noted that there is a grain-boundary contribution of Ho5.4Zr0.6MoO12.3 in wet air and Ar.