Investigations into Sr3CaZr0.5Ta1.5O8.75, a novel proton conducting perovskite oxide

A new complex triple perovskite of formula Sr3CaZr0.5Ta1.5O8.75 has been synthesized. X-ray diffraction indicates a tetragonal unit cell for an as-prepared sample, with a=5.8049(6) and c=8.220(1), although high-resolution neutron diffraction reveals a slight monoclinic distortion. Thermogravimetric analysis (TGA) shows that water is taken up to fill 55% of the vacant oxygen sites when a vacuum-treated sample is subjected to wet 5% H2/Ar atmosphere at 380°C for 24 h. X-ray diffraction simply shows a decrease in the degree of tetragonal distortion on hydration. In wet H2 atmosphere (4% H2/3% H2O), the bulk and grain boundary conductivities were found to be σb=4.64×10−4 S cm−1 (Eact=0.66 eV) and σgb=7.04×10−5 S cm−1 at 300°C (Eact=0.88 eV), an improvement over Sr3Ca1.18Nb1.82O8.73 under the same conditions. Proton conduction has been verified by changing the hydration level of a 5% H2/Ar atmosphere, while AC impedance conductivity measurements were made. On changing from a dry gas atmosphere to a wet gas atmosphere, bulk proton conductivity increases by a factor of 3.5 when exposed to a wet 5% H2/Ar mixture for 24 h at 644°C. The process is reversible, i.e. the resistance is seen to increase, on annealing in a dry atmosphere. The half-life of the water (proton) loss process is approximately 72 min at 440°C.