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% H-2/Ar atmosphere at 380 degreesC for 24 It. X-ray diffraction simply shows a decrease in the degree of tetragonal distortion on hydration. In wet H-2 atmosphere (4% H-2/3% H2O), the bulk and grain boundary conductivities were found to be sigma (b) = 4.64 x 10(-4) S cm(-1) (E-act = 0.66 eV) and sigma (gb) = 7.04 x 10(-5) S cm(-1) at 300 degreesC (E-act = 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% H-2/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% H-2/Ar mixture for 24 h at 644 degreesC. 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 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.