Ionic and p-type electronic transport in zircon-type Ce1 − xAxVO4 ± δ(A = Ca, Sr)

Abstract

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce1 − xAxVO4 (A = Ca, Sr; x = 0–0.2) leads to higher p-type electronic conductivity, while the tetragonal unit cell volume and Seebeck coefficient decrease due to increasing concentration of electron holes localised on cerium cations. The oxygen ion transference numbers of Ce1 − xCaxVO4 in air, determined by faradaic efficiency measurements, vary in the range from 2 × 10−4 to 6 × 10−3 at 973–1223 K, increasing with temperature. The ionic conductivity is essentially independent of calcium content and decreases with reducing oxygen partial pressure. The activation energy for ionic transport in Ce(Ca)VO4 is 90–125 kJ mol−1. Doping with calcium enhances the stability of cerium orthovanadate at reduced oxygen pressures, shifting the phase decomposition limits down to oxygen activity values of 10−16–10−14 atm at 1023 K. The results on structure, Seebeck coefficient, and the partial p-type electronic and oxygen ionic conductivities suggest the presence of hyperstoichiometric oxygen in the Ce1 − xAxVO4 + δ lattice. The hyperstoichiometry, estimated from Seebeck coefficient data in the p(O2) range from 10−19 to 0.75 atm at 923–1223 K, may achieve 2–3% of the total oxygen content and weakly depends on the temperature and oxygen pressure variations within the zircon phase existence domain. Thermal expansion coefficients of Ce1 − xAxVO4 + δ ceramics in air, calculated from dilatometric data, are in the narrow range (5.6–5.9) × 10−6 K−1 at 400–800 K.