Is ReO₃ a mixed ionic-electronic conductor? A DFT study of defect formation and migration in a B^VIO₃ perovskite-type...

Density-functional-theory (DFT) calculations within the generalised gradient approximation (GGA) were used to examine the behaviour of point defects in the cubic B^VIO₃ perovskite-type oxide, ReO₃. Energies of reduction and of hydration were calculated, and the results are compared with literature data for ABO₃ perovskite oxides. The activation energies of migration were determined for O^2-, H⁺, Li⁺, Na⁺, K⁺ and H₃O⁺. An occupied A site in ReO₃ is found to be beneficial to oxide-ion migration by a vacancy mechanism as well as to proton migration by a Grotthuss mechanism. Na⁺, K⁺ and H₃O⁺ exhibit activation energies of migration higher than 2 eV, whereas Li⁺ is characterised by a very low migration barrier of 0.1 eV. Reasons for this behaviour are discussed. Our results suggest that H⁺, O^2-, and especially Li⁺, are highly mobile ions in ReO₃.