Phase Behaviour and Mixed Ionic-Electronic Conductivity of Ba4Sb2O9

Abstract

The 6H-type perovskite phase Ba4Sb2O9, which decomposes in air below 600 K, is found to survive to room temperature in a CO2-free atmosphere. It shows substantial mixed protonic, oxide ionic and electronic conductivity. However, compared to Ba4 Nb2 O9 and Ba4 Ta2 O9 , Ba4Sb2O9 shows higher ionic conductivity due to the relatively easy reducibility of Sb5+, but lower electronic conductivity due to the predominantly n-type conductivity provided by the Sb5+/Sb3+ redox couple which leads to reduced hole concentration under oxidising conditions. Variable temperature synchrotron x-ray and neutron powder diffraction studies carried out in tu under controlled atmospheres reveal a strong monoclinic distortion below 1150 K. The hexagonal → monoclinic transition is slow, does not show second-order behaviour, is strongly dependent on atmosphere, and coincides with the loss of ∼0.4 molecules of H2O per formula unit of Ba4Sb2O9. All of this suggests an important structural role for protons or hydroxide ions in the monoclinic phase.