Enhanced bulk conductivity of A-site divalent acceptor-doped non-stoichiometric sodium bismuth titanate

Bismuth-deficient sodium bismuth titanate (nominally Na0.5Bi0.49TiO2.985, NB0.49T) is a good oxide-ion conductor. Here we report the influence of A-site divalent ions, M2+ = Ca2+, Sr2+ and Ba2+, on the electrical properties of NB0.49T. A-site divalent doping for Bi3+ enhances the bulk (grain) conductivity by similar to one order of magnitude without changing the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on the doping mechanism Bi3+ + 1/2 O2- -> M2+. Among these three dopants, Sr2+ is the most effective in increasing the bulk conductivity due to a combination of its smaller mismatch in ion size with Bi3+, its intermediate polarisability and lower bond strength to oxygen compared to Ca2+ and Ba2+. Doping strategies for further improvements to bulk conductivity of NBT materials are discussed based on these results. Comparison with other oxide-ion conductors and initial stability test under reducing atmosphere show the doped non-stoichiometric NBT materials are promising for low and intermediate temperature applications. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).