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

AbstractBismuth-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 ~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++½ 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