From insulator to oxide-ion conductor by a synergistic effect from defect chemistry and microstructure: acceptor-doped Bi-excess sodium bismuth titanate Na0.5Bi0.51TiO3.015

Abstract

The influence of Ti-site acceptor-doping (Mg²⁺, Zn²⁺, Sc³⁺, Ga³⁺ and Al³⁺) on the electrical conductivity and conduction mechanism of a nominally Bi-excess sodium bismuth titanate perovskite, Na_0.5Bi_0.51TiO_3.015 (NB_0.51T), is reported. Low levels of acceptor-type dopants can introduce appreciable levels of oxide-ion conductivity into NB_0.51T, i.e., 0.5% Mg-doping for Ti⁴⁺ can enhance the bulk conductivity of NB_0.51T by more than 3 orders of magnitude with the oxide-ion transport number going from <0.1 for NB_0.51T to >0.9 at 600 °C. The intriguing electrical behaviour in acceptor-doped NB_0.51T dielectrics is a synergistic effect based on the defect chemistry and ceramic microstructure in these materials. NB_0.51T ceramics with extremely low levels of doping show an inhomogeneous microstructure with randomly distributed large grains embedded in a small grained matrix. This can be considered as a two-phase composite with large grains as a conductive phase and small grains as an insulating phase based on an empirical conductivity – grain size relationship. Variation in the fraction of the conductive, large grained phase with increasing doping levels agrees with the oxide-ion transport number. This electrical two-phase model is supported by finite element modelling. This study reveals the significance of ceramic microstructure on the electrical conduction behaviour of these materials and can provide a guideline for selecting suitable doping strategies to meet the electrical property requirements of NBT-based ceramics for different applications.