From insulator to oxide-ion conductor by a synergistic effect from defect chemistry and microstructure: acceptor-doped Bi-excess sodium bismuth titanate Na 0.5 Bi 0.51 TiO 3.015

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2020-12, Vol.8 (47), p.25120-25130
• Main Authors: Yang, Fan, Dean, Julian S., Hu, Qiaodan, Wu, Patrick, Pradal-Velázquez, Emilio, Li, Linhao, Sinclair, Derek C.
• Format: Article
• Language: English
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/D0TA10071D

The influence of Ti-site acceptor-doping (Mg 2+ , Zn 2+ , Sc 3+ , Ga 3+ and Al 3+ ) on the electrical conductivity and conduction mechanism of a nominally Bi-excess sodium bismuth titanate perovskite, Na 0.5 Bi 0.51 TiO 3.015 (NB 0.51 T), is reported. Low levels of acceptor-type dopants can introduce appreciable levels of oxide-ion conductivity into NB 0.51 T, i.e. , 0.5% Mg-doping for Ti 4+ can enhance the bulk conductivity of NB 0.51 T by more than 3 orders of magnitude with the oxide-ion transport number going from 0.9 at 600 °C. The intriguing electrical behaviour in acceptor-doped NB 0.51 T dielectrics is a synergistic effect based on the defect chemistry and ceramic microstructure in these materials. NB 0.51 T 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.