Towards understanding the defect properties in the multivalent A-site Na${}_{0.5}$Bi${}_{0.5}$TiO${}_3$-based perovskite ceramics

A defect model involving cation and anion vacancies and anti-site defects is proposed that accounts for the non-stoichiometry of multi-valent $A$-site Na${}_{0.5}$Bi${}_{0.5}$TiO${}_3$ based perovskite oxides with $AB{O}_3$ composition. A series of samples with varying $A$-site non-stoichiometry and $A$:$B$ ratios were prepared to investigate their electrical conductivity. The oxygen partial pressure and temperature dependent conductivities where studied with direct current (dc) and alternating current (ac) techniques, enabling to separate between ionic and electronic conduction. The Na-excess samples, regardless of the $A$:$B$ ratio, exhibit dominant ionic conductivity and $p$-type electronic conduction, with the highest total conductivity reaching $4\times {10}^{-4}$ S/cm at 450${}^{\circ }$C. In contrast, the Bi-excess samples display more insulating characteristics and $n$-type electronic conductivity, with conductivity values within the 10${}^{-8}$ S/cm range at 450${}^{\circ }$C. These conductivity results strongly support the proposed defect model, which offers a straightforward description of defect chemistry in NBT-based ceramics and serves as a valuable guide for optimizing sample processing to achieve tailored properties.