Enhanced oxide ion conductivity in sodium niobate-based ceramics

Abstract

In the past decades, there have been increasing in-depth studies on the defect chemistry of perovskite-structured lead-free piezoelectric ceramics such as Na_1/2Bi_1/2TiO₃ (NBT), revealing the effects of chemical doping on their electrical conductivity and conduction mechanisms. The electrical conduction mechanisms of another important family of lead-free piezoelectric ceramics based on K_1/2Na_1/2NbO₃ (KNN) are however still much less studied. NaNbO₃ (NN) is an end member of KNN. In this work, the electrical conductivity of NN ceramics is tailored by doping different ions (Mg²⁺, Al³⁺, Sc³⁺, Ga³⁺, and Zr⁴⁺) at the Nb-site. The effects of acceptor doping on the crystal structure, microstructure, and electrical conduction mechanisms of NN ceramics are investigated. Specifically, doping of Mg²⁺ and Ga³⁺ leads to the transition of electrical conduction mechanism of NN from mixed oxide ion and electron conduction to oxide ion dominated conduction, and the oxide ion transport number increases to 0.92 at 700 °C for Ga³⁺ doped NN. Ga³⁺ doped NaNbO₃ also exhibits good stability of phase structure and electrical properties even if samples are annealed in a reducing atmosphere. The electrical conduction mechanisms underlying the enhanced oxide ion conductivity of NN are discussed based on the physical properties of acceptor dopants. The enhanced oxide ion conductivity of NN-based materials mostly originates from the increased concentration of oxygen vacancies, which are created by acceptor doping and are confirmed by electron paramagnetic resonance (EPR) measurements. The weak M–O bond strength and the large polarisability of the dopants facilitate oxide ion conduction in NN-based materials. This work provides theoretical guidance for the regulation of the electrical properties and conductivity mechanisms of NN-based ceramics.