How crystal structure and microstructure can influence the sodium-ion conductivity in halide perovskites

The perovskite crystal structure with nominal composition ABX3 offers a very flexible framework for sodium halide ionic conductors, an aspect not well defined in the current literature. This structure can accommodate a variety of sizes and oxidation states of cations, as well as different contents of cation vacancies. Different studies have shown that substitution of trivalent by tetravalent cations in the structure of some halides 'double' perovskites significantly improve their ionic conductivity, which can be explained by the creation of cation vacancies in the B-sites. The understanding of the structure opens the possibility to create cation vacancies, not only in the B sites- but also in the A-sites, by the replacement of the trivalent cations by pentavalent cations and to study their impact on the ionic transport of sodium halide materials. In this work, we show a study of the Na3−2xIn(III)1−xTa(V)xCl6 system with respect to their structure, microstructure, and ionic transport properties, demonstrating the coupling among these three aspects. This work aims to provide a detailed description of the current halide ionic conductors in the framework of the perovskite structure. By fully describing sodium ion conducting halides as perovskites, we hope to offer a reliable guidance to design improved solid-electrolyte materials.