Structures and Ionic Transport Properties of Perovskite-Related BIAIIX3 and BIA2IIX5 Halides

Abstract

The structure family of perovskites and related phases contains a large variety of compounds with versatile properties and applications. While perovskite structures of the A^IB^IIX₃ type usually are categorized based on geometrical considerations like the Goldschmidt tolerance factor, perovskite-related and distorted structure types need to be classified by the more general approach of structure field diagrams. By synthesizing LiSr₂X₅ with X = Cl, Br, and I, LiSr₂Br_3.9Cl_1.1 and LiEu₂X₅ with X = Br and I, and LiSm₂I₅ and LiM^III₃ with M^II = Sr, Ba, Eu, and Sm as well as KCdBr₃, we were able to add several new compounds exhibiting different structure types to the structure field diagrams of perovskite-related ABX₃ and B^IA₂^IIX₅ compounds. According to the size of lithium ions, these compounds exhibit inverse structure types of B^IA^IIX₃ or B^IA₂^IIX₅, where the monovalent lithium ion resides on the lower-coordinated B-site and the divalent metal cation occupies the higher-coordinated A-site. Using in situ variable-temperature powder X-ray diffraction and differential scanning calorimetry, we investigated the relationship between different structure types exemplarily for LiEuI₃. Additionally, we examined the ionic transport properties of the different structure types by means of electrochemical impedance spectroscopy and bond valence sum calculations and found restricted dimensionalities of the ion percolation pathways in the investigated structure types, generally limiting the ionic transport properties. Furthermore, the size and softness of the underlying anion lattice, as well as the size and bonding situation of the divalent metal cations, can influence the charge transport properties in LiM₂X₅ and LiMX₃ compounds significantly, where ionic conductivities range between 10^–12 and 10^–7 S cm^–1 at 25 °C.