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 AIBIIX3 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, LiSr2Br3.9Cl1.1 and LiEu2X5 with X = Br and I, and LiSm2I5 and LiMIII3 with MII = 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 BIA2IIX5 compounds. According to the size of lithium ions, these compounds exhibit inverse structure types of BIAIIX3 or BIA2IIX5, 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.