Structures and Ionic Transport Properties of Perovskite-Related B^IA^IIX₃ and B^IA₂^IIX₅ Halides

High Resolution Image Download MS PowerPoint Slide 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 I B II X 3 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 LiSr 2 X 5 with X = Cl, Br, and I, LiSr 2 Br 3.9 Cl 1.1 and LiEu 2 X 5 with X = Br and I, and LiSm 2 I 5 and Li M II I 3 with M II = Sr, Ba, Eu, and Sm as well as KCdBr 3, we were able to add several new compounds exhibiting different structure types to the structure field diagrams of perovskite-related ABX 3 and B I A 2 II X 5 compounds. According to the size of lithium ions, these compounds exhibit inverse structure types of B I A II X 3 or B I A 2 II X 5, 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 LiEuI 3 . 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 Li M 2 X 5 and Li MX 3 compounds significantly, where ionic conductivities range between 10 –12 and 10 –7 S cm –1 at 25 °C.