Polymorphism, Structural Frustration, and Electrical Properties of the Mixed Conductor Ag10Te4Br3

Abstract

Ag10Te4Br3 is polymorphic with four polymorphs in the temperature range from 3 to 450 K. It represents the first member of a formerly unseen class of materials featuring covalently and ionically bonded tellurium substructures. Thermal analyses (DSC and Cp) prove the reversibility of the α−β, β−γ, and γ−δ phase transitions at 355, 317, and 290 K, respectively. The existence of the low-temperature δ-phase is substantiated by Cp measurements down to 3 K. Temperature-dependent single-crystal structure analysis and nonharmonic refinements of the silver distribution for all polymorphs reveal a high silver mobility over the whole temperature range. A significant change in the dimensionality of the silver distribution, from an exclusively 2D (δ, γ) to a 3D (β, α) arrangement, can be observed for Ag10Te4Br3 with the increase in temperature. The enhanced silver mobility causes a structural frustration and disorder phenomena of the predominantly covalently bonded tellurium substructure for the high-temperature α- and β-polymorphs. The increase in disorder comes along with the occurrence of diffuse scattering in form of Kagomé nets, indicating a structural frustration in a rod packing arrangement. An ionic conductivity, approaching the values of most of the known silver super ion conductors and only 1 order of magnitude lower than that in RbAg4I5, are a remarkable feature of air-, photo-, and moisture-stable Ag10Te4Br3.