Polymorphism, structural frustration, and electrical properties of the mixed conductor Ag10Te4Br3

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 C-p) prove the reversibility of the alpha-beta, beta-gamma, and gamma-delta phase transitions at 355, 317, and 290 K, respectively. The existence of the low-temperature delta-phase is substantiated by C-p 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 (delta, gamma) to a 3D (beta, alpha) 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 alpha- and beta-polymorphs. The increase in disorder comes along with the occurrence of diffuse scattering in form of Kagome 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.