Silver ion dynamics in the Ag5Te2Cl-polymorphs revealed by solid state NMR lineshape and two- and three-time correlation spectroscopies

The relation between structure and ion dynamics in the three polymorphs of Ag5Te2Cl has been investigated using Ag-109, Te-125, and Cl-35 NMR spectroscopies. Specifically, the influence of the structural phase transitions observed near 240 K (P2(1)/c <-> P2(1)/n) and near 332 K (P2(1)/n <-> I4/mcm) upon silver ion mobilities has been studied by temperature dependent Ag-109 NMR lineshapes and spin-lattice relaxation times. While the superionic high temperature phase alpha-Ag5Te2Cl is characterized by a molten cationic sublattice, fast ion dynamics in the medium temperature phase beta-Ag5Te2Cl occurs in spatially restricted regions comprising all the crystallographically distinct silver sites. Temperature dependent magic-angle-spinning linewidths yield an activation energy of 0.38 eV, consistent with 0.44 eV measured from dc electric conductivities. For the low-temperature gamma-modification, results of two- and three-time Ag-109 correlation spectroscopies provide a detailed insight into the nature of the silver ionic hopping motion. Temperature dependent jump rates measured by two-time correlation functions yield an activation energy E-a = 0.48 eV. Ag-109 NMR three-time correlation functions indicate that the non-exponential relaxation behavior of the silver ions can be attributed to a broad distribution of jump rates rather than correlated forward-backward jumps. Nevertheless, all the silver ions are mobile down to temperatures of about 185 K.