Thursday, March 23, 2006

Physical Chemistry Chemical Physics, 2006, 8, 369 - 378DOI: 10.1039/b512236h
Title: Silver ion dynamics in the Ag5Te2Cl-polymorphs revealed by solid state NMR lineshape and two- and three-time correlation spectroscopies

Authors: Christian Brinkmann, Sandra Faske, Michael Vogel, Tom Nilges, Andreas Heuer and Hellmut Eckert

Abstract: The relation between structure and ion dynamics in the three polymorphs of Ag5Te2Cl has been investigated using 109Ag, 125Te, and 35Cl NMR spectroscopies. Specifically, the influence of the structural phase transitions observed near 240 K (P21/cP21/n) and near 332 K (P21/nI4/mcm) upon silver ion mobilities has been studied by temperature dependent 109Ag NMR lineshapes and spin–lattice relaxation times. While the superionic high temperature phase -Ag5Te2Cl is characterized by a molten cationic sublattice, fast ion dynamics in the medium-temperature phase -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 -modification, results of two- and three-time 109Ag 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 Ea= 0.48 eV. 109Ag 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.

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