Silver dynamics in crystalline and glassy silver ion conductors studied by one- and two-dimensional 109Ag NMR

We demonstrate that one-(1D) and two-dimensional (2D) Ag-109 NMR is well suited to study diffusion in crystalline and glassy silver ion conductors. In 1D Ag-109 NMR, especially when applying magic angle spinning, the local environments of the silver sites can be characterized. 2D Ag-109 NMR allows one to record two-time correlation functions which directly monitor dynamics of single silver ions. These techniques are used to elucidate microscopic properties of polycrystalline Ag7P3S11. In this silver ion conductor, Ag+ motion results in non-exponential correlation functions which decay to a finite plateau value. At all temperatures, the data are well described by a modified Kohlrausch function f(t) = (1 - C)exp[-(t/tau)(beta)] +C, where beta approximate to 0.42 and and C approximate to 0.12. The temperature dependence of the mean correlation time obeys an Arrhenius law with activation energy E-a = 0.37 eV. Our results indicate that Ag-109 NMR probes elementary silver ion jumps which are the basis of macroscopic ion transport. In addition, we investigate for the first time dynamics in glassy ion conductors by means of 2D NMR. In the glasses (AgI)(0.3)-(AgPO3)(0.7) and (AgI)0.3-(1/3)(Ag4P2O7)(0.7), silver motion manifests itself in correlation functions which decay in an extremely stretched manner, i.e., beta = 0.21, to C = 0. Hence, there is no evidence for domain segregation into silver phosphate glass and AgI clusters, in which case a bimodal rate distribution would be expected. (C) 2002 Elsevier Science B.V. All rights reserved.