Connection between NMR and electrical conductivity in glassy chalcogenide fast ionic conductors
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Pulsed Nuclear Magnetic Resonance measurements and electric conductivity measurements have been performed in glassy fast ionic conductors, lithium thiogermanates ( xLi2S+(1-x)GeS2) and lithium thioborates ( xLi2S+(1-x)B2S3) in order to investigate the microscopic mechanism of ion dynamics. Both 7Li nuclear spin lattice relaxation rate (NSLR) versus temperature at several resonance frequencies (4 MHz to 135 MHz) and conductivity, [sigma]([omega]) versus temperature, in the frequency range from 1 Hz to 4 MHz have been measured. The 7Li NSLR show BPP-type behavior with strong asymmetry of the bell shaped curve on the two sides of the maximum. A sizable dependence of the NSLR on the type and composition of the glass is found. The results have been analyzed in terms of both a phenomenological model using a stretched exponential correlation function and a model based on a distribution of activation energies. The model based on a simple hopping of the free ions over the barriers of a given distribution gave a good fits of the NSLR data. By using the distribution obtained from NMR we could also account for the value and the temperature dependence of the dc. conductivity. In the framework of this model we can explain the different correlation times obtained from NMR and conductivity as a consequence of percolation effects in the conductivity. It is shown that the distribution of barriers should lead to small but detectable deviations from Arrhenius behavior of dc conductivity. This deviation has been indeed observed in lithium thiobrate glasses which have lower activation energy. The phenomenological model based on the stretched exponential was compared with the experiments in the light of the interpretation of the "coupling model" and found in partial disagreement with our data. In thioborate glasses it was found that the 7Li NSLR has two maxima as a function of temperature. The 11B NSLR on the other hand displays two relaxation rates associated with trigonal and tetragonal structural groups respectively. It is shown that the NSLR of the mobile (7Li) ion and immobile (11B) ion are indirectly related to each other and from this one can deduce information about energy barriers associated to each structure for the mobile ion and immobile ion.