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At low temperatures, nuclear spin relaxation (NSR) can be related to electrical conductivity in various inorganic oxide glasses by the fluctuation-dissipation theorem. The results indicate a common physical origin of the relaxation mechanism owing to fluctuating charges. In comparison, however, in heavy metal fluoride glasses, NSR is shown to detect additional fluctuations which are not observed by. conductivity experiments. The underlying relaxation process is caused by nuclear magnetic fluctuations without any accompanying charge motions. Both dielectric and NSR responses can be explained in terms of thermally activated excitations of asymmetric double-well potential (ADWP) configurations intrinsic to highly disordered solids. The ADWP configurations are characterized from the effect of alkali concentration and size on the low-temperature conductivity. The asymmetry of ADWP configurations is determined primarily by the elastic strains in the structure.

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