23Na NMR study of the local static and dynamic properties of the orientational glass Na(CN)xCl1-x

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Abstract
The local static and dynamic properties of the mixed crystals Na(CN)xCl1-x are studied by 23Na NMR over a wide concentration range. Special emphasis is laid upon systems which transform into a low-temperature glass state (x23Na spin-lattice relaxation time, the temperature dependence of the correlation times describing the motions of the CN ions in their local potentials are deduced for various concentrations x. In the temperature region where the transition to the glass state takes place the motions remain fast compared with the timescale of the NMR experiment (reciprocal Larmor frequency) but the type of motions changes gradually. A qualitative model is developed which basically assumes that decreasing temperature deviations from the cubic symmetry of the local potentials occur progressively as a consequence of cooperative phenomena. Measurements of the 23Na satellite T1 exclude slow fluctuations in kHz regime.