Ionic Dynamics in molten salts and molten salt hydrates

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The Raman spectra of the CN stretching mode of <SCN^-> ions were measured in molten thiocyanates, KSCN and NaSCN, at temperatures 450-600K. The vibrational and rotational correlation functions were calculated, and the dynamics of <SCN^-> ions in the molten state are compared with those in aqueous solutions. The observed vibrational correlation functions are analysed by the stochastic line shape theory of Kubo, in which homogeneous and inhomogeneous broadening are treated simultaneously. Both broadening contributions to the isotropic spectra are extracted. The homogeneous broadening increases with increasing temperature, while inhomogeneous broadening remains constant in molten KSCN, which is considered to lead to the temperature broadening of the isotropic Raman bandwidth. Rotational correlation functions of <SCN^-> ions in melts show the behavior of short time inertial rotation (t <<![> 0.15ps), which is a little slower than the free rotation of a single ion. The long time exponential decay reflects the ultimate diffusional behavior of the ionic reorientation. The rotational relaxation rate is found to increase with temperature in both melts and aqueous solutions.The Raman spectra of the <(nu)_1> mode of <NO(_3^-)> ions in molten nitrates and their binary mixtures are now measured and analysed. The mechanisms of rotational relaxation and the roles of fluctuations of coordination number and concentration in the vibrational dephasing will be studied in the near future.