Raman spectral studies of the dynamics of ions in molten LiNO3–RbNO3 mixtures. II. Vibrational dephasing: Roles of fluctuations of coordination number and concentration

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Abstract
The isotropic Raman spectra of the ν1 (A’1) mode of NO−3 ions are measured in molten LiNO3–RbNO3 mixtures at various concentrations and at temperatures from 478 to 750 K. The isotropic Raman bandwidth Γiso is found to increase almost linearly with the mole fraction c(Li+) of LiNO3, from 9–13 cm−1 for molten RbNO3 to 26–27 cm−1 for molten LiNO3. Vibrational correlation functions are analyzed on the basis of a model of simultaneous homogeneous and inhomogeneous vibrational dephasing. The contribution of inhomogeneous dephasing was found to be negligibly small in mixtures of c(Li+)≲0.33. While in mixtures of concentrations c(Li+)≳0.5, homogeneous and inhomogeneous processes are found to give comparable contributions to the total vibrational widths, where the latter contribution increases as the temperature decreases. The observed isotropic Raman bands are asymmetric. The asymmetry parameter α=Γ1/Γh , where Γ1 and Γh are the half‐widths at half‐maximum height of the low and high frequency sides of the band, is larger than one in molten LiNO3 and RbNO3, while α is smaller than one in molten mixtures of concentrations c(Li+)=0.33–0.67. This concentration dependent asymmetry is found to arise predominantly from coordination number fluctuation, i.e., fluctuation of the environmental cation number of a reference NO−3 ion, while concentration fluctuation in the coordination sphere plays a minor role. A model of local environmental states in a molten mixture, which assumes a linear change of vibrational frequency with neighboring cation numbers, is presented. The observed spectral asymmetry can be interpreted by assuming that the equilibrium distribution has a peak at the small coordination number side in pure molten LiNO3 and RbNO3, while the distribution peak shifts to the high coordination number side in molten mixtures.