Raman scattering study of the protonic order–disorder transition in SnCl2⋅2H2O

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Abstract
Raman spectra of anhydrous, monohydrated and dehydrated SnCl2 solids have been measured. The hydrogen bond vibrations in the two dimensional intervening H2O layers in SnCl2⋅2H2O (SCD) have been identified. Polarized Raman spectra of SCD in several scattering geometries and at several temperatures above and below Tc have been obtained. The results of spectral intensity near the phase transition have been discussed on the basis of proton population at various sites. The peak frequency and spectral linewidth of a hydrogen bond vibration are discussed in terms of anharmonicity in the hydrogen bond potential. No soft optical phonon has been observed in the Raman spectra of SCD, despite the fact that several bonds are affected by the phase transition. A relaxational rather than resonant soft mode may be important in SCD.