Inelastic light scattering studies of the protonic order–disorder transition in SnCl2⋅2H2O

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Abstract
Raman and Brillouin scattering spectra of SnCl2⋅2H2O in several scattering configurations were studied as a function of temperature, with special emphasis on the phase transition at 218 K. The O–H stretching vibration region was investigated for the first time. A sharp band at 3500 cm−1 has been assigned as the O–H stretching vibration involving the proton not participating in the hydrogen bond formation. The spectral line width of this band shows anomaly near the phase transition temperature and has been interpreted as due to the reorientation of H2O molecules of type II. The low frequency Raman spectra were also studied. The bands at 250 and 320 cm−1 both display a similar striking frequency shift with respect to temperature change. These frequency versus temperature data have been interpreted in terms of the proton population at position 5. The Brillouin spectra obtained at various scattering configurations do not show any anomaly near Tc. Combination of the Raman and Brillouin scattering results leads to the conclusion that the phase transition in SnCl2⋅2H2O is associated with a Debye‐type relaxation mode associated with the reorientation of H2O molecules.