Frequencies and Temperature-Effects in the Vibrational Spectra of the Metal-Hydrate Complexes in NiSnCl6 · 6 aqu and in NiSO4 · 6 aqu

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Abstract
We have investigated the internal vibrations of metal hydrate complexes in ionic single crystals and their interactions with the phonon system of the lattice, by observing the Raman spectra and infrared reflection spectra with polarized light at different temperatures T(2 K ≦ T ≦ 300 K). As special systems the octahedral nickel hydrate complexes in NiSnCl6 · 6 H2O, NiSnCl6·6 D2O and in NiSO4 · 6H2O. NiSO4 · 6D2O have been chosen. For NiSnCl6·6 aqu a complete interpretation of the spectra is given, starting from the assumption of isolated metal hydrate complexes and SnCl6 -- -ions as structural units. The discussion of the spectra of the sulfate crystal is based on the corresponding assumption. However, here an unambiguous assignment of all lines observed was not possible. The most prominent feature of all spectra is the strong temperature dependence of the line widths which is much more pronounced than in anhydrous crystals in the same temperature interval. This effect is qualitatively explained in terms of the anharmonicity of the potential by which the water molecules are bound in the crystal lattice. Simple models for this potential are discussed.

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