Low temperature vibrational spectroscopy. III. Structural aspects and detection of phase transitions in crystalline alkali metal and tetramethylammonium hexabromotellurates and platinates

The low‐frequency infrared and Raman spectra of A₂[TeBr₆] with A=K, Rb, Cs, NH₄, (CH₃)₄N, (CD₃)₄N and of A₂[PtBr₆] with A=K, (CH₃)₄N, and (CD₃)₄N were recorded as a function of temperature down to ∼100 K. The spectra of the Rb and Cs compounds changed little. For the other salts new, and in some cases sharp, bands were observed at low temperatures. The spectra have been assigned, and in most cases a confirmation of previous results was obtained. The assignment ambiguity in the literature on the ν₄ mode of [PtBr₆]²⁻ has been solved, placing it at ∼ 130 cm⁻¹. The majority of the new low temperature bands were interpreted in accordance with the known occurrence of phase transitions breaking the ideal antifluorite K₂[PtCl₆]‐type symmetry, but it appears that the method of vibrational spectroscopy does not invariably reflect the presence of phases of lower symmetry. In the spectra of the tetramethylammonium compounds, methyl torsional IR bands were observed with increasing sharpness at lower temperatures. This behavior can be correlated with a gradual ordering of methyl torsional disorder. The potential energy barrier against methyl group rotation was found to be of the order 4–5 kcal/mol, showing that the methyl groups are not pseudo freely rotating. The anharmonicity of the vibrations in tetramethylammonium hexabromotellurate seems to increase abnormally at lower temperatures, possibly due to enhanced methyl–bromine interaction.