Spectroscopy study of [Me(NH3)6]X2 compounds. II—Low frequency infrared and Raman spectra at various temperatures

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Abstract
Infrared and Raman spectra of [Ni(NH3)6]X2 compounds were measured at temperatures from 80 K to 300 K in the frequency range below c. 400 cm−1. Besides the simple bands previously reported additional ones were seen, especially at lower temperatures. The broad band which appeared between the lattice vibration and the δ(NNiN) deformation vibration was interpreted as partly representing the phonon density distribution from the whole Brillouin zone. Its appearance is induced by crystal imperfections connected with NH3 vacancies. Additíonal bands between δ(NNiN) deformation vibration and ν(NiN) stretching, and those above the ν(NiN) stretching were interpreted as combination bands in which the lattice vibration combines with the deformation in the former case and with the stretching mode in the latter. A satellite band close to the ν(NiN) stretching was interpreted as a combination of the stretching and libration mode of the whole [Ni(NH3)6]2+ ion. A model correlation function describing the librations was fitted to the experimental correlation function obtained from the i.r. spectra.