INFRARED (700-100 CM-1) VIBRATIONAL-SPECTRA OF THE LAYERED TRANSITION-METAL THIOPHOSPHATES, MPS3 (M = MN, FE AND NI)

The infrared (700-100 cm-1) vibrational spectra of single crystals and powders of the transition metal thiophosphates, MPS3 (M = Mn, Fe and Ni) have been studied as a function of temperature. The spectra of the three compounds show strong similarities both in the position and intensity of the various bands. The only metal dependent feature is a band at approximately 260 cm-1. A detailed analysis of the selection rules for the MPS3 lattice has been carried out using the correlation method. From the expected orientational dependence it was possible to assign the various spectral features to the normal modes of the MPS3 lattice. It was found that the spectra in the 700-200 cm-1 limit could be consistently interpreted within an ionic model, M-2+(2) (P2S6)4-. The normal modes being separable into the internal modes of the ''ethane like'' P2S6 group, which occur in the 700-300 cm-1 spectral region, those to the vibrations of the MS6 polyhedra (approximately 260 cm-1) and to coupled modes occurring below 200 cm-1. The model successfully accounts for the gross similarity in the vibrational spectra of MnPS3, FePS3 and NiPS3.