Infrared $(700 - 100 cm^{-1})$ vibrational spectra of the layered transition metal thiophosphates, $MPS_3$ (M = Mn, Fe and Ni)

The infrared $(700 - 100 cm^{-1})$ vibrational spectra of single crystals and powders of the transition metal thiophosphates, $MPS_3$ (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 $\sim 260 cm^{-1}$. A detailed analysis of the selection rules for the $MPS_3$ 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 $MPS_3$ lattice. It was found that the spectra in the $700 - 200 cm^{-1}$ limit could be consistently interpreted within an ionic model, $M^{2+}_2 (P_2S_6)^{4-}$. The normal modes being separable into the internal modes of the ethane like $P_2S_6$ group, which occur in the $700 - 3OOcm^{-1}$ spectral region, those to the vibrations of the $MS_6$ polyhedra $(\sim260cm^{-1})$ and to coupled modes occurring below $200cm^{-1})$. The model successfully accounts for the gross similarity in the vibrational spectra of $MnPS_3$, $FePS_3$ and $NiPS_3