Infrared (700-100 cm⁻¹) vibrational spectra of the layered transition metal thiophosphates, MPS₃ (M = Mn, Fe and Ni)

Abstract

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 ~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, M₂²⁺ (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 (~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.