Optical-absorption spectra of the layered transition-metal thiophosphates $MPS_3$ (M=Mn, Fe, and Ni)

Optical-absorption spectra of single crystals of manganese, iron, and nickel thiophosphates have been recorded at 300 and 120 K. The spectral features have been assigned to transitions within the d-electron manifold of the corresponding metal ion in octahedral symmetry. The spectra have been analyzed with use of the weak-field coupling scheme of crystal-field theory and the transition energies have been calculated by diagonalization of the appropriate weak-field matrices. The crystal-field-splitting parameter Dq has been found to be similar in magnitude $(\sim 850 cm^{-1})$ for all three compounds. The calculated values of the Racah interelectron repulsion parameter B for $MnPS_3$, $FePS_3$, and $NiPS_3$ are 560, 900, and 950 $cm^{-1}$, respectively, which are close to that of the corresponding free ion and that in other ionic compounds. The crystal field parameters have thus been used to establish the covalency or ionicity of the M-S linkage in these compounds. All three compounds are ionic with the d states localized in character