OPTICAL-ABSORPTION SPECTRA OF THE LAYERED TRANSITION-METAL THIOPHOSPHATES MPS3 (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 (approximately 850 cm-1) for all three compounds. The calculated values of the Racah interelectron repulsion parameter B for MnPS3, FePS3, and NiPS3 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.