Analysis of the mid-IR electronic absorption spectra of Fe₂P₂S₆ and Co₂P₂S₆

Single-crystal FTIR spectra of Fe₂P₂S₆, Co₂P₂S₆, Cd₂P₂S₆, and Mn₂P₂S₆ were recorded at room temperature. The Fe₂P₂S₆ and Co₂P₂S₆ spectra exhibit a very broad band which was assigned to an electronic transition between the trigonal field components of the ground electronic state. A point-charge crystal-field model was used to relate the trigonal field splitting to crystal-field parameters (Dq and Cp) and to determine the relative energy of the trigonal field components. The FTIR spectrum of Fe₂P₂S₆ intercalated with n-propylamine was analyzed in terms of intercalation-induced changes in the crystal-field parameters.