FTIR electronic spectra of Fe2P2S6 and Co2P2S2: Trigonal field splitting and lithium intercalation effects

Abstract

The FTIR electronic absorption spectra of single crystals of Fe2P2S6, Co2P2S6 and their lithium intercalation compounds have been obtained. The spectrum of Fe2P2S6 exhibits a broad, weak absorption feature at 1885 cm-1 and a second, stronger band at 8870 cm-1. These are assigned to transitions between the 5Eg (ground state) and the 5A1g and 5Eg trigonal crystal-field components of the Fe2+ free-ion 5D ground term. The broad absorption bands in the spectrum of Co2P2S6 are assigned to the transitions between the 4A2g (ground state) and the 4Eg (1224cm-1) and 4A1g (7124 cm-1) trigonal crystal-field components of the Co2+ free-ion 4F ground term. A point-charge crystal-field model was used to relate the crystal-field splitting parameters (Dq and Cp) to the trigonal lattice distortion.Lithium intercalation completely suppresses the trigonal field transition in Co2P2S6 and causes a dramatic shift of the absorption edge to lower frequencies. However, lithium intercalation has little effect upon the spectrum of Fe2P2S6.