FTIR electronic spectra of Fe 2P 2S 6 and Co 2P 2S 2: Trigonal field splitting and lithium intercalation effects

Abstract

The FTIR electronic absorption spectra of single crystals of Fe 2P 2S 6, Co 2P 2S 6 and their lithium intercalation compounds have been obtained. The spectrum of Fe 2P 2S 6 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 5 E g (ground state) and the 5 A 1 g and 5 E g trigonal crystal-field components of the Fe 2+ free-ion 5 D ground term. The broad absorption bands in the spectrum of Co 2P 2S 6 are assigned to the transitions between the 4 A 2 g (ground state) and the 4 E g (1224cm −1) and 4 A 1 g (7124 cm −1) trigonal crystal-field components of the Co 2+ free-ion 4 F 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 Co 2P 2S 6 and causes a dramatic shift of the absorption edge to lower frequencies. However, lithium intercalation has little effect upon the spectrum of Fe 2P 2S 6.