Lattice vibration spectra, XXXIII: Far-infrared reflection spectra, TO and LO phonon frequencies, optical and dielectric constants, and effective charges of the spinel-type compounds MCr 2S 4( M =Mn, Fe, Co, Zn, Cd, Hg), MCr 2Se 4(M =Zn, Cd, Hg), and MIn 2S 4( M =Mn, Fe, Co, Ni, Cd, Hg)

Abstract

The far-infrared reflection spectra of hot-pressed samples of the spinels MCr 2S 4 with M =Mn, Fe, Co, Zn, Cd, and Hg, MCr 2Se 4 with M =Zn, Cd, and Hg, and MIn 2S 4 with M =Mn, Fe, Co, Ni, Cd, Hg have been recorded in the range from 40 to 700 cm −1. The spectra show four reststrahlen bands in the case of normal spinels and up to five in the case of inverse spinels. The transverse and longitudinal optical phonon frequencies at wave vector ∣ q∣ ⋍ 0 are determined by Kramers-Kronig analysis and the classical oscillator-fit method. Effective ionic charges (normalized splitting, transverse effective charge, Szigeti charge) were calculated from the experimentally determined TO–LO splittings and from the high frequency dielectric constants. The obtained data reveal an increasing covalency of the chalcide spinels in the order selenides > sulfides, chromium compounds > indium compounds, inverse spinels > normal spinels,Ni M′ 2X 4 > Co M′ 2X 4 > Fe M′ 2X 4 ,Mn M′ 2X 4 > Hg M′ 2X 4 > Zn M′ 2X 4 >CD M′ 2X 4 .