Lattice Vibration Spectra XXIV*. Far-Infrared Reflection Spectra, Optical and Dielectric Constants, and Effective Charges of the Pyrite Type Compounds FeS2, MnS2, MnSe2, and MnTe2

From the far-infrared reflection spectra of the pyrite type compounds FeS2, MnS2, MnSe2, and MnTe2 we determined the TO and LO phonon frequencies at wave vector | q | ≈ 0 by both Kramers-Kronig analysis and classical oscillator fit method. In the case of MnSe2 and MnTe2 we found for the first time all five group-theoretically allowed reststrahlen bands. The phonon frequencies obtained from both single crystal and pressed pellet reflection measurements do not show any significant differences. The calculated optical and dielectric constants however are strongly affected by the surface quality of the sample. Effective ionic charges (Born effective charge, Szigeti charge) were calculated from the experimentally determined TO/LO splittings. They demonstrate an increasing ionicity in the order FeS2<MnTe2<MnSe2<MnS2.