Far‐Infrared Reflection Spectra, Optical and Dielectric Constants, Effective Charges, and Lattice Dynamics of the Skutterudites CoP3, CoAs3, and CoSb3

Abstract

AbstractFrom the far‐infrared reflection spectra of the skutterudite type compounds CoP3, CoAs3, and CoSb3 the TO and LO phonon frequencies at wave vector |q| ≈ 0 are determined by both Kramers‐Kronig analysis and classical oscillator fit method. In the case of CoP3 large free carrier contributions (plasma resonance frequency ω′p ⩽ Ω+ ≈ 800 cm−1) with strong plasmon–phonon interactions are observed. Attempts to calculate the decoupled phonon frequencies of CoP3 using a three and a four parameter model of the dielectric function failed. Lattice dynamical calculations are done by setting up a valence force field consisting of M–X, two X–X and X4–X4, and M–M stretching constants. The obtained force constants, e.g. f(CoSb): 85, f(SbSb): 87 and 59 N m−1, respectively, and also the calculated effective dynamical charges (from TO/LO splittings) show the mainly covalent character of the skutterudites under consideration. The lattice modes of CoSb3 are found to be both mainly internal vibrations of the planary Sb4‐units, v4 and v5, at 174 and 144 cm−1, respectively, and external ones, v1–v3 and v6–v7, at higher and lower wavenumbers than v4 and v5.