Far‐Infrared Reflection Spectra, Optical and Dielectric Constants, Effective Charges, and Lattice Dynamics of the Skutterudites CoP 3 , CoAs 3 , and CoSb 3

From the far‐infrared reflection spectra of the skutterudite type compounds CoP 3 , CoAs 3 , and CoSb 3 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 CoP 3 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 CoP 3 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 X 4 –X 4 , 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 CoSb 3 are found to be both mainly internal vibrations of the planary Sb 4 ‐units, v 4 and v 5 , at 174 and 144 cm −1 , respectively, and external ones, v 1 – v 3 and v 6 – v 7 , at higher and lower wavenumbers than v 4 and v 5 .