Lattice vibrations of CdCl_2, CdBr_2, MnCl_2, and CoCl_2: infrared and Raman spectra*

The fundamental lattice vibrations of the trigonal CdCl2 structure have been analyzed group theoretically. All k=0 optical modes are either infrared or Raman active, so a complete vibrational analysis is possible. Results of far-infrared measurements on crystal and powder samples using a Fourier-transform spectrometer together with single-crystal Raman spectra are presented for CdCl2, CdBr2, MnCl2, and CoCl2. Symmetry assignments of the observed frequencies are made on the basis of polarization behavior. Second-order infrared and Raman spectra are also presented; peaks in the two-phonon axial infrared absorption spectra are assigned to allowed fundamental combinations at k=0. The infrared absorption spectra exhibit additional features in the range 200–4000 cm−1 that are ascribed to impurities.