Vibrational structure of crystal-field spectra in layered3d-metal dihalides

The vibrational structures observed in the absorption spectrum associated with the crystal-field transitions in Mn, Fe, Co, and Ni dichlorides and dibromides are studied in detail at 5 K. The choice of these crystals is motivated by their appreciable molecular character resulting in a small dispersion of optical branches and hence in the appearance of relatively sharp phonon progressions in the crystal-field spectra. Most of these phonon structures are replicas of Raman-active $A_{1g}$ and $E_g$ frequencies and readily interpreted in terms of many-phonon processes induced by the first-order linear electron-phonon interaction. Furthermore, in Mn and Ni dihalides some intraconfigurational transitions display interesting two-phonon progressions which are attributed to the second-order linear electron-phonon interaction. The $q=0\mathrm{}$ phonon frequencies of even symmetry, deduced from vibronic or Raman spectra, and those of odd symmetry, measured by far-infrared transmission, are interpreted in the framework of a deformation-dipole model. Information on the partial covalency of these compounds and on the magnitude of three-body interactions is derived.