Effect of ferromagnetic ordering on phonons in KCo₂Se₂

Results of the density functional theory studies of the phonon dynamics in the ternary layered cobalt diselenide are reported. The partial phonon densities of states due to vibrations of K, Co, and Se atoms are analysed in detail. They indicate that phonons associated with the dynamics of Co and Se ions within the [Co₂Se₂] structural blocks span the entire spectral range extending to 260 cm⁻¹, whereas phonons from the K-sublattice remain limited to the frequency range of 80–150 cm⁻¹. The phonons conform with structural features of the quasi-2D layered structure of KCo₂Se₂. Ferromagnetic order in the Co-sublattice is shown to determine to a great extent the phonon densities of states, the Raman and infrared spectra of KCo₂Se₂. The in-planar magnetic interactions are responsible for pronounced softening of the high-frequency phonon modes and lead to disappearance of the low-frequency Raman-active mode of the E _g symmetry. The observed behavior of the Raman-active and infrared-active modes suggests rather strong spin-phonon coupling in KCo₂Se₂. Results of the present investigations allow to clarify the origin of substantial differences between dynamical properties of the ferromagnetic Co-based and the paramagnetic Ni-based ternary layered dichalcogenides, both adopting the ThCr₂Si₂-type structure. (paper)