Vibrational spectra of KPb2Cl5 and KPb2Br5 crystals

Lattice dynamics of KPb2Cl5 and KPb2Br5 crystals is simulated in the framework of ab initio model, taking into account multipole moments of electron envelopes of ions. Results are compared with experimental polarized Raman spectra; good agreement with results of simulations is shown. Extremely low frequencies of phonon spectra are explained by big mass of anions together with considerable participation of heavy cations in the eigenvectors of upper frequency lattice modes.