YEVYCH R. Lattice dynamics and phase transitions in complex phosphorous-containing chalcogenides

The thesis is dedicated to the elastic properties in the region of the hypersound waves and phonons spectra studies of the MM'P2X6 (M – Sn, Cu, In; X – S, Se) crystals family. The elastic modulus tensor of crystals under investigation in the different phases (ferroelectric and paraelectric) was obtained, and acoustic properties anisotropy was investigated by Brillouin scattering spectroscopy techniques. The temperature anomalies of the hypersound propagation characteristics and the role of acoustic phonons at ferroelectric phase transitions were studied. The lattice dynamics of the Sn2P2S6, Sn2P2Sе6 crystals and their solid solutions with a three-dimensional morphology of the crystal lattice in the polarizable ion model and the SnP2S6 crystal with a two-dimensional morphology of the crystal lattice in the rigid ion model was calculated. The modeled dynamical instability by ions recharging in the Sn2P2S6 and Sn2P2Sе6 crystals corresponds the experimental observed temperature behavior of the phonons spectra at the phase transitions in these compounds. The possibility of “two-mode” concentration behavior of the soft phonons dispersion branches was obtained.