Raman spectra and lattice dynamics of single-crystal

Polarized Raman spectra of oriented monoclinic single crystals of have been obtained at temperatures of 20 - 650 K in the region . The assignment of the Raman bands to the and modes was given. The photoinduced reduction of the Raman intensity was observed after heating a crystal above 500 K, and cooling it down to room temperature, under laser irradiation. A lattice dynamical analysis based on two-particle interatomic potentials and a rigid-ion model was performed. These potentials were consistent with the Raman and IR data, besides which they showed a stable dynamics throughout the Brillouin zone, and yielded the minimum of internal forces and pressures in the crystal lattice. The phonon dispersion curves, and the projected and total densities of phonon states were calculated.