Lattice dynamics simulation of Cs2CdBr4 crystal

Lattice dynamics calculations for the Cs2CdBr4 crystal in the high-temperat ure orthorhombic phase (Pnma) and in the monoclinic phase (P2(1)/n11) at si milar to 195 K were carried out. The model was based on the atom-atom poten tial function which comprised the long-range Coulombic, short-range and cov alent interactions. Comparison of the calculated phonon frequencies with th e experimental ones obtained from far-infrared reflectivity and Raman scatt ering measurements gives a reasonable agreement in most cases. A possible o rigin of the incommensurate phase transition was discussed on the basis of a coupling between low-lying Sigma(2) phonon branches. Using group-theory a nalysis and the calculated phonon dispersion branches, it is shown that the P2(1)/n 1 1-P (1) over bar phase transition is very probably induced by th e condensation of a phonon mode at the Brillouin zone boundary.