Far-infrared spectroscopy investigation of commensurate and incommensurateCs2CdBr4 crystal

Far-infrared reflectivity measurements of Cs2CdBr4 crystal were performed o ver the frequency region 15-600 cm(-1) and temperature region 10-297 K. All transverse lattice modes have frequencies below 230 cm(-1) because of the heavy atoms in the Cs2CdBr4 crystal. A sequence of phase transitions can be identified on the basis of the activation of new modes below the phase tra nsition temperatures due to the lowering of the crystal symmetry on cooling . Factor-group analysis was carried out and selection rules found for the i nfrared activity of phonon modes in the incommensurate and commensurate pha ses, and the results were compared with the experimental data.