The orientational phase transitions of hexammine cadmium (II) halides studied by Raman spectroscopy

The Raman spectra of all normal and deuterated hexammine cadmium (II) halides are presented. Evidence is given for the occurrence of both dynamic and static changes at the phase transitions. Dynamic changes are reflected in all spectra as Tc is approached by the critical increase in the intensity of internal modes involving ammonia group vibrations and rotations and torsional oscillations. These results are related to the slowing down of reorientations of the H3 proton triangles. This slowing down clearly continues through the temperature of the structural transition Tc for the chloride and bromide salts and therefore these transitions have a first-order component. Temperature dependence of the Raman-active lattice mode is also observed for the iodide salts and this is discussed in terms of anharmonic coupling with a soft librational mode. Analysis of the static changes in the spectra caused by the structure distortion at Tc indicates that the iodides become trigonal and the chlorides and bromides become monoclinic in their low-temperature phases.