Raman and infrared hard mode spectroscopy of the internal vibrations in Sn2P2Se6

The high frequency internal valence vibrations of the P2Se6 molecular units in Sn2P2Se6 are studied by Raman and infrared hard mode spectroscopy. Thes e internal modes reflect the structural changes on a local bond length scal e, related to the phase transition sequence paraelectric-incommensurate-fer roelectric. Clear frequency and intensity changes are observed in relations hip to the phase transitions, in quantitative agreement with mode coupling theory and symmetry selection rules. The relationship to the structural cha nges is discussed. Additional Raman activity is observed in the incommensur ate phase related to phonon folding processes, and in the ferroelectric pha se because of deviations from a centro-symmetric structure. (C) 1999 Elsevi er Science Ltd. All rights reserved.