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.