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Abstract
Fully deuterated lithium acetate dihydrate undergoes a structural phase transition at low temperatures. We have undertaken a systematic study of the low-frequency lattice mode dispersion curves, along a previously determined path in the Brillouin zone, in the high and the low temperature phases, as well as in the vicinity of T c . A set of low frequency optical modes was detected and their symmetries attributed. In accordance with previous results, we propose assigning some of them to complex vibrations involving acetate and the water molecule librations.We propose thus a coupling mechanism between the two possible reorientations of the methyl groups and one of the low frequency lattice vibrations in the vicinity of T c .
Acknowledgments
Pedro Teles thanks F.C.T.(Portugal) for his grant SFRH/BD/2002/11300, and the L.L.B.(CEA-CNRS, Saclay, France) for providing facility and financial support. We would like to thank the L.L.B.(CEA-CNRS, Saclay, France) and the E.S.R.F.(Grenoble, France) for the beam time. We also thank P Baroni and P Boutrouille for technical support. Finally, we thank JM Perez-Mato for the fruitful discussions and suggestions.