Lattice dynamics of BCCD

The paper summarizes the recent studies of lattice dynamics in Betaine Calc ium Chloride Dihydrate (BCCD), which are not included in the review by G. S chaack and M. Le Maire (1988). The hard harmonic mode method used previousl y for determination of amplitudon and pseudo-phason modes in BCCD is formul ated in a general manner. It is shown why and how the well-known coupled da mped harmonic oscillator model can be systematically used for determination of the bare soft mode parameters directly from a single experimental spect rum. Transmission maximum in IR spectra used previously for monitoring the modulation vector is assigned to the anti-resonance effect caused by coupli ng between the soft and the transverse acoustic mode.