FIR REFLECTIVITY, PYROELECTRIC EFFECT AND DIELECTRIC-CONSTANT IN 8-PERCENT BROMINATED BCCD

Betaine calcium chloride dihydrate is known to exhibit a rich number o f commensurate and incommensurate phases (k = delta(T)c) between a ro om temperature paraelectric phase (Pnma) and a lower temperature ferro electric phase (Pn2(1)a). Partial bromination of this compound induces important changes in the phase diagram of the material. This work rep orts recent results of FIR reflectivity, dielectric dispersion and pyr oelectric effect measured on 8% brominated BCCD. In agreement with pre vious Raman results reported by Schaack and coworkers, the FIR. reflec tivity (E parallel to b) spectra (20 cm(-1) < omega < 300 cm(-1)) show that the sequence of commensurate and incommensurate phases observed in tile pure compound is shifted towards lower temperatures and that t he structure remains modulated at lower temperatures (modulation waven umber delta approximate to 1/4 at T = 20 K). The temperature and frequ ency dependence of the dielectric constants epsilon(a) and epsilon(b), as well as the behaviour of the pyroelectric coefficient lambda(b) ob served along a sequence of thermal cycles with and without a bias fiel d, indicate a progressive deviation from thermal equilibrium and a sub sequent freezing of the structure at low temperatures. The experimenta l results are discussed within the framework of a phenomenological mod el based on a Landau-type free energy expansion, previously reported t o account for the phase transition sequence observed in the pure cryst al under a positive hydrostatic pressure.