EFFECTS OF LATTICE DEFECTS ON THE PHASE TRANSITIONS OF BETAINE CALCIUM CHLORIDE DIHYDRATE

We have studied the effects of Br and Mn impurities and gamma-irradiation defects on the phase transition sequence of BCCD using dielectric and Raman spectroscopic measurements at hydrostatic pressures p < 420 MPa and temperatures 20 K < T < 250 K. The ( T, p)-phase diagrams of 5% Mn doped BCCD and three gamma-irradiated BCCD crystals have been determined and compared to those of 2%, 7% and 28% Br doped BCCD. We have localized the incommensurate phases and obtained the modulations delta of the commensurate phases of the doped crystals and an irradiated one by Raman spectroscopy. The effects of the different types of defects on the phase transition sequence of BCCD are discussed. A freezing process in the dipole system appears to be present at least in some of the studied substances.