PRESSURE-TEMPERATURE PHASE-DIAGRAM OF PARTIALLY BROMINATED BETAINE CALCIUMCHLORIDE DIHYDRATE

We have investigated the pressure and temperature dependence of the dielectric constant (Ɛ (p.T)) in a sample of betaine calciumchloride-dihydrate (BCCD), where 6.4% of the chlorine ions have been substituted by bromine (BC[C0.934B0.064]D). We have also studied the temperature dependent Raman spectra (8K ≤ T ≤ 200K) at ambient pressure of this compound. The results are compared with the (p-T)phase diagram of pure BCCD.The expansion of the lattice due to the Br--ions with larger ionic radius corresponds to a “negative hydrostatic pressure” of 61.3 MPa, the local strain at Br--sites causes strong global hysteresis effects and suppresses most of the commensurate lock-in phases found in pure BCCD. Only the most stable phases of BCCD (δ=1/4, 1/6(?), 0 (ferroelectric)) can be identified by dielectric data at high pressure. A t ambient pressure our results indicate the persistence of the 1/4-phase down to very low temperatures. © 1990, Taylor & Francis Group, LLC. All rights reserved.