Pressure-temperature phase diagram of partially brominated betaine calciumchloride dihydrate

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Abstract
We have investigated the pressure and temperature dependence of the dielectric constant (ϵ (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. At ambient pressure our results indicate the persistence of the 1/4-phase down to very low temperatures.