On the transition to the X-phase in highly brominated Betaine Calcium Chloride Dihydrate (BCCD)

We report dielectric and optical birefringence measurements in 38% brominated BCCD, which confirm the transition to the X-phase, at ambient pressure, around T-c=80 K. These results are compared with elastic neutron scattering data. A coexistence of the parent Pnma phase with the X-phase is observed in a large range of temperature centered at about T-c. It is found that the mosaic spread of the crystal varies strongly and irreversibly when a temperature cycle Pnma - X-phase - Pnma is performed. A 30% brominated BCCD has also been investigated by elastic neutron scattering. For this compound, an intermediate incommensurate phase occurring between T(1)approximate to 111 K and T(c)approximate to 50 K is observed.