Structure of the X-phase of 38% brominated betaine calcium chloride dihydrate

The structures of the high- and low-temperature phases of 38% brominated BC CD [betaine (trimethylammonioacetate) calcium chloride dihydrate], the latt er being known as the X-phase, have been determined by single-crystal neutr on diffraction at 295 and 20 K, respectively. The symmetry of the X-phase i s described by the P2(1)2(1)2(1) space group. The distortion with respect t o the high-temperature Pnma phase is characterized by anti-symmetric displa cements of the betaine molecules as well as of the Ca octahedra. On the bas is of a symmetry-mode analysis, we propose an interpretation of the direct phase transition that occurs around 80 K between these two phases.