Acta Crystallographica Section B

Structural Science

Volume 57, Part 3 (June 2001)

research papers

Acta Cryst. (2001). B57, 296-302 [ doi:10.1107/S0108768101001525 ]

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

L. G. Vieira, O. Hernandez, J. L. Ribeiro, A. Cousson, J.-M. Kiat, M. R. Chaves, A. Almeida and A. Klöpperpieper

Abstract: The structures of the high- and low-temperature phases of 38% brominated BCCD [betaine (trimethylammonioacetate) calcium chloride dihydrate], the latter being known as the X-phase, have been determined by single-crystal neutron diffraction at 295 and 20 K, respectively. The symmetry of the X-phase is described by the P2₁2₁2₁ space group. The distortion with respect to the high-temperature Pnma phase is characterized by anti-symmetric displacements of the betaine molecules as well as of the Ca octahedra. On the basis of a symmetry-mode analysis, we propose an interpretation of the direct phase transition that occurs around 80 K between these two phases.

Keywords: betaine; symmetry.