Commensurately modulated 1/4 and 1/5 phases of deuterated betaine calcium chloride dihydrate: a neutron structural study

The structures of the commensurate 1/4 and 1/5 phases of the displacively m odulated compound D-BCCD [deuterated betaine (trimethylammonioacetate) calc ium chloride dihydrate, i.e. (CD3)(3)NCD2COOCaCl2(D2O)(2) or CaCl2. C5D11NO 2. 2D(2)O] have been determined by single-crystal neutron diffraction at 10 0 and 68 K, respectively. The structural model of the 1/4 phase is found to be quite different from that obtained previously from X-ray diffraction da ta of the hydrogenated compound. This discrepancy comes from the fact that X-ray irradiation induces in this compound an unusal time-dependent decreas e of the intensity of high-order satellite diffraction peaks. As a conseque nce and due to the commensurate nature of the phases investigated, X-ray di ffraction failed to detect the large anharmonicity of the structural modula tion which is clearly present in the two structures determined by neutron d iffraction.