STABLE AND X-RAY-INDUCED COMMENSURATE AND INCOMMENSURATE PHASES IN BETAINE CALCIUM-CHLORIDE DIHYDRATE

We report a high-resolution x-ray-diffraction study of commensurate an d incommensurate phases of betaine calcium chloride dihydrate [(CH3)(3 )NCH2COO . CaCl2 . 2H(2)O or (BCCD)] above 80 K. Our results show that very strong pinning of the modulation can be induced due to x-ray irr adiation, similar to the way it has been reported in some incommensura te organic compounds such as thiourea or TMATC-Co (N(CH3)4)(2)CoCl4. U sing a procedure of direct cooling in order to avoid these pinnings, s ome results regarding the intrinsic behavior of the modulation and lat tice parameters are given. in particular, direct evidence of the 3/11 and 4/15 commensurate phases is observed. In the corresponding tempera ture range, the modulation wave-vector evolution is characterized by a sequence of direct passages (first order) from 3/11 to 4/15 and then to 1/4, with coexistence of these phases in large temperature ranges.