ELASTIC-NEUTRON-SCATTERING STUDY OF THE DEVILS-STAIRCASE BEHAVIOR IN DEUTERATED BETAINE CALCIUM-CHLORIDE DIHYDRATE

An elastic-neutron-scattering study of partially deuterated dihydrate betaine calcium chloride (D-BCCD) is reported. We obtain direct diffraction evidence for the occurrence of a commensurate phase with wave vector q = delta-c* = (2/9)c* and for the absence of modulation of the lowest-temperature phase. The existence of another incommensurate phase adjacent to the delta = 2/9 commensurate phase is pointed out and some indications of the existence of the delta = 2/7 commensurate phase are described. Diffraction data relative to the temperature dependence of the modulation wavelength and to its amplitude are discussed in the light of the present controversy relative to the theoretical scheme relevant to BCCD.