Influence of bromine on the incomplete devil's staircase behavior of betaine calcium chloride dihydrate - art. no. 174106

The results of a comparative Raman study of pure and Br--doped betaine calc ium chloride dihydrate in the temperature interval 12-292 K are presented. The analysis of the low-frequency Raman spectra at room temperature reveale d that the lowest-frequency phonons are associated with the betaines' vibra tions whereas the lattice modes sensitive to Cl-/Br- substitution are found above 44 cm(-1). The mechanisms responsible for the incommensurate phase a re not strongly affected by bromination in contrast with what happens in th e commensurate phases. It is shown that bromine weakens hydrogen bonding, p reventing the localization of protons related to the onset of the nonmodula ted ferroelectric phase. A quantitative analysis of low-frequency spectra p rovides evidence for a correlation between the anomalous increase of the da mping parameter of the activated acoustic mode and the freezing of the rand om distribution of solitons at low temperatures.