Abstract
The results of a comparative Raman study of pure and ${\mathrm{Br}}^{\ensuremath{-}}$-doped betaine calcium chloride dihydrate in the temperature interval 12--292 K are presented. The analysis of the low-frequency Raman spectra at room temperature revealed that the lowest-frequency phonons are associated with the betaines' vibrations whereas the lattice modes sensitive to ${\mathrm{Cl}}^{\ensuremath{-}}/{\mathrm{Br}}^{\ensuremath{-}}$ substitution are found above 44 ${\mathrm{cm}}^{\ensuremath{-}1}.$ The mechanisms responsible for the incommensurate phase are not strongly affected by bromination in contrast with what happens in the commensurate phases. It is shown that bromine weakens hydrogen bonding, preventing the localization of protons related to the onset of the nonmodulated ferroelectric phase. A quantitative analysis of low-frequency spectra provides evidence for a correlation between the anomalous increase of the damping parameter of the activated acoustic mode and the freezing of the random distribution of solitons at low temperatures.
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