On the T_s-Anomaly in Betaine Calcium Chloride Dihydrate

Abstract

Recent experimental investigations of the dielectric anomaly inside the fourfold modulate phase of betaine calcium chloride dihydrate by Le Maire et al. (T_s-anomaly) is analysed in the framework of Landau theory. Suggested symmetry for the structure is P2₁ca (above T_s) and P1c1, P11a or P2₁11(below T_s), respectively.It is shown that biquadratic coupling between primary order parameter with the B₂ mode (inducing the P2₁ca to P11a phase transition ) may provide consistent explanation of Le Maire et al.'s results.