Phenomenological description of the sevenfold phase of betaine calcium chloride dihydrate. I. Coupling to the electric field

The sevenfold phase of Betaine Calcium Chloride Dihydrate (q(c)/c* = 2/7) is analyzed in the framework of the conventional phenomenological model for the type I commensurate phase. Parameters of the potential are determined from a set of selected experimental data. Quantitative results are obtained for various physical properties, including the relaxation frequency of the ("pseudo") phason gap and the mobility of the domain walls in the commensurate phase, the thickness and mutual distance of the phase solitons in the incommensurate phase, field-temperature phase diagram and the amplitude of the order parameter.