PHASE-DIAGRAM OF BETAINE CALCIUM-CHLORIDE DIHYDRATE IN AN APPLIED ELECTRIC-FIELD

The molecular crystal betaine calcium chloride dihydrate (BCCD) exhibits the largest number of structurally incommensurate and commensurate phases yet found. A model recently introduced by two of us reproduced the sequence and temperature stability interval of the commensurate phases, as well as the complex temperature dependence of the spontaneous polarization. We generalize the free energy of this model to include the effect of an applied electric field. The phase diagram of BCCD in an applied field is then calculated and the result compared with experiments.