ACCUMULATION OF STRUCTURE COMBINATION BRANCHING-PROCESSES IN BETAINE CALCIUM-CHLORIDE DIHYDRATE

The dielectric constants of betaine calcium chloride dihydrate have been measured under hydrostatic pressure (190 less-than-or-equal-to p less-than-or-equal-to 320 MPa) and at varying temperatures (140 less-than-or-equal-to T less-than-or-equal-to 210 K) in the region of structure branching processes between the [22] and [23] commensurate phases. Along the various phase boundaries the dielectric permittivity exhibits maxima, which have been attributed to an accumulation of structure branching processes, as predicted by statistical spin models. The accumulation points are quasitricritical points, which have not been observed before. Our results compare favorably with recent calculations based on the axial next-nearest-neighbor Ising model.