TRANSITIONS BETWEEN PHASES WITH EQUAL WAVE-NUMBERS IN A DOUBLE ISING SPIN MODEL - APPLICATION TO BETAINE CALCIUM-CHLORIDE DIHYDRATE

A double Ising spin model for uniaxially structurally modulated materi als exhibits as a special feature phase transitions between phases wit h equal wavenumbers but different pseudospin configurations. The chara cter of these 'internal' transitions is investigated in the mean-held approximation, with the mean-field transfer-matrix method, and in Mont e Carlo simulations. The structural changes at the transitions are cha racterized by different strengths of harmonics in a Fourier analysis o f the spatial modulation. A dielectric anomaly in the phase diagram of betaine calcium chloride dihydrate (BCCD) and seemingly contradictory structure analyses are explained.