DIELECTRIC-PROPERTIES OF BETAINE PHOSPHITE NEAR THE FERROELECTRIC PHASE-TRANSITION

The dielectric behaviour of a betaine phosphite [BPI: (CH3)(3)NCH2COO . H3PO3] crystal is investigated around its ferroelectric phase transi tion. The temperature dependence of the dielectric constant is measure d for de bias fields up to 150 kV/m. The spontaneous polarization is m easured using the Diamant-Drenck-Pepinsky circuit. The results are dis cussed on a basis of the quasi-one-dimensional Ising model. The parame ters obtained in this experiment have considerable anisotropy of inter actions: J(parallel to)/k = 270 K, J(perpendicular to)/k = 21 K, and J (parallel to)/J(perpendicular to) = 12.8. The model explains well the temperature dependences of spontaneous polarization only in the temper ature range 0 < (T-C - T) < 10 K. The field dependence of the maximum dielectric constant epsilon(max) as well as the spontaneous polarizati on close to T-C, can be described starting from the classical equation of state. The equation coefficients are determined. However, the best fit to the reciprocal permittivity data, in the paraelectric phase, i s obtained for a second-order polynomial. In our opinion the macroscop ic properties of BPI are considerably confused by the real structure o f the crystals - defects, space charge, surface layers.