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 transition. The temperature dependence of the dielectric constant is measured for de bias fields up to 150 kV/m. The spontaneous polarization is measured using the Diamant-Drenck-Pepinsky circuit. The results are discussed on a basis of the quasi-one-dimensional Ising model. The parameters obtained in this experiment have considerable anisotropy of interactions: 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 temperature range 0 < (T-C - T) < 10 K. The field dependence of the maximum dielectric constant epsilon(max) as well as the spontaneous polarization 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, is obtained for a second-order polynomial. In our opinion the macroscopic properties of BPI are considerably confused by the real structure of the crystals - defects, space charge, surface layers.