Dielectric properties of betaine phosphite near the ferroelectric phase transition

The dielectric behaviour of a betaine phosphite [BPI: (CH3)3NCH2COO · H3PO3] crystal is investigated around its ferroelectric phase transition. The temperature dependence of the dielectric constant is measured for dc 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∥/k = 270 K, J⟂/k = 21 K, and J∥/J⟂ = 12.8. The model explains well the temperature dependences of spontaneous polarization only in the temperature range 0 < (Tc − T) < 10 K. The field dependence of the maximum dielectric constant emax as well as the spontaneous polarization close to Tc, 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. On a etudie les proprietes dielectrique du cristal (CH3)3NCH2COO · H3PO3 (BPI) autour de la transition de phase ferroelectrique. La constante dielectrique a ete mesure en fonction de la temperature avec un champ electrique constant de valeur entre 0 et 150 kV/m. On a mesure aussi la polarization spontanee en utilisant le circuit de Diamant, Drenck et Pepinsky. Nous discutons nos resultats sur la base de modele anisotrope d'Ising. Les parametres obtenus dans cette experience montrent l'anisotropie importante des interactions: J∥/k = 270 K, J⟂/k = 21 K et J∥/J⟂ = 12.8. Ce modele explique bien la dependence de la polarization spontanee en temperature dans la region 0 < (Tc − T) < 10 K. Evolution en champ electrique du maximum de la constante dielectrique et la polarization spontanee, au voisinage de la transition de phase, peuvent etre decrites sur la base d'equation d'etat classique. On a determine les coefficients de cette equation. Pour la constant dielectrique en fonction de la temperature, dans la phase paraelectrique, on a obtenu des meilleurs ajustements avec un polynǒme de deuxieme ordre. Nos observations suggerent que les proprietes macroscopiques de BPI sont tres vraisemblablement liees a la structure reelle du cristal — les defaux, le charge d'espace, les couches de surfaces.