Dielectric and Acoustic Properties of BPI1-x BPx (x = 0−0.1) Crystals in the Region of Antiferrodistortive and Ferroelectric Phase Transitions

Abstract

Temperature dependence of the dielectric constant and longitudinal acoustic wave velocity were studied in betaine phosphite—betaine phosphate single crystals (BPI1-xBPx: x = 0−0.1) in temperature regions of antiferrodistortive (Tc1) and ferroelectric (Tc2) phase transitions. The admixture of BP to BPI leads to increase of the antiferrodistortive phase transition temperature Tc1(x) (Tc1(0.1) = 360 K, Tc1(0) = 355 K) and decrease of the dielectric anomaly at Tc1(x). The nonlinear temperature dependence of the inverse dielectric constant in the antiferrodistortive phase, the electric field effect on the dielectric constant and acoustic anomalies at the ferroelectric phase transition are described in the frames of the Landau-Ginzburg-Devonshire-type thermodynamic model with the biquadratic coupling between nonpolar and polar order parameters.