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Temperature dependences of the dielectric permittivity of betaine phosphite crystals are studied both without and under application of an electric bias. It is shown that, in view of the fact that the high-temperature improper ferroelastic (antiferrodistorsive) phase transition at T[sub c1] = 355 K is nearly tricritical, the nonlinear temperature dependence of inverse dielectric permittivity in the paraelectric phase and the effect of the field on the dielectric permittivity can be described within a phenomenological model containing two coupled (polar and nonpolar) order parameters with a negative coupling coefficient. An analysis of the model revealed that, in the case where two phase transitions, a nonpolar and a ferroelectric one, can occur in the crystal, all of its dielectric properties, including the polarization response in a field, can be described by one dimensionless parameter a. For the crystal under study, we have a = -2.5. This value of the parameter corresponds to a second-order ferroelectric transition far from the tricritical point, at which a = -1. It is shown that the polarization response in the paraelectric phase in an electric field calculated within this model differs radically from that in the ferroelectric phase-transition model for which the Curie-Weiss law holds in the paraelectric phase. [ABSTRACT FROM AUTHOR]

A study is reported of acoustic anomalies in the vicinity of the antiferrodistortive and ferroelectric phase transitions in nominally pure betaine phosphite crystals and a crystal with 3% betaine phosphate impurity. The observed anomalies in the velocity and damping rate of longitudinal acoustic waves propagating along different crystallographic axes are interpreted within Landau theory. It is shown that the phase transitions in these crystals are characterized by a relatively weak correlation of the order parameter to strain and by a considerable contribution to the acoustic anomalies of an interaction higher in order than striction. [ABSTRACT FROM AUTHOR]