Ultrasonic Studies of Ferroelectric Phase Transition in Gly-H3P03 Crystals

Crystals of glycine phosphite G l y H 3 P 0 3 (abbreviated as GPI ) belong to the low-symmetry representatives of hydrogen-bonded crystals. At room temperature the unit cell of this compound is monoclinic (space group P21 /a) and contains four formula units [1]. The crystal structure is characterised by the presence of infinite chains of hydrogen-bonded phosphite anions that are directed along the crystallographic c-axis. Two kinds of symmetrical hydrogen bonds with a double potential well for protons exist in this crystal, and ordering of protons could be expected below room temperature. DSC and dielectric measurements [2] have revealed a ferroelectric phase transition at Tc = 225 K. Although the proton ordering is expected along the c-axis, the dielectric investigations showed the appearance of spontaneous polarization below Tc along the 6-axis with the Curie-Weiss type anomaly of the corresponding dielectric permittivity in the vicinity of this phase transition. In this sense, G P I differs from betaine phosphite (BPI) in which the ferroelectricity is connected with ordering of protons along ferroelectric axis [3]. Thus, the mechanism of the ferroelectric phase transition in G P I crystals is not yet completely clear. In the present paper we report the results of ultrasonic studies of the paraferroelectric phase transition in G P I crystals.