Pressure-temperature phase diagrams of betaine-arsenate, -phosphate, and -phosphite

Abstract

Betaine arsenate (BA), betaine phosphate (BP), and betaine phosphite (BPI) are paraelectric materials at ambient temperature, displaying structural phase transitions to ferro- or antiferroelectrically ordered low-temperature states. The crystals are closely related with respect to their chemical compositions, their crystal structures, and due to the types of intermolecular chemical bonds. Hydrogen bonds play a decisive role. By measuring the dielectric constant ε(p, T) in the temperature interval 40 K ≤ T ≤ 250 K we have observed linear shifts of the transition temperatures T_c on applying hydrostatic pressure p in the range p ≤ 450 MPa. In all three materials the low temperature ordered phase is destabilized with increasing p, dT_c/dp < 0, however with different slopes. These results are closely related to similar findings in the KH₂PO₄-group.

In BP the two transitions into the antiferroelectric phase (T_{c 2} = 86 K) and into the cell-doubled phase (T_{c 3} = 81 K) coincide at p > 8 MPa, i.e., the cell-doubled transition is triggered by the anti-ferroelectric transition. In BA a characteristic dielectric dispersion with frequency is observed at low temperatures, accompanied by considerable dielectric losses, which indicates a dipole-glass behavior, presumably due to pressure induced lattice defects.