Competing interactions and phase transitions in betaine arsenate-betaine phosphate (BA_xBP_1-x)

Abstract

We have studied mixed crystals of ferroelectric (f) betaine arsenate (BA) and antiferroelectric (af) betaine phosphate (BP), BA_xBP_1-x, x=0.07, 0.24, 0.39 by observing the anomalies in the real part of the dielectric function (ϵ) and the dissipation factor (tan δ) at various temperatures (23 < T < 300 K) and hydrostatic pressures (p<300 MPa) at frequencies F=0.5, 1, 5, 10, 50, 100 kHz. For x=0.24 and x=0.39, ϵ exhibits a double peak structure near T_c mirrored in the tanδ behavior of each substance. The transition into the af phase at T_c2 shifts to lower temperatures, the ordered phase is destabilized. Interesting phase diagrams are obtained with a probable glassy state preceded by some mixed phase. The frustrated interactions af-f seem to play the major role in this behavior.