Phase diagrams and structural order in mixed crystals BA_xBP_1−x for x > 0.80

Abstract

In order to complete the pressure-temperature-concentration phase diagrams we have extended earlier measurements made on mixed crystals of ferroelectric (f) betaine arsenate (BA) and antiferroelectric (af) betaine phosphate (BP), BA, BA_xBP_1−x, by observing the anomalies in the real and imaginary parts 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 v = 0.1, 0.5, 1, 5, 10, 50, 100 kHz for x = 0.86, 0.94, and 1.00. With increasing pressure the transition into the f phase at T_c2 shifts to lower temperatures; the ordered phase is destabilized. The anomalous behavior found in tan δ and ε” point out the existence of a distribution of relaxation times and competing interactions. A low temperature glassy phase is supposed to exist in this system.

Key Words:

• Phase diagrams
• mixed crystals
• BA_xBP_1−x
• competing interactions
• glassy phases