Alternative phase diagram for the betaine phosphate-betaine arsenate mixed system

Abstract

The mixed solid solutions of ferroelectric betaine arsenate (BA) and antiferroelectric betaine phosphate (BP) are known examples of H-bonded materials with competing ferro and antiferro interactions. Dispersive anomalies detected in the dielectric loss at low temperatures have been tentatively associated with the stabilization of disordered phases such as re-entrant dipolarlike glassy phases. Despite extensive experimental studies of $(\mathrm{BP}{)}_{\mathrm{x}}(\mathrm{BA}{)}_{1\ensuremath{-}\mathrm{x}}$ mixed systems, the existence of such phases is doubtful so far. In an attempt to clarify the nature of the low temperature phase transition sequence of $(\mathrm{BP}{)}_{\mathrm{x}}(\mathrm{BA}{)}_{1\ensuremath{-}\mathrm{x}}$, we have undertaken detailed pyroelectric, dielectric, polarization reversal, Raman, and infrared studies as a function of temperature. In this work we shall present the experimental results for $x=0.25$ (BP25) and compare them with the ones previously reported for the two other intermediate compositions. We shall then propose a phase diagram alternative to the one already published for this system; moreover, we shall tentatively identify the possible underlying mechanisms, and discuss how such mechanisms evolve as a function of the BP concentration.