Random fields versus random bond effects in betaine arsenate/phosphate mixed crystals

Abstract

Ferroelectric (F) betaine arsenate (BA) and antiferroelectric (AF) betaine phosphate (BP) are structurally very closely related and mixed crystals BA _x BP_1-x can be grown over the full concentration range. The phase diagrams of several concentrations 0 ≤ x ≤ 1 have been established by investigating dielectric and structural properties by changing external parameters like temperature, hydrostatic pressure, electric bias field, and frequency. This system exhibits interesting pressure-temperature-concentration phase diagrams in which the transition in concentration from an AF to a F phase can be observed with the appearance of some mixed regions and a probably incommensurate phase in the intermediate concentration range. At low temperatures the transition in concentration from a domain to an orientational glass state can be observed besides the appearance of some mixed phases in which different phases coexist. Together with a small review of the most striking features of the system we shall study in detail some new experimental results and their possible interpretation in the pure crystals and in the regions of low impurity content in which the random field effects can be observed in contrast to the random bond region at intermediate concentrations.

Keywords:

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