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Abstract
Ferroelectric betaine arsenate (BA) and antiferroelectric betaine phosphate (BP) are structurally very closely related and mixed crystals BA x BP1−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 FE 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.
The experimental results were compared with some theoretical models. The high temperature transition into the ordered phase was discussed in the framework of the quasi-1D Ising, the tunnel, and a phenomenological models. The low temperature transition into the reentrant glassy phase is treated according to the frequency dependent shift of its indicating anomaly by means of an Arrhenius versus Vogel-Fulcher analysis.