Review of experimental and theoretical results for the betaine arsenate/phosphate mixed crystals system

Ferroelectric betaine arsenate (BA) and antiferroelectric 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 seve ral 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 exhi bits interesting pressure-temperature -concentration phase diagrams in whic h 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 pha se in the intermediate concentration range. At low temperatures the transit ion in concentration from a domain to an orientational glass state can be o bserved besides the appearance of some mixed phases in which different phas es coexist. The experimental results were compared with some theoretical models. The hi gh temperature transition into the ordered phase was discussed in the frame work of the quasi-1D Ising, the tunnel, and a phenomenological models. The low temperature transition into the reentrant glassy phase is treated accor ding to the frequency dependent shift of its indicating anomaly by means of an Arrhenius vel sus Vogel-Fulcher analysis.