Raman and infrared study of the quasi-one-dimensional betaine arsenate-phosphate mixed-crystal system

Abstract

Ferroelectric (FE) betaine arsenate (BA) and antiferroelectric (AF) betaine phosphate (BP) are structurally very closely related and mixed crystals ${\mathrm{BA}}_{x}{\mathrm{BP}}_{1\ensuremath{-}x}$ can be grown over the full concentration range. 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 a concentration-dependent appearance of some mixed regions. At low temperatures different relaxation processes have been observed that might be related to the transition from a domain state to an orientational glass depending on concentration. In some regions of the phase diagram different phases coexist. The origin of the different transitions and relaxation processes is still unclear and recent x-ray-diffraction studies reveal that the peculiar behavior of the mixed crystals cannot be explained from the structural point of view. We have performed Raman and infrared investigation of samples across the whole (x, T) phase diagram, at various hydrostatic pressures, in order to shed light on the structural origin of the different processes. It is concluded that the FE or AF character of the ordered phase is related to the degrees of freedom of the ${\mathrm{AsO}}_{4}$ and ${\mathrm{PO}}_{4}$ tetrahedra. In the same way these units determine the type and the modes triggering the phase transitions for each concentration. The latter seems to occur together with a participation of the betaine molecule, especially of the methyl and buthyl groups.