Dielectric and Spectroscopic Studies of Betaine Phosphate-Betaine Arsenate Mixed System

Abstract

Betaine arsenate (BA) and betaine phosphate (BP) are addition compounds of the amino acid betaine [(CH3)3N+CH2COO−] and arsenic and phosphoric acids, respectively. The mixed system (BP)x(BA)1 − x displays, at low temperatures, dielectric anomalies that have been interpreted as due to additional transitions to re-entrant glass phases originated by frustrated FE and AF interactions. In this work, Raman and infrared spectroscopic data are analysed and compared with results of complex dielectric constant, pyroelectric measurements performed during thermal cycles under different bias electric fields and low frequency polarization hysteresis loops. The results obtained for the x = 0.75 composition do not give any evidence for the stabilisation of a re-entrant glass phase or a non-ergodic regime, and a long-range order seems to prevail in the whole temperature range studied. The inclusion of BA is however capable of destabilising the AF order, giving rise to a slightly polar ferroelectric phase at low temperatures. Moreover spectroscopic results reveal that methyl groups of the betaine molecules act as excellent probes for the detailed study of the phase transition sequence in this system.