BETAINE ARSENATE AS A SYSTEM WITH 2 INSTABILITIES

Using our experimental dielectric and acoustic data and those available in the literature (the small-signal dielectric response, the temperature evolution of the dielectric hysteresis loops, the electric-field-temperature phase diagram, the sequence and the types of the observed phase transitions, the temperature-deuterium-concentration phase diagram and partial temperature loops in the sound velocity), we analyze the properties of a (CH3)NCH2COOH3AsO4-(CH3) NCH2COOD3AsO4 solid solution in the framework of a unique phenomenological scheme corresponding to a system with two competing instabilities (ferroelectric and structural). We average the competition on a macroscopical level, where the coexistence of the two types of ordering is, in principle, possible, but the appearance of one of the order parameters suppresses the instability related to the other. It is shown that betaine arsenate-deuterated betaine arsenate represents a good example of such a system. © 1995 The American Physical Society.