PHASES IN ANTIFERROELECTRIC-SIDE RB1-X(ND4)(X)D2ASO4 MIXED-CRYSTALS BY LIGHT-SCATTERING

The Raman spectra of A(1), B-2, and E symmetries and the longitudinal [100] Brillouin backscattering have been measured as a function of tem perature in the mixed ferroelectric (FE)-antiferroelectric (AFE) syste m Rb1 - x(ND4)(x)D2AsO4 (DRADA-x) for ammonium concentrations x = 0.39 , 0.55, 0.69, and 1.0. Successive phase transitions [paraelectric (PE) phase-->coexistence of PE and AFE phases-->AFE phase] were observed i n both x = 0.55 and 0.69 as the temperature decreases. Taking into acc ount earlier dielectric results, a phase coexistence of the deuteron-g lass state and the AFE order is confirmed in x = 0.39. In addition, a broad damping peak (that is associated with a Landau-Khalatnikov-like maximum in x = 0.55 and 0.69) was observed in the Brillouin phonon spe ctra of x = 0.39, 0.55, and 0.69. Such an anomaly is attributed to dyn amic order-parameter fluctuations. Using previous results for ferroele ctric-side DRADA-x, a phase diagram (ND4 concentration x vs T) with qu alitative phase boundaries is also given.