Phases in antiferroelectric‑side Rb1-x(ND4)xD2AsO4 mixed crystals by light scattering

Abstract

The Raman spectra of A1,B2, and E symmetries and the longitudinal [100] Brillouin backscattering have been measured as a function of temperature in the mixed ferroelectric (FE)-antiferroelectric (AFE) system Rb1−x(ND4)xD2AsO4 (DRADA-x) for ammonium concentrations x=0.39, 0.55, 0.69, and 1.0. Successive phase transitions [paraelectric (PE) phase⃗ coexistence of PE and AFE phases⃗ AFE phase] were observed in both x=0.55 and 0.69 as the temperature decreases. Taking into account earlier dielectric results, a phase coexistence of the deuteron-glass 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 spectra of x=0.39, 0.55, and 0.69. Such an anomaly is attributed to dynamic order-parameter fluctuations. Using previous results for ferroelectric-side DRADA-x, a phase diagram (ND4 concentration x vs T) with qualitative phase boundaries is also given.