Brillouin scattering in single crystals of Rb1-x(ND4)xD2AsO4

The LA[100] (a-axis) Brillouin backscattering phonon spectra have been measured as a function of temperature (20-370 K) in the mixed deuterated ferroelectric (FE)-antiferroelectric (AFE) system Rb_1-x (ND₄)_xD₂AsO₄ (DRADA-x) for ammonium concentrations x=0. 0.10 and 0.28. The Brillouin frequency shift with decreasing temperature shows hardening (positive coupling) whose steepness decreases with higher ND₄ content. For RbD ₂AsO₄ (DRDA), a Landau-Khalatnikov-like maximum (which persists in weaker form for x=0.10) was observed and the polarization relaxation time is estimated to be τ-3.8×10^-12/(T_c-T) sec, where T_c is the ferroelectric transition temperature. For both x=0.10 and 0.28, a broad damping peak anomaly which is stronger in x=0.28 was observed and can be connected with the dynamic order parameter fluctuations. Taking into account earlier NMR and dielectric results, we conclude that the local structure competition between FE ordering and AFE ordering is the origin of these broad damping anomalies