HYPERSONIC STUDIES IN CSD2ASO4 AND CS-0.84(ND4)(0.16)D2ASO4 SINGLE-CRYSTALS

The longitudinal (LA) Brillouin back-scattering spectra along the [1 0 0] (a-axis) phonon direction have been measured as a function of temp erature (80-300 K) for both CsD2AsO4(DCDA) and Cs-0.84(ND4)(0.16)D2AsO 4 (DCADA-0.16) single crystals. As the temperature decreases the acous tic phonon frequencies of both DCDA and DCADA-0.16 show a hardening (w hose steepness is weaker in DCADA-0.16). A Landau-Khalatnikov-like max imum was observed in a pure crystal of DCDA and the polarization relax ation time is estimated to be tau similar to 2.0 x 10(-11)/T-c-T/s, wh ere T-c similar to 206 K is the ferroelectric transition temperature. In DCADA-0.16, the temperature-dependent frequency shift shows a gradu ally rising anomaly associated with a damping maximum near similar to 175 K, This phenomenon is attributed to coexistence of ferroelectric a nd paraelectric phases. (C) 1998 Elsevier Science B.V. All rights rese rved.