TEMPERATURE-DEPENDENT RAMAN-SPECTRA OF RB1-X(ND4)(X)D2ASO4 MIXED-CRYSTALS

The Raman spectra (50-1000 cm(-1)) of B-2-y(xy)x symmetry have been me asured as a function of temperature (80-300 K) in the mixed ferroelect ric-antiferroelectric system Rb1-x(ND4)(x)D2AsO4 (DRADA-x) for ammoni um concentrations x=0.55, 0.69 and 1.0. As temperature decreases, x= 0 .55 and 0.69 begin to show similar B-2 spectra with the pure DADA. It reveals that x= 0.55 and 0.69 possess an antiferroelectric order in t he low-temperature region. The stretching modes nu(1) (near 755 cm(-1) ) of the AsO4 group and the in-plane bending modes delta(O-D) (near 82 4 cm(-1)) of x=0.55 and 0.69 exhibit a gradual softening and hardening with decreasing temperature, respectively. These phenomena may relate to the formation of phase coexistence of antiferroelectric and parael ectric clusters.