TEMPERATURE AND CONCENTRATION DEPENDENCES OF RAMAN VIBRATIONAL-MODES IN RB-1-X(ND4)(X)D2ASO4 MIXED-CRYSTALS

The Raman vibrations (10-10(3) cm(-1)) of A(1), B-2, and E symmetries along the [110] phonon direction have been measured as a function of t emperature (80-293 K) in the mixed ferroelectric (FE) antiferroelectri c system Rb-1-x(ND4)(x)D2AsO4 (DRADA-x) for ammonium concentrations x= 0, 0.10, and 0.28. With decreasing temperature, the B-2 Raman shifts o f RbD2AsO4 show both softening and hardening anomalies for four select ed modes. In DRADA-0.10, the bending mode v(2)(B-2) (near 280 cm(-1)) of the AsO4 group exhibits a gradual softening down to T similar to 12 5 K and then has a rapid drop. This phenomenon is attributed to a quic k development of long-range FE ordering. In DRADA-0.28; the in-plane b ending mode delta(O-D) (B-2) shows a softening near 220 K, whereas the bending mode v(2)(B-2) Of the AsO4 group exhibits a hardening. We ass ociate these anomalies with the onset of glass formation. A doublet wi th splitting of about 10 cm(-1) was observed in the v(1) modes and was attributed to a slight deformation of the v(1) vibration due to AsO4 groups having different nearby ions (ND4 and Rb).