Temperature and concentration dependences of Raman vibrational modes in Rb1-x(ND4)xD2AsO4 mixed crystals

Abstract

The Raman vibrations (10–103 cm−1) of A1, B2, and E symmetries along the [110] phonon direction have been measured as a function of temperature (80–293 K) in the mixed ferroelectric (FE) antiferroelectric system Rb1−x(ND4)xD2AsO4 (DRADA-x) for ammonium concentrations x=0, 0.10, and 0.28. With decreasing temperature, the B2 Raman shifts of RbD2AsO4 show both softening and hardening anomalies for four selected modes. In DRADA-0.10, the bending mode ν2(B2) (near 280 cm−1) of the AsO4 group exhibits a gradual softening down to T∼125 K and then has a rapid drop. This phenomenon is attributed to a quick development of long-range FE ordering. In DRADA-0.28, the in-plane bending mode δ(O-D) (B2) shows a softening near 220 K, whereas the bending mode ν2(B2) of the AsO4 group exhibits a hardening. We associate these anomalies with the onset of glass formation. A doublet with splitting of about 10 cm−1 was observed in the ν1 modes and was attributed to a slight deformation of the ν1 vibration due to AsO4 groups having different nearby ions (ND4 and Rb).