Browsing by Author "Gao, S.-S."

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Phases in antiferroelectric‑side Rb1-x(ND4)xD2AsO4 mixed crystals by light scattering
(1998-09-01) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Xue, Z.-Q.; Hwa, L.-G.; Schmidt, V. Hugo
The Raman spectra of A1,B2, and E symmetries and the longitudinal [100] Brillouin backscattering have been measured as a function of temperature in the mixed ferroelectric (FE)-antiferroelectric (AFE) system Rb1−x(ND4)xD2AsO4 (DRADA-x) for ammonium concentrations x=0.39, 0.55, 0.69, and 1.0. Successive phase transitions [paraelectric (PE) phase⃗ coexistence of PE and AFE phases⃗ AFE phase] were observed in both x=0.55 and 0.69 as the temperature decreases. Taking into account earlier dielectric results, a phase coexistence of the deuteron-glass state and the AFE order is confirmed in x=0.39. In addition, a broad damping peak (that is associated with a Landau-Khalatnikov-like maximum in x=0.55 and 0.69) was observed in the Brillouin phonon spectra of x=0.39, 0.55, and 0.69. Such an anomaly is attributed to dynamic order-parameter fluctuations. Using previous results for ferroelectric-side DRADA-x, a phase diagram (ND4 concentration x vs T) with qualitative phase boundaries is also given.
Temperature dependent Raman spectra of Rb1-x(ND4)xD2AsO4 mixed crystals
(1998) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Hwa, L.-G.; Schmidt, V. Hugo; Brandt, Dan; Chien, R.-M.
In the mixed ferroelectric (FE)-antiferroelectric (AFE) systemA1−x(ND4)xD2BO4 [A=Rb(orK,Cs)andB=As (or P)], there is competition between the FE and the AFE orderings, each characterized by speciﬁc conﬁgurations of the acid deuterons. [1–8] The random distribution of the Rb and ND4 ions is the main source to produce frustration which can increase local structural competition such that the long-range order of electric dipole disappears. Instead of a typical sharp FE or AFE phase transition, the phase coexistence becomes a characteristic in this type of mixed compounds. By a group theoretical analysis for the KDP-type structure (which contains two molecular units in a primitive unit cell); at zero wavevector, the vibrational modes in the tetragonal symmetry (space group I¯ 42d − D12 2d) can be decomposed into the following irreducible representations: Γvib = 4A1(R) + 5A2(Silent) + 6B1(R) + 6B2(R,IR) + 12E(R,IR). [9] The symmetry species A1, B1, B2 and E are Raman active. The situation in the mixed system D*RADA-x is more complicated than one in the parent crystals, because some Rb (or ND4) ions have been substituted by ND4 (or Rb) ions. In this case, the selection rule of the free AsO4 group is expected to be broken much easily than in the pure crystal. In the recent years, many measurements in D*RADAx system have been achieved on ferroelectric-side crystals x = 0.1, 0.10 and 0.28. [2–5] However, only a few experiments were done on antiferroelectric-side compounds (x ≥0.50).[6,7]A complete understanding for this mixed system is still lacking. This motivated us to carry out the polarized Raman scattering on D*RADA-0.55, 0.69 and 1.0. Here, we pay special attention to the stretching mode ν1 (near 755 cm−1) and the in-plane bending mode δ(O-D) (near 825 cm−1).