DEUTERATION EFFECT ON THE STRUCTURAL PHASE-TRANSITIONS OF HYDROGEN-BONDED DIELECTRIC TLH2ASO4 - A VIBRATIONAL STUDY

Infrared and Raman spectra of TID2AsO4, highly deuterated (> 95 D%), a re studied over the 10-450 K temperature range. Two phase transitions are evidenced near 370 K and near 90 K. The phase sequence is isomorph ous with the TID2PO4 one. The two-dimensional hydrogen bond network is looser and more symmetric than in TIH2AsO4. In contrast to TIH2PO4 an d TIH2AsO4, the disordering of the hydrogen bonds occurs in a one-step process. The order-disorder transition, at the high temperature, is d riven by reorientations of the tetrahedra. The structure of the ordere d phase of TID2AsO4, assumed to be antiferroelectric by analogy with T ID2PO4, is different from that of the quasi-antiferroelectric phase of TIH2AsO4. The order-order transition at low temperature, distinctly o f first order, implies clear structural changes. When the hydrogen bon ds strengthen at low temperature (T < 90 K), both isotope compounds be come isomorphous, as they are above 370 K. Although the OD modes appea r very little coupled with the heavy-atom modes, they appear more sens itive to the order-disorder transition than OH modes. The band-width o f AsO4 modes are a probe to reveal the paraelectric-antiferroelectric transition. (C) 1997 Elsevier Science B.V.