Deuteration effect on the structural phase transitions of hydrogen bonded-dielectric T1H2AsO4: a vibrational study Original Research Article

Infrared and Raman spectra of TlD2AsO4, highly deuterated (>95 D%), are studied over the 10–450 K temperature range. Two phase transitions are evidenced near 370 K and near 90 K. The phase sequence is isomorphous with the TlD2PO4 one. The two-dimensional hydrogen bond network is looser and more symmetric than in TlH2AsO4. In contrast to TlH2PO4 and TlH2AsO4, the disordering of the hydrogen bonds occurs in a one-step process. The order-disorder transition, at the high temperature, is driven by reorientations of the tetrahedra. The structure of the ordered phase of TlD2AsO4, assumed to be antiferroelectric by analogy with TlD2PO4, is different from that of the quasi-antiferroelectric phase of TlH2AsO4. The order-order transition at low temperature, distinctly of first order, implies clear structural changes. When the hydrogen bonds strengthen at low temperature (T < 90 K), both isotope compounds become isomorphous, as they are above 370 K. Although the OD modes appear very little coupled with the heavy-atom modes, they appear more sensitive to the order-disorder transition than OH modes. The band-width of AsO4 modes are a probe to reveal the paraelectric-antiferroelectric transition.