Vibrational study of various crystalline phases of thallium dihydrogen arsenate TlH2AsO4. Comparison with thallium dihydrogen phosphate TlH2PO4 Original Research Article

Infrared and Raman spectra of polycrystalline thallium dihydrogen arsenate (TDA) have been investigated in the 10–4000 cm−1 wave number range as a function of temperature between 10 and 300 K. Comparison with the original inelastic neutron scattering spectrum of a polycrystalline sample at 30 K allows a detailed assignment. The proton dynamics appear very weakly coupled to the heavy atom dynamics. The spectra are consistent with the doubling of the unit cell below Tc. The order-disorder transition is associated with the reorientation of the AsO4 tetrahedra; the stabilization of these motions is attained at about 150 K below Tc. TIH2AsO4 at low temperature appears ferrielectric. The vibrational features are similar to those of TlH2PO4, which leads to the conclusion that the hydrogen bond frameworks are also similar.