Vibrational study by inelastic neutron scattering, infrared absorption and Raman scattering of potassium, rubidium and cesium dihydrogenarsenate crystals: Comparison with thallium dihydrogenarsenate

Article abstract of DOI:10.1016/S0301-0104(97)00210-3

The vibrational study of polycrystalline potassium, rubidium and cesium dihydrogenarsenates, KH₂AsO₄, RbH₂AsO₄ and CsH₂AsO₄, has been undertaken in the 20-4000 cm^-1 range. The inelastic neutron scattering spectra obtained at 20 K allow a precise assignment of the OH vibrations. The infrared absorption and Raman scattering spectra of the paraelectric and ferroelectric phases have been analyzed. The three compounds are isomorphous with KH₂PO₄. The results are compared to those obtained for the thallium dihydrogenarsenate T1H₂AsO₄. In all compounds the proton dynamics appear weakly coupled to heavy atoms motions. The order-disorder transition at T_c is associated to relaxational motions of anions and protons in the tetragonal potassium, rubidium and cesium dihydrogenarsenates and to collective reorientational motions of anions in monoclinic thallium derivative. The spectroscopic features show the participation of cations K⁺, Rb⁺ and Cs⁺ to the dynamics of the three-dimensional lattice in both phases. In contrast, heavy-cation T1⁺ motions are decoupled from the dynamics of hydrogen bond layers. No correlation between ordering of lattice and OH stretching wavenumber is observed.