VIBRATIONAL STUDY BY INELASTIC NEUTRON-SCATTERING, INFRARED-ABSORPTION AND RAMAN-SCATTERING OF POTASSIUM, RUBIDIUM AND CESIUM DIHYDROGENARSENATE CRYSTALS - COMPARISON WITH THALLIUM DIHYDROGENARSENATE

The vibrational study of polycrystalline potassium, rubidium and cesiu m dihydrogenarsenates, KH2AsO4 RbH2AsO4 and CsH2AsO4, has been underta ken in the 20-4000 cm(-1) range. The inelastic neutron scattering spec tra obtained at 20 K allow a precise assignment of the OH vibrations. The infrared absorption and Raman scattering spectra of the paraelectr ic and ferroelectric phases have been analyzed. The three compounds ar e isomorphous with KH2PO4. The results are compared to those obtained for the thallium dihydrogenarsenate TlH2AsO4. in ail compounds the pro ton dynamics appear weakly coupled to heavy atoms motions. The order-d isorder transition at T-c is associated to relaxational motions of ani ons and protons in the tetragonal potassium,rubidium and cesium dihydr ogenarsenates and to collective reorientational motions of anions in m onoclinic thallium derivative. The spectroscopic features show the par ticipation of cations K+, Rb+ and Cs+ to the dynamics of the three-dim ensional lattice in bath phases. In contrast, heavy-cation Tl+ motions are decoupled from the dynamics of hydrogen bond layers. No correlati on between ordering of lattice and OH stretching wavenumber is observe d. (C) 1997 Elsevier Science B.V.