Vibrational study of H3OUO2PO4.3 H2O (HUP) and related compounds. Phase transitions and conductivity mechanisms

Infrared and Raman spectra of polycrystalline KUO2PO4.3 H2O (KUP) and its isotopic derivatives KUO2P18O4.3 H2O and KUO2PO4.3 D2O have been investigated in the 4000-10-cm-1 range at different temperatures. An assignment of the bands in terms of UO2, PO4 and H2O vibrations has been proposed. Combined differential scanning calorimetry and spectroscopic data show two diffuse phase transitions near 130 and 230 K. Comparison of the vibrational spectra of phase I at 300 K and phase IV at 100 K indicates that ordering of the water molecules with subsequent ordering of PO4 tetrahedra on a site with lower symmetry appears to be the main mechanism responsible for the phase transformation. All the six O-H distances of water molecules in phase IV are found to be crystallographically nonequivalent. Conducting ion frequencies and the corresponding force constants have been determined for the analogous compounds MUP with M = K+, Na+, Ag+, NH4+, Tl+ and H3O+ and compared with other properties of these ionic conductors. Conductivity mechanisms in these materials are discussed. (author)