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

Infrared and Raman spectra of polycrystalline H3OUO2PO4.3 H2O (HUP) and its D and P18O4 derivatives, in the form of dense transparent disks and wet powder, have been investigated at various temperatures in the 100 to 300 K region. The bands due to framework vibrations are similar to those of KUP, whereas those for the protonic species are different. OH stretching and bending bands of the oxonium ion have been identified at 2920, 1740 and 1160 cm-1 in the low-temperature spectrum of HUP. Differential scanning calorimetry and infrared intensity investigations show a phase transition between 274 and 260 K. The mechanism of the phase transition consists, as in the case of KUP, of ordering of the protonic species, which induces ordering of PO4 tetrahedra. The ordering can be influenced by excess water content, stacking faults and stress (ferroelastic behavior is evidenced). The conductivity mechanism in HUP is discussed. (author)