Abstract
Single crystals of LiDy(PO3)4, NaNd(PO3)4, NH4Dy(PO3)4 and AgLa(PO3)4 polyphosphates have been grown using the flux method. A mixture of monovalent carbonate (M2CO3), rare earth oxide (Ln2O3) and phosphoric acid (H3PO4 85%) was heated at 285°C during a week. Single crystals were obtained and characterized by X-ray diffraction, infrared and Raman spectroscopy. Their investigation was carried out by thermal analysis (ATG/ATD). Their electric conductivity was measured by impedance spectroscopy. The conductivity, σ, vary between $1{,}5\times10^{-8}~\Omega^{-1}$.cm-1 at 300 °C and $2\times10^{-5}~\Omega^{-1}$.cm-1 at 500 °C. The activation energies increase from 0,31 for NH4Dy(PO3)4 to 1,26 eV for NaNd(PO3)4. The basic structural units of these phosphates are ribbons (PO3)n formed by corner-sharing of PO4 tetrahedra with variable geometry, which are unfavourable to the ionic conduction. The ionic transport was mainly assured by M+ ions; moreover, the hydrogen bond established in the structure of NH4Dy(PO3)4 and the proton exchange allow to explain the relatively high conductivity of this phosphate. The results are discussed and correlated to the structural properties of these phosphates.