Étude des propriétés de conduction électrique des phosphates condensés mixtes éléments monovalents-terres rares : LiDy(PO$_3$)$_4$, NaNd(PO$_3$)$_4$, AgLa(PO$_3$)$_4$, NH$_4$DyPO$_3$)$_4$

Single crystals of LiDy(PO 3 ) 4, NaNd(PO 3 ) 4, NH 4 Dy(PO 3 ) 4 and AgLa(PO 3 ) 4 polyphosphates have been grown using the flux method. A mixture of monovalent carbonate (M 2 CO 3 ), rare earth oxide (Ln 2 O 3 ) and phosphoric acid (H 3 PO 4 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 1,5x10 -8 Ω -1 .cm -1 at 300 °C and 2x10 -5 Ω -1 .cm -1 at 500 °C. The activation energies increase from 0,31 for NH 4 Dy(PO 3 ) 4 to 1,26 eV for NaNd(PO 3 ) 4 . The basic structural units of these phosphates are ribbons (PO 3 ) n formed by corner-sharing of PO 4 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 NH 4 Dy(PO 3 ) 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.