Enhancement of Oxide Ion Conductivity in CuspidineType Materials

Two oxy-cuspidine series, RE4(Ga2O7)O2 (RE = La, Nd, Sm) and La4(Ga2-xGexO7+x/2)O2 (x = 0.2, 0.4, 0.6), have been prepared as single phases. The roomtemperature crystal structures have been determined from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data for La4 (Ga2O7)O2 and La4(Ga1.4Ge0.6O7.3)O2. La4(Ga2O7)O2 is monoclinic (space group P21/c) with a = 7.9791(1) A, b = 11.2054(2) A, c = 11.6276(2) A, = 109.468(1), and V = 980.18(3) A3; and the Rietveld refinement converged to RFNPD = 0.8% and RFLXRPD = 2.1%. La4 (Ga1.4Ge0.6O7.3)O2 is also monoclinic with a = 8.0097 (1) A, b = 11.1601(2) A, c = 11.6426(4) A, = 110.107 (4), and V = 977.29(5) A3; and the Rietveld refinement converged to RFNPD = 1.9% and RFLXRPD = 3.3%. The main structural result is the location of the extra-oxygen between the tetrahedral digallate/germanate groups. The insertion of this oxygen transforms the isolated pyrogroups to infinite distorted trigonal bipyramid chains with some interruptions due to the partial occupancy of oxygens. The electrical results from impedance spectroscopy in static air and under reducing atmosphere indicate that the samples are pure oxide ion conductors. The activation energies for these oxy-cuspidine materials are close to 1.2 eV. The ionic conductivity slightly increases along the RE4A (Ga2O7)O2 series as the unit cell volume increases. The enhancement in the overall oxide conductivity along the La4(Ga2-xGexO7+x/2)O2 series is large, as increases from 6.1(9)·10-6 S·cm-1 for La4(Ga2O7)O2 to 7.0(4)10-4 S·cm-1 for La4(Ga1.4Ge0.6O7.3)O2 at 1073 K. The ion transference number for La4(Ga1.4Ge0.6O7.3)O2 determined by combining the impedance data and the electromotive force measurements, using Gorelovʹs method, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10-20 atm.