Enhancement of Oxide Ion Conductivity in Cuspidine-Type Materials

Two oxy-cuspidine series, RE 4 (Ga 2 O 7 )O 2 (RE = La, Nd, Sm) and La 4 (Ga 2 - x Ge x O 7+ x /2 )O 2 ( x = 0.2, 0.4, 0.6), have been prepared as single phases. The room-temperature crystal structures have been determined from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data for La 4 (Ga 2 O 7 )O 2 and La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 . La 4 (Ga 2 O 7 )O 2 is monoclinic (space group P 2 1 / c ) with a = 7.9791(1) Å, b = 11.2054(2) Å, c = 11.6276(2) Å, β = 109.468(1)°, and V = 980.18(3) Å 3; and the Rietveld refinement converged to R F NPD = 0.8% and R F LXRPD = 2.1%. La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 is also monoclinic with a = 8.0097(1) Å, b = 11.1601(2) Å, c = 11.6426(4) Å, β = 110.107(4)°, and V = 977.29(5) Å 3; and the Rietveld refinement converged to R F NPD = 1.9% and R F LXRPD = 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 RE 4 (Ga 2 O 7 )O 2 series as the unit cell volume increases. The enhancement in the overall oxide conductivity along the La 4 (Ga 2 - x Ge x O 7+ x /2 )O 2 series is large, as increases from 6.1(9)·10 - 6 S·cm - 1 for La 4 (Ga 2 O 7 )O 2 to 7.0(4)10 - 4 S·cm - 1 for La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 at 1073 K. The ion transference number for La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 determined by combining the impedance data and the electromotive force measurements, using Gorelov's method, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10 - 20 atm.