Combined Effect of Germanium Doping and Grain Alignment on Oxide-Ion Conductivity of Apatite-Type Lanthanum Silicate Polycrystal

The oxide-ion conductivity was successfully improved for the apatite-type lanthanum silicate polycrystal by combining the beneficial use of germanium doping and texturing methods. We synthesized the highly c -axis-oriented polycrystalline material La 9.33 (Si 0.87 Ge 0.13 ) 6 O 26 by isothermal heating of the sandwich-type La 2 Si 2 O 7 /La 2 (Si 0.833 Ge 0.167 )O 5 /La 2 Si 2 O 7 diffusion couples at 1873 K for 50 h. The resulting polycrystal was subsequently characterized using optical microscopy, micro-Raman spectroscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the c -axis steadily increased from 2.0 × 10 –3 S/cm to 6.92 × 10 –2 S/cm with increasing temperature from 673 to 973 K, with the empirical activation energy of conduction being 0.75 eV. The crystal structure at ambient temperature (space group P 6 3 / m ) showed the appreciable positional disordering of O atoms (12 i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4 f and 6 h sites). The disordered structure might be closely related to the fast oxide-ion conduction at high temperatures.