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

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Abstract

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 La9.33(Si0.87Ge0.13)6O26 by isothermal heating of the sandwich-type La2Si2O7/La2(Si0.833Ge0.167)O5/La2Si2O7 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 P63/m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The disordered structure might be closely related to the fast oxide-ion conduction at high temperatures.

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