Anisotropy of oxide-ion conduction in apatite-type lanthanum silicate

The anisotropy of oxide-ion conduction was clarified for the apatite-type La9.33 + 2x(SiO4)6O2 + 3x (0.01 ≤ x ≤ 0.13). We synthesized the polycrystalline materials of apatite by the isothermal heating of the sandwich-type La2Si2O7/La2SiO5/La2Si2O7 diffusion couples at 1873 K for 100 h. The resulting polycrystals were subsequently characterized using optical microscopy, X-ray diffractometry and impedance spectroscopy. The individual c-axes of the crystallites were appreciably aligned along the diffusion direction, while their a-axes were, around the common c-axis direction, oriented randomly. We obtained the impedance spectroscopy data of the polycrystalline electrolyte, the planar electrodes of which were parallel to the common c-axis. Thus, the obtained bulk conductivity ( σ ¯ bulk ⊥ c) corresponds to the average value of those perpendicular to the c-axis, which steadily increased from 1.6 × 10− 5 to 3.8 × 10− 4 S/cm with increasing temperature from 623 to 848 K. The bulk conductivity parallel to the c-axis (σbulk ‖ c) that was determined in a previous study was, when compared at the same temperature, necessarily higher than the corresponding σ ¯ bulk ⊥ c value; the magnitude of anisotropy (σbulk ‖ c / σ ¯ bulk ⊥ c) steadily decreased with increasing temperature from 2.1 × 102 at 623 K to 4.5 × 101 at 823 K. The activation energy of σ ¯ bulk ⊥ c was 0.71 eV, which is almost twice that of σbulk ‖ c (0.35 eV).