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

Abstract

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 1873K for 100h. 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 3.8×10−4S/cm with increasing temperature from 623 to 848K. 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 623K to 4.5×101 at 823K. The activation energy of σ¯bulk ⊥ c was 0.71eV, which is almost twice that of σbulk ‖ c (0.35eV).