Enhancement of lanthanum ion conductivity by (40−2) plane orientation of polycrystalline La₄Ga₂O₉

To enhance the conductivity of La3+, the (402) plane aligned polycrystalline La4Ga2O9 (space group P21/c) was prepared by sintering at 1673 K for 2 h after colloidal processing under a high magnetic field of 12 T. The textured polycrystal was characterized by X-ray diffraction and impedance spectroscopy with respect to the grain alignment direction. The (402) plane normal of each constituent crystal grain of the polycrystal was found to be almost parallel to the applied magnetic field. The texture fraction of (402), expressed as the Lotgering factor f402, was 0.235. A comparison was made of the conductivities parallel (σ∥) and perpendicular (σ⊥) to the aligned plane normal at temperatures ranging from 673 to 1073 K. These were also compared with the conductivity (σr) of randomly grain-oriented La4Ga2O9 polycrystal. The σ∥, ranging from 1.63 × 10−7 S cm−1 at 623 K to 8.52 × 10−4 S cm−1 at 1073 K, demonstrated the highest value at each temperature, followed by σr and σ⊥ in that order. The σ∥/σ⊥ ratios ranged from 10.0 at 673 K to 15.2 at 1073 K, and the σ∥/σr ratios ranged from 6.7 at 873 K to 8.3 at 1073 K. Since the a-axis is almost parallel to the (402) plane normal, the enhanced La3+ conductivity of the La4Ga2O9 polycrystal has confirmed, for the first time, the prediction by the bond valence energy landscape method in the literature that La3+ conduction is preferential along the a-axis.