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

Abstract

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 (σ_r) 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 σ_r 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_∥/σ_r 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.