Calcium ion conduction anisotropy of b-axis-aligned CaAl₄O₇ polycrystal

Abstract

To investigate the anisotropy of Ca²⁺ conduction, the b-axis-aligned CaAl₄O₇ (space group C2/c) polycrystal was prepared by colloidal processing under high magnetic field of 12 T followed by sintering at 1773 K for 2 h. The textured polycrystal was characterized by X-ray diffractometry and impedance spectroscopy with respect to the grain alignment direction, which was parallel to the applied magnetic field. The texture fraction of {0k0}, expressed as the Lotgering factor f_0k0, was 0.63. The 〈101〉 directions of individual crystal grains were randomly distributed around the grain-alignment direction of the polycrystal. The conductivities perpendicular (σ_⊥) and parallel (σ_∥) to the grain alignment direction were compared with the conductivity (σ_random) of random grain oriented polycrystal between 773 and 1073 K. The σ_⊥, ranging from 3.09 × 10⁻⁷ to 1.80 × 10⁻⁵ S cm⁻¹, showed the highest value at each temperature, followed by σ_random and σ_∥ in that order. The results have confirmed for the first time the anisotropy of Ca²⁺ conduction and strongly supported the preferential conduction in the 〈101〉 direction predicted in the literature by the bond valence method.