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

To investigate the anisotropy of Ca 2+ conduction, the b-axis-aligned CaAl 4 O 7 (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 Ca 2+ conduction and strongly supported the preferential conduction in the <101> direction predicted in the literature by the bond valence method.