Discovery of Fast Calcium-Ion Conduction in Grossite-Type Compounds CaAl₄O₇ and CaGa₄O₇ by Bond Valence Screening Method

Bond valence method is a powerful technique for exploring new ion conductors. Using this technique, we have discovered that Ca ions conduct one-dimensionally along the ⟨101⟩ directions in grossite-type compounds CaAl4O7 and CaGa4O7. The randomly grain-oriented polycrystals were prepared to evaluate the ion conductivity. The transference number at 1073 K was 0.973 for CaAl4O7 and 0.958 for CaGa4O7. With increasing temperature from 723 to 1073 K, the bulk conductivity of CaGa4O7 steadily increased from 1.26 × 10–7 to 4.40 × 10–5 S cm–1, which was 2.7–4.0 times higher than that of CaAl4O7 when compared at the same temperatures. These conductivities were compared with those of NASICON-type compounds in which Ca ions conduct in three dimensions. The total conductivities were superior to that of CaZr4(PO4)6, while the bulk conductivities were inferior to that of (Ca0.05Hf0.9)4/3.9Nb(PO4)3. The grossite polycrystals with the constituent grains aligned in the conduction direction and/or grossite single crystals cut in the relevant orientations can be expected to further improve the conductivity.