Ba₃₃Zn₂₂Al₈O₆₇ with a Framework of ([O(Zn/Al)₄]/Ba)(Zn/AlO₄)₄ Motifs

Single crystals of a new oxide, Ba₃₃Zn₂₂Al₈O₆₇ (melting point = 1452 K), were grown in a melt-solidified sample prepared by heating a compact of a BaCO₃, ZnO, and Al₂O₃ mixed powder in a dry airflow. Ba₃₃Zn₂₂Al₈O₆₇ can be handled in dry air, but it decomposes into carbonates, hydroxides, and hydrates in humid air. Single-crystal X-ray structure analysis clarified that Ba₃₃Zn₂₂Al₈O₆₇ crystallizes in a cubic cell (a = 16.3328 (3) Å, space group F23) having a three-dimensional Zn/AlO₄ framework in which {([OZn₄]/Ba)­(Zn/AlO₄)₄} motifs are connected to each other by bridging Zn/AlO₄ tetrahedra. A Ba atom or a [OZn₄] cluster is statistically situated at the center of the motif with a probability of 0.5. Motifs of another type, {([O­(Zn/Al)₄])­(Zn/AlO₄)₄}, are isolated from the Zn/AlO₄ framework. These motifs, {([OZn₄]/Ba)­(Zn/AlO₄)₄} and {([O­(Zn/Al)₄])­(Zn/AlO₄)₄}, are alternatingly arranged along the a axis like a checkered cube, and Ba atoms are situated between the motifs. A linear thermal expansion coefficient of 10.4 × 10^–6 K^–1 was measured in an Ar gas flow at 301–873 K for a sintered Ba₃₃Zn₂₂Al₈O₆₇ polycrystalline sample with a relative density of 73%. A relative permittivity of 31 and a temperature coefficient of 15 ppm K^–1 at 301 K were obtained for another sintered sample (relative density = 70%) in a dry airflow. The electrical conductivity at 1073 K and the activation energy for conduction at 923–1073 K measured for the sintered samples in dry and wet airflows were 6.2 × 10^–7 S cm^–1 and 0.65 eV and 2.9 × 10^–6 S cm^–1 and 0.59 eV, respectively.