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.