The crystal structure of Ba₃CuRu₂O₉ and comparison to Ba₃MRu₂O₉ (M=In, Co, Ni, and Fe)

The crystal structure of Ba₃CuRu₂O₉, determined by powder neutron diffraction, is reported for temperatures of 18-295 K. Structural refinements by powder neutron diffraction for the previously reported compounds Ba₃MRu₂O₉ for M=Fe, Co, Ni, and In are also reported. All structures consist of pairs of face-shared RuO₆ octahedra interconnected by corner-sharing MO₆ octahedra. For Ba₃CuRu₂O₉, Jahn-Teller distortion of the CuO6 octahedra leads to an orthorhombic symmetry structure of space group Cmcm with a=5.6723(1), b=10.1722(3), and c=14.1575(3) Å at 295 K. For Ba₃FeRu₂O₉, site occupancy disorder is found. Magnetic ordering is observed below 100, 105, and 110 K for M=Cu, Ni, and Co, respectively. The magnetic structures at low temperatures for M=Co and Ni are described and compared to previous reports.