The crystal and magnetic structures of Ba₃NiRu₂O₉, Ba₃CoRu₂O₉, and Ba₃ZnRu₂O₉

The crystal structures of the 6H perovskites Ba₃NiRu₂O₉, and Ba₃CoRu₂O₉ have been refined from neutron powder diffraction data collected at 5 or 2 K. Ba₃NiRu₂O₉ and Ba₃ZnRu₂O₉ have hexagonal symmetry, space group P6₃ mmc: a_Ni = 5.7256(2). c_Ni = 14.0596(2) Å; a_Zn = 5.7549(2), c_Zn = 14.1328(2) Å. Ba₃CoRu₂O₉ undergoes a phase transition from hexagonal symmetry at room temperature to orthorhombic symmetry at 2 K, space group Cmcm: a_Co = 5.7456(1), b_Co = 9.9177(2), c_Co = 14.0862(3) Å. In all three compounds the cation sites within the face-sharing octahedra of the 6H structure are occupied by Ru⁵⁺; the corner-sharing octahedra are occupied by Zn²⁺, Ni²⁺, or Co²⁺. The neutron diffraction data show that Ba₃NiRu₂O₉ is magnetically ordered at 5 K, with antiferromagnetic coupling between Ru⁵⁺ ions in the Ru₂O₉ dimers and ferromagnetic coupling along approximately linear Ni²⁺ORu⁵⁺ superexchange pathways. The magnetic moments align along z. Ba₃CoRu₂O₉ is also magnetically ordered with antiferromagnetic coupling in the Ru₂O₉ dimers, but in this case antiferromagnetic superexchange between Co²⁺ ions leads to a more complex magnetic structure.