Abstract

Magnetic properties of quaternary oxides Ba₃LnRu₂O₉ (Ln = Ce, Pr and Tb) are reported. These compounds adopt the 6H-perovskite structure with space group P6₃/mmc, in which the cation sites within the face-sharing octahedra are occupied by Ru ions and those within the corner-sharing octahedra are occupied by Ln ions. The oxidation states of both Ru and Ln ions were found to be tetravalent. These compounds are semiconductors over the temperature range 100–400 K. Measurements of the magnetic susceptibility and specific heat were carried out. The Ln⁴⁺ antiferromagnetic transition occurred at 9.5 K for Ln = Tb and 10.5 K for Ln = Pr. Powder neutron diffraction measurements for Ba₃TbRu₂O₉ were performed at 2 and 15 K, and at room temperature. It was found that Ba₃TbRu₂O₉ has a long range antiferromagnetic ordering of Tb⁴⁺ ions at 2 K. The moments of the Tb⁴⁺ ions order ferromagnetically in the c plane, and these ferromagnetic planes are stacked antiferromagnetically along the c axis. The direction of the ordered moments is parallel to the c axis, and the ordered magnetic moment of the Tb⁴⁺ ions is 6.84(4) μ_B.