Crystal structures and magnetic properties of the 6H-perovskites Ba3LnRu2O9 (Ln=Ce, Pr and Tb)

Magnetic properties of quaternary oxides Ba(3)LnRu(2)O(9) (Ln = Ce, Pr and Tb) are reported. These compounds adopt the 6H-perovskite structure with space group P6(3)/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 Ln(4+) 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) mu (B).