Spin orders and lattice distortions of geometrically frustrated 6H-perovskites Ba sub(3) B' Ru sub(2) O sub(9)[B' = La super(3+), Nd super(3+), and Y super(3+)

The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the B' = La and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the B' cation. An anomalous 120 K structural transition in Ba sub(3) NdRu sub(2)O sub(9) is attributed to Jahn-Teller distortion, evidenced by large-amplitude [Gamma] sub(6) deformations of the NdO sub(6) octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba sub(3) NdRu sub(2) O sub(9). Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and [1/3]-magnetization steps in the magnetic hysteresis loop.