Magnetic and electrical properties of quadruple perovskites with 12 layer structures Ba4LnM3O12 (Ln=rare earths; M=Ru, Ir): The role of metal-metal bonding in perovskite-related oxides

Structures and magnetic and electrical properties of quadruple perovskites containing rare earths Ba(4)LnM(3)O(12) (Ln = rare earths; M=Ru, Ir) were investigated. They crystallize in the 12L-perovskite-type structure. Three MO6 octahedra are connected to each other by face-sharing and form a M3O12 trimer. The M3O12 trimers and LnO(6) octahedra are alternately linked by corner-sharing, forming the perovskite-type structure with 12 layers. For Ln:=Ce, Pr, and Tb, both the Ln and M ions are in the tetravalent state (Ba(4)Ln(4+)M(3)(4+)O(12)), and for other Ln ions, Ln ions are in the trivalent state and the mean oxidation state of M ions is +4.33 (Ba(4)Ln(3+)M(3)(4.33+)O(12)). All the Ba(4)Ln(3+)Ru(3)(4.33+)O(12) compounds show magnetic ordering at low temperatures, while any of the corresponding iridium-containing compounds Ba(4)Ln(3+)Ir(3)(4.33)O(12) is paramagnetic down to 1.8K. Ba4Ce4+Ir34+O12 orders antiferromagnetically at 10.5 K, while the corresponding ruthenium-containing compound Ba4Ce4+Ru34+O12 is paramagnetic. These magnetic results were well understood by the magnetic behavior of M3O12. The effective magnetic moments and the entropy change for the magnetic ordering show that the trimers Ru34.33+O12 and Ir34+O12 have the S=1/2 ground state, and in other cases there is no magnetic contribution from the trimers Ru34+O12 or Ir34.33+O12. Measurements of the electrical resistivity of Ba(4)LnM(3)O(12) and its analysis show that these compounds demonstrate two-dimensional Mott-variable range hopping behavior. (C) 2010 Elsevier Inc. All rights reserved.