MICROSTRUCTURE AND MAGNETIC-PROPERTIES OF R2BANIO5 (R=SM, GD, YB AND LU)

The magnetic behavior found for R2BaNiO5 (R = Sm, Gd, Yb and Lu) compo unds has been explained by taking into account the two structural type s in which they crystallize. For both Sm and Gd isostructural oxides a bove room temperature, the nickel ions are 1D-antiferromagnetically or dered along the chains of octahedra characteristic of the Nd2BaNiO5 st ructure type. Below 30 K both rare earth and nickel sublattices are si multaneously 3D-ordered. In the case of Yb and Lu oxides, belonging to the SM2BaCUO5 structure type, the existence of isolated distorted squ are pyramids (NiO5) justifies the assumption that antiferromagnetic in teractions in the nickel sublattice are not operative until the temper ature is as low as 10 K. Moreover, no evidence of order in the ytterbi um compound has been found down to a temperature of 4.2 K. On the othe r hand, high-resolution electron microscopy reveals that both types of material are well ordered, without any local structural defects.