EVIDENCE OF LOW-DIMENSIONAL ANTIFERROMAGNETIC ORDERING AND CRYSTAL-STRUCTURE IN THE Y,ER-2BANIO5 OXIDES

Crystals of R2BaNiO5 (R=Y,Er) have been grown, and their structures have been established by single-crystal x-ray diffraction. Both compounds crystallize in the Nd2BaNiO5 structure type, with one-dimensional chains of vertex-sharing NiO6 octahedra in the direction of the a axis. These octahedra show an unusual twofold distortion: The Ni-O distances to the two axial oxygen atoms are considerably shorter, 0.3, than those to the four equatorial oxygens, and these oxygens are distorted from the right angles of a regular octahedron to 79.0(2)°or 77.7(6)°, respectively. As a result of this, Ni-O(axial)-Ni distances are very short, 3.76 and 3.75 for R=Y and Er, respectively. X-ray powder diffraction data and the results of magnetic measurements for both oxides are given. The structural features mentioned elucidate why Ni2+ ions in polycrystalline Y2BaNiO5 behave as a monodimensional system in which they become antiferromagnetically ordered below 300 K. Besides that, the ferromagnetic interactions that operate below 40 K can be due to tridimensional interchain interactions and/or the presence of ferromagnetic impurities. The estimated Néel temperature for Y2BaNiO5, higher than that reported for Y2BaCuO5, is explained by the promotion of the superexchange Ni-O-Ni interactions along the chains of flattened NiO6 octahedra sharing corners. In Er2BaNiO5 both effects are masked by the strong paramagnetic signal of Er3+, and a maximum observed at 15.6 K for the susceptibility is attributed to tridimensional ordering of the Er3+ cations. © 1990 The American Physical Society.