Evidence of low-dimensional antiferromagnetic ordering and crystal structure in theR2BaNiO5(R=Y,Er) oxides

Crystals of ${\mathit{R}}_2$ ${\mathrm{BaNiO}}_5$ (R=Y,Er) have been grown, and their structures have been established by single-crystal x-ray diffraction. Both compounds crystallize in the ${\mathrm{Nd}}_2$ ${\mathrm{BaNiO}}_5$ structure type, with one-dimensional chains of vertex-sharing ${\mathrm{NiO}}_6$ 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 Å, 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 Å 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 ${\mathrm{Ni}}^{2+}$ ions in polycrystalline ${\mathrm{Y}}_2$ ${\mathrm{BaNiO}}_5$ 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 ${\mathrm{Y}}_2$ ${\mathrm{BaNiO}}_5$, higher than that reported for ${\mathrm{Y}}_2$ ${\mathrm{BaCuO}}_5$, is explained by the promotion of the superexchange Ni-O-Ni interactions along the chains of flattened ${\mathrm{NiO}}_6$ octahedra sharing corners. In ${\mathrm{Er}}_2$ ${\mathrm{BaNiO}}_5$ both effects are masked by the strong paramagnetic signal of ${\mathrm{Er}}^{3+}$, and a maximum observed at 15.6 K for the susceptibility is attributed to tridimensional ordering of the ${\mathrm{Er}}^{3+}$ cations.