Neutron diffraction evidence for a cationic order in the REMn0.5Ni0.5O3 (RE = La, Nd) and YMn0.5Co0.5O3 perovskites

The REMn0.5Ni0.5O3 (RE = La, Nd) and YMn0.5Co0.5O3 compounds, prepared by solid-state reaction under air, were investigated by X-ray diffraction (XRD) and neutron powder diffraction (NPD). The La-compound contains some lanthanum vacancies decreasing the lattice parameters and increasing the crystal distortion. Nearly stoichiometric compositions were found for the other compounds prepared under similar conditions. The NPD data of all compounds indicate some ordering of the mixed transition ions over the octahedral site, since the crystal structure is better described in monoclinic space group P2(1)/n that accounts for two distinguishable octahedral sites, rather than in orthorhombic Pbnm. Such results contrast with those obtained from X-ray diffraction where the mixed ions appear to be randomly distributed over the only octahedral site of the Pbnm space group. The low-temperature NPD patterns of YMn0.5Co0.5O3 exhibit some magnetic peaks, below 70 K. The magnetic structure at 1.4 K consists of two collinear ferromagnetic sublayers; with saturated magnetic moments of 3.26(2) mu(B) and 2.14(2) mu(B) per ion, at the 2c- and 2d-site, respectively. The low-temperature NPD patterns of LaMn0.5Ni0.5O3 and NdMn0.5Ni0.5O3 exhibit an obvious increase in intensity over the only nuclear peak, at 20 similar to 36.89 degrees and 36.40 degrees, respectively. Although such behavior indicates a ferromagnetic ordering, the magnetic structure could not be refined because of the faint magnetic properties, more likely due to the incomplete atomic ordering. (C) 2007 Elsevier Ltd. All rights reserved.