Crystal structure and magnetism of YbFeMnO5: A neutron diffraction and Mössbauer spectroscopy study

We have studied the crystal structure and magnetic properties of Y bFeMnO5 obtained by substituting Fe3+ for Mn3+ in the parent Y bMn2O5 compound, through x-ray (XRD) and neutron (NPD) powder diffraction, magnetometry and Mössbauer spectroscopy. The samples were prepared in polycrystalline form by a soft chemistry route, followed by thermal treatments under high-oxygen pressure. The Rietveld analysis of diffraction data shows that Y bFeMnO5 is isostructural with the oxides of stoichiometry RMn2O5 (R=rare earth, Y or Bi); the crystal structure is orthorhombic, Pbam space group, formed by chains of edge-sharing Mn4+O6 octahedra linked together by dimer groups of square pyramids Fe3+O5 and Y b3+O8 scalenohedra.A low level of disorder was established between the two transition metal positions 4f and 4h, occupied ideally by Mn4+ and by Fe3+: about 6% of Mn cations is replaced by Fe and 16% of Fe by Mn. Mössbauer spectroscopy data confirm the existence of two distinct crystallographic sites for Fe3+. One of them corresponds to almost regular octahedra (at 4ff positions), characterized by nearly equal Mn/Fe–O distances of 1.890 Å at RTRT (from NPD data), giving a quadrupole doublet in the Mössbauer spectra at RTRT, broadened by the Fe/Mn disorder over this site. The second environment for Fe3+ contributes to a less broadened, but more intensive doublet in the Mössbauer spectra, which corresponds to a distorted square pyramid Fe3+O5 (at 4h4h sites), for which NPD data demonstrates an axial distortion with three sets of Fe–O distances at 2.010(2) Å, 1.859(5) Å and 1.925(3) Å.Magnetic studies and the thermal evolution of the NPD patterns show that below a transition temperature Tc∼178K a long-range magnetic order is developed, resolved from NPD data as a ferrimagnetic structure with propagation vector k=0. The spin arrangements for the Mn4+ ions (4f4f site) and Fe3+ ions (4h4h site) are given by the basis vectors (0,0,Fz0,0,Fz) and (0,0,Fz′), respectively. The Mn and Fe ordered magnetic moments are oriented along the cc axis. At lower temperatures, the magnetic moment of the Y b3+ cation also participates in the magnetic structure, adopting a parallel arrangement with the Mn4+ spins; at T=1.8 K the magnitudes of the magnetic moments of the effective scatterers are 2.00(8), −3.04(17) and 0.35(14) μB, over the 4f4f, 4h4h and 4g4g sites respectively.