Synthesis, structural study and magnetic properties of TbFeMnO5

We describe the preparation and characterization of a new ferrimagnetic oxide of composition TbFeMnO5. It has been prepared by a soft-chemistry procedure, involving the thermal treatment at moderate temperatures (600 °C) of citrate precursors, subsequently annealed at 900 °C under high oxygen pressure (200 bar). The crystallographic and magnetic structures have been studied by neutron powder diffraction (NPD) experiments, in complement with magnetization and susceptibility measurements. TbFeMnO5 is an orthorhombic, Pbam space group; the crystal structure contains infinite chains of Mn4+O6 edge-sharing octahedra running along the cc axis and interconnected by Fe3+O5 pyramids and TbO8 units. Some anti-site disorder is observed from the NPD refinements: about 9% of the 4f4f octahedral sites are occupied by Fe3+ cations and about 18% of the pyramidal 4h4h positions contain Mn4+. These structural features are confirmed by Mössbauer spectroscopy: two sites for Fe3+ are identified; one of them corresponds to almost regular octahedra (at 4f4f positions), characterized by nearly equal Mn/Fe–O distances of 1.906Å at RT (from NPD data), giving a quadrupole doublet in the Mössbauer spectra at RT; the second contributes to a more intensive doublet corresponding to a distorted square pyramid Fe3+O5 (at 4h4h sites), for which NPD data demonstrates an axial distortion with three sets of Mn/Fe–O distances at 1.981(4), 1.851(5) and 1.944(3) Å. Both doublets are broadened by the Fe/Mn disorder over these sites. TbFeMnO5 undergoes long-range magnetic ordering below TC≈175K, as shown from magnetization, NPD and Mössbauer spectroscopy data. The propagation vector k=0 defines a ferrimagnetic structure that initially concerns the Mn4+ and Fe3+ cations, in a magnetic arrangement defined by the basis vectors (0,0,Fz)(0,0,Fz) and (0,0,Fz′) respectively. The Tb3+ sublattice is ordered at 2 K with a spin arrangement given by (0,0,Fz″). The refined magnetic moments at this temperature are 2.66(12)μB for the Mn4+, −0.8(2)μB for the Fe3+ and −2.0(3)μB for the Tb3+ cations.