Moderate-pressure Synthesis and Neutron Diffraction Study of New Metastable Oxides

We have synthesized two new series of metastable oxides, namely REMn2O5 and RECu3Mn4O12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn3+ by Fe3+ in the parent REMn2O5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn4+O6 octahedra connected via dimeric groups of Fe3+O5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below TC ≈ 165 K, characterized by the propagation vector k = 0. Immediately below TC, the Fe3+ and Mn4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe3+ spins. Some new derivatives of CaCu3Mn4O12 have been prepared at moderate pressures of 2 GPa by replacing Ca2+ by RE3+ cations in the series RECu3Mn4O12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to TC. The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu3Mn4O12, as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn3+/4+ and Cu2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.