Enhancement of the Curie Temperature along the Perovskite Series RCu3Mn4O12 Driven by Chemical Pressure of R3+ Cations (R = Rare Earths)

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Abstract
The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 °C in the presence of KClO4 as oxidizing agent. The crystal structures are cubic, space group Im3̅ (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) Å (R = La) to a = 7.2409(1) Å (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca2+ with R3+ cations in the parent CaCu3Mn4O12 oxide leads to a substantial increment of the ferrimagnetic Curie temperature (TC). An essential ingredient is supplied by the internal pressure of the R3+ cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO6 octahedral units for the small rare earths provides progressively shorter Mn−O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing TC by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO6 octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t−e orbital hybridization, for the observed increment of TC.

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