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

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journal contribution
posted on 21.06.2010, 00:00 by Javier Sánchez-Benítez, José Antonio Alonso, María Jesús Martínez-Lope, Alicia de Andrés, María Teresa Fernández-Díaz
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 te orbital hybridization, for the observed increment of TC.