Abstract
New complex perovskites of the series CaCu3(Mn4−xRex)O12 have been prepared from citrate precursors under moderate pressure conditions of 2 GPa and 1000 °C, in the presence of KClO4 as oxidizing agent to stabilize Mn3+/Mn4+ mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. The crystal structures are cubic, space group Im-3. The unit-cell parameters increase from a = 7.2379(2) Å for the parent (x = 0) compound to a = 7.2420(4) Å for CaCu3(Mn3.9Re0.1)O12. Both oxides adopt a superstructure of the perovskites ABO3 with long-range 1:3 ordering for Ca2+ and Cu2+ ions at the A sublattice. For the compound doped with Re (x = 0.1), the result of a NPD study shows that Re6+ ions are randomly located at the octahedral positions, being the (Mn,Re)O6 octahedra strongly tilted, with superexchange (Mn,Re)-O-(Mn,Re) angles of 142.03°. Neutron diffraction data clearly show that some Mn3+ ions are located together with Cu2+ at the square-planar 6b positions. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Mn,Re)8c spins at octahedral positions and weak antiferromagnetism with (Cu2+,Mn3+)6b spins. Interestingly, an enhancement of the magnetoresistance effect is observed for the Re-doped compound, well beyond that found for the parent perovskite.
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