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The A-site-ordered cubic perovskite LaCu3Cr4O12, where a partial Cu-Cr intersite charge transfer transition occurs at TCT=220K, was found to be an unconventional metallic and G-type antiferromagnetic oxide. Neutron powder diffraction revealed a G-type antiferromagnetic ordering at the Cr sites and no ordered moments at the Cu sites. Ab initio electronic structure calculations revealed that the narrowing of the Cr-O-Cr bands due to heavy tilting of the CrO6 octahedra and the strong hybridization of the Cu-3d, Cr-3d, and O-2p orbitals near the Fermi level give an unusual electronic structure in the vicinity of a localized-electron regime. The G-type magnetic structure is primarily stabilized by nearest-neighbor antiferromagnetic superexchange interactions of the near-localized Cr spins. The ferromagnetic {111} layers of the G-type antiferromagnetic Cr-spin sublattice allow the spin-polarized electron transfer through the strongly hybridized Cu orbitals. [ABSTRACT FROM AUTHOR]