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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]

B-site-disordered perovskite CaFe0.5Mn0.5O3 with unusually high valence Fe4+ was synthesized using a high-pressure technique. Fe4+ randomly distributed at half of the B sites shows charge disproportionation to Fe3+ and Fe5+. The spins of Fe3+,Fe5+, and Mn4+ order below 90 K. Analysis of low-temperature neutron powder-diffraction data revealed a G-type antiferromagnetic structure--where all the nearest-neighboring spins of Fe3+,Fe5+, and Mn4+ couple antiparallel--and the small ordered moment of 0.58μB reveals local charge ordering that gives rise to predominant Fe3+-O-Fe5+ antiferromagnetic arrangements. Despite the identical chemical compositions of CaFe0.5Mn0.5O3 and Ca2FeMnO6, the magnetic structure of the present CaFe0.5Mn0.5O3 is very different from the noncollinear one of layered B-site-ordered Ca2FeMnO6. [ABSTRACT FROM AUTHOR]

The coupling between spins and charge disproportionation states has been investigated in the LaCa2Fe3O9 oxide with neutron powder diffraction. This A -site layer ordered triple perovskite LaCa2Fe3O9 undergoes charge disproportionation on cooling and shows two different charge ordering patterns. At 230 K, Fe 3.67+ disproportionates into a 2:1 ratio of Fe3+: Fe5+, which order in a layered manner along the <010> direction of the pseudocubic unit cell. At lower temperatures(T< 170 K), the charge ordering pattern changes to a layered arrangement along the <111> direction. Neutron powder diffraction data show that in the intermediate temperature range(170 K layered magnetic structure. [ABSTRACT FROM AUTHOR]

The perovskite Ca0.5Bi0.5FeO3 undergoes a remarkable sequence of charge-disproportionation (CD) and charge-transfer (CT) transitions on cooling due to competing electronic instabilities: Ca0.5Bi3+0.5Fe3.5+O3→Ca0.5Bi3+0.5Fe3+0.67Fe4.5+0.33O3(CDphase)→Ca0.5Bi3+0.25Bi5+0.25Fe3+O3(CTphase). The accompanying changes in charge and spin ordering have been determined from neutron diffraction and physical property measurements. The CT phase adopts a simple G-type antiferromagnetic structure of Fe3+ spins but the CD phase adopts an unusual charge and magnetic arrangement in which Fe3+ spins are antiferromagnetically ordered but the Fe4.5+ moments have no long-range order due to magnetic frustration and form a spin glass at low temperatures. [ABSTRACT FROM AUTHOR]