Coexistence of Localized and Itinerant Electrons in the Double‐Perovskite Ba 2 Fe 2/3 Mo 4/3 O 6

A polycrystalline Ba 2 Fe 2/3 Mo 4/3 O 6 double‐perovskite has been prepared by decomposition of citrate precursors and subsequent thermal treatment under a reducing atmosphere. The crystal structure has been studied by combined X‐ray and neutron powder diffraction (XRPD and NPD). At room temperature, the structure is cubic ( Fm $\bar {3}$ m ) with lattice parameter a = 8.0710(1) Å. It is unchanged between 3 and 320 K. The crystallographic formula is Ba 2 [Fe 0.52(2) Mo 0.48(2) ] 4 a [Fe 0.14(2) Mo 0.86(2) ] 4 b O 5.9(2) . NPD, electron spin resonance (ESR), and magnetization measurements show spontaneous magnetization below the Curie temperature ( T C ) = 310 K. The ESR behavior is associated with the presence of localized Fe 3+ ions, whereas the transport properties (electrical conductivity and Seebeck effect) suggest the presence of highly correlated electrons in a metallic band with disorder, which can be associated with the Mo t 2g electrons. The coexistence of localized and itinerant electrons leads to non‐negligible magnetoresistance properties.