Electronic structure study of double perovskites A₂FeReO₆ (A=Ba,Sr,Ca) and Sr₂MMoO₆ (M=Cr,Mn,Fe,Co) by LSDA and LSDA+U

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We have implemented a systematic LSDA and LSDA+U study of the double perovskites A₂FeReO₆ (A=Ba,Sr,Ca) and Sr₂MMoO₆ (M=Cr,Mn,Fe,Co) for understanding of their intriguing electronic and magnetic properties. The results suggest a ferrimagnetic (FiM) and half-metallic (HM) state of A₂FeReO₆ (A=Ba,Sr) due to a pdd-π coupling between the down-spin Re⁵⁺/Fe³⁺ t₂g orbitals via the intermediate O 2pπ ones, also a very similar FiM and HM state of Sr₂FeMoO₆. In contrast, a decreasing Fe t₂g component at Fermi level (EF) in the distorted Ca₂FeReO₆ partly accounts for its nonmetallic behavior, while a finite pdd-σ coupling between the down-spin Re⁵⁺/Fe³⁺ eg orbitals being present at EF serves to stabilize its FiM state. For Sr₂CrMoO₆ compared with Sr₂FeMoO₆, the coupling between the down-spin Mo⁵⁺/Cr³⁺ t₂g orbitals decreases as a noticeable shift up of the Cr³⁺ 3d levels, which is likely responsible for the decreasing TC value and weak conductivity. Moreover, the calculated level distributions indicate a Mn²⁺(Co²⁺)/Mo⁶⁺ ionic state in Sr₂MnMoO₆ (Sr₂CoMoO₆), in terms of which their antiferromagnetic insulating ground state can be interpreted. While orbital population analyses show that owing to strong intrinsic pd covalence effects, Sr₂MMoO₆ (M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts for the similar M-independent spectral features observed in them.