FeMo double perovskite: From small clusters to bulk material

To understand the differences in behaviour between up- and down-spin electrons observed in the half-metallic Sr₂FeMoO₆ double perovskite, the density of states (DOS) was studied for the (FeO ₆)^-4 and (MoO₆)^-6 octahedral clusters using first-principles density functional theory within the generalised gradient approximation (GGA) scheme and the Perdew-Burke-Ernzerhof (PBE) functional. Our results reveal that half-metallic character is present, even starting from an isolated (FeO₆)^-4 cluster, and is a consequence of spin decoupling of antibonding hybridisations between iron t _2g states and oxygen p states (t2ga states), i.e., t2ga states lie below the Highest Occupied Molecular Orbital (HOMO) in the up-spin channel, whereas they lie above the HOMO level in the down-spin channel. The spin-induced shifting between up-spin and down-spin DOS situates the HOMO in such a way that the molecular orbitals oxygen p states (p bands) are fully spin-paired by octet electrons. Thus, the down-spin channel has metallic character because the HOMO lies just at the p bands, and the up-spin channel is semiconducting because the HOMO falls within the energy gap between the t2ga and ega bands. Finally, the (MoO₆)^-6 octahedron does not inhibit the perovskite half-metallic character since this cluster has a zero total spin.