Strain and Spin-Orbit Coupling Induced Orbital-Ordering in Mott Insulator BaCrO3

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Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite BaCrO₃ with a formal 3d² configuration, the volume of which is apparently substantially enhanced by a strain due to SrTiO₃ substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects leads to a metal-insulator transition and in-plane zigzag orbital-ordering (OO) of alternating singly filled dxz+idyz and dxz-idyz orbitals, which results in a large orbital moment Mₗ -0.78 μB antialigned to the spin moment Mₛ 2|Mₗ| in Cr ions. Remarkably, this ordering also induces a considerable Mₗ for apical oxygens. Our findings show metal-insulator and OO transitions, driven by an interplay among strain, correlation, and SOC, which is uncommon in 3d systems.