Octahedral engineering of orbital polarizations in charge transfer oxides

Negative charge transfer ABO₃ oxides may undergo electronic metal-insulator transitions (MIT) concomitant with a dilation and contraction of nearly rigid octahedra. On both sides of the MIT are in-phase or out-of-phase (or both) rotations of adjacent octahedra that buckle the B-O-B bond angle away from 180^°. Using density functional theory with the PBEsol +U approach, we describe an octahedral engineering avenue to control the B 3d and O 2p orbital polarization through enhancement of the BO₆ rotation "sense" rather than solely through conventional changes to the B-O bond lengths, i.e., crystal field distortions. Using CaFeO₃ as a prototypical material, we show the flavor of the octahedral rotation pattern when combined with strain-rotation coupling and thin film engineering strategies offers a promising avenue to fine tune orbital polarizations near electronic phase boundaries.