Octahedral Engineering of Orbital Polarizations in Charge Transfer Oxides

Abstract

Negative charge transfer $AB$O${}_3$ 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${}^{\circ }$. Using density functional theory with the PBEsol$+U$ approach, we describe a novel octahedral engineering avenue to control the $B$ 3d and O $2p$ orbital polarization through enhancement of the $B$O${}_6$ rotation "sense" rather than solely through conventional changes to the $B$--O bond lengths, \emph{i.e.} crystal field distortions. Using CaFeO${}_3$ 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.

6 pages, 5 figures

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• Metal-insulator transition and local-moment collapse in negative charge-transfer CaFeO${}_3$ under pressure
• Inverted orbital polarization in strained correlated oxide films
• Structure, strain, and control of ground state property in LaTiO${}_3$/LaAlO${}_3$ superlattice