Octahedral Engineering of Orbital Polarizations in Charge Transfer Oxides

TL;DR

This paper introduces a novel method to control orbital polarizations in charge transfer oxides by engineering octahedral rotations, offering new ways to tune electronic phases in materials like CaFeO3.

Contribution

It demonstrates that octahedral rotation patterns can be used to manipulate orbital polarization, expanding beyond traditional bond length modifications.

Findings

Octahedral rotations influence orbital polarization in charge transfer oxides.

Strain and thin film engineering can fine-tune orbital states near phase boundaries.

The approach provides a new pathway for electronic phase control in 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.