From Charge to Orbital Ordered Metal-Insulator Transition in Alkaline-Earth Ferrites

TL;DR

This paper uses first-principles simulations to elucidate the structural mechanisms behind the metal-insulator transition in CaFeO$_3$ and proposes ways to induce insulating phases in related ferrites through strain and oxygen rotation engineering.

Contribution

It reveals oxygen rotation motions as the structural trigger for the transition in CaFeO$_3$ and predicts methods to induce insulating phases in SrFeO$_3$ and CaFeO$_3$ thin films.

Findings

Oxygen rotations trigger the metal-insulator transition in CaFeO$_3$.

Strain can switch CaFeO$_3$ from charge- to orbital-ordered insulating states.

SrFeO$_3$ can potentially become insulating through oxygen rotation engineering.

Abstract

While CaFeO$_3$ exhibits upon cooling a metal-insulator transition linked to charge ordering, SrFeO$_3$ and BaFeO$_3$ keep metallic behaviors down to very low temperatures. Moreover, alkaline-earth ferrites do not seem prone to orbital ordering in spite of the d$^4$ formal occupancy of Fe$^{4+}$. Here, from first-principles simulations, we show that the metal-insulator transition of CaFeO$_3$ is structurally triggered by oxygen rotation motions as in rare-earth nickelates. This not only further clarifies why SrFeO$_3$ and BaFeO$_3$ remain metallic but allows us to predict that an insulating charge-ordered phase can be induced in SrFeO$_3$ from appropriate engineering of oxygen rotation motions. Going further, we unveil the possibility to switch from the usual charge-ordered to an orbital-ordered insulating ground state under moderate tensile strain in CaFeO$_3$ thin films. We rationalize the competition between charge and orbital orderings, highlighting alternative possible strategies to produce such a change of ground state, also relevant to manganite and nickelate compounds.