Role of oxygen vacancy in the spin-state change and magnetic ordering in SrCoO$_{3-\delta}$

TL;DR

This study uses first-principles calculations to explore how oxygen vacancies influence the spin states and magnetic orderings in SrCoO$_{3- ext{ extdelta}}$, revealing the role of hybridization and structural changes in electronic transitions.

Contribution

It provides the first detailed theoretical analysis of the structural, electronic, and magnetic properties across different oxygen contents in SrCoO$_{3- ext{ extdelta}}$, highlighting the impact of oxygen vacancies.

Findings

Oxygen vacancies increase Co-Co distances, reducing hybridization.

Brownmillerite SrCoO$_{2.5}$ exhibits high-spin Co$^{3+}$ and antiferromagnetic insulating state.

SrCoO$_{2.5}$ has stable I2mb symmetry and exhibits ferroelectricity.

Abstract

We present the first-principles investigation of the structural, electronic, and magnetic properties of SrCoO$_{3-\delta}$ ($\delta=0, 0.25, 0.5$) to understand the multivalent nature of Co ions in SrCoO$_{3-\delta}$ along the line of topotactic transition between perovskite SrCoO$_{3}$ and brownmillerite SrCoO$_{2.5}$. From the on-site Coulomb interaction $U$-dependent ground state of stoichiometric SrCoO$_{3}$, we show the proximity of its metallic ferromagnetic ground state to other antiferromagnetic states. The structural and magnetic properties of SrCoO$_{3-\delta}$ depending on their oxygen-content provide an interesting insight into the relationship between the Co-Co distances and the magnetic couplings so that the spin-state transition of Co spins can understood by the change of $pd$-hybridization depending on the Co-Co distances. The \emph{strong} suppression of the $dp\sigma$-hybridization between Co $d$ and O $p$ orbitals in brownmillerite SrCoO$_{2.5}$ brings on the high-spin state of Co$^{3+}$ $d^{6}$ and is responsible for the antiferromagnetically ordered insulating ground state. The increase of effective Co-Co distances driven by the presence of oxygen vacancies in SrCoO$_{3-\delta}$ is consistent with the reduction of the effective $pd$-hybridization between Co $d$ and O $p$ orbitals. We conclude that the configuration of neighboring Co spins is shown to be crucial to their local electronic structure near the metal-to-insulator transition along the line of the topotactic transition in SrCoO$_{3-\delta}$. Incidentally, we also find that the \textit{I2mb} symmetry of SrCoO$_{2.5}$ is energetically stable and exhibits ferroelectricity via the ordering of CoO$_{4}$ tetrahedra, where this polar lattice can be stabilized by the presence of a large activation barrier.