We present the first-principles investigation of the structural, electronic, and magnetic properties of ${\mathrm{SrCoO}}_{3-\delta }$ ($\delta =0,0.25,0.5$) to understand the multivalent nature of Co ions in ${\mathrm{SrCoO}}_{3-\delta }$ along the line of topotactic transition between perovskite ${\mathrm{SrCoO}}_3$ and brownmillerite ${\mathrm{SrCoO}}_{2.5}$. From the onsite Coulomb interaction $U$-dependent ground state of stoichiometric ${\mathrm{SrCoO}}_3$, we show the proximity of its metallic ferromagnetic ground state to other antiferromagnetic states. The structural and magnetic properties of ${\mathrm{SrCoO}}_{3-\delta }$ 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 be understood by the change of $pd$ hybridization depending on the Co-Co distances. The strong suppression of the $dp\sigma$ hybridization between Co $d$ and O $p$ orbitals in brownmillerite ${\mathrm{SrCoO}}_{2.5}$ brings on the high-spin state of ${\mathrm{Co}}^{3+}{d}^6$ 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 ${\mathrm{SrCoO}}_{3-\delta }$ 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 ${\mathrm{SrCoO}}_{3-\delta }$. Incidentally, we also find that the I2mb symmetry of ${\mathrm{SrCoO}}_{2.5}$ is energetically stable and exhibits ferroelectricity via the ordering of ${\mathrm{CoO}}_4$ tetrahedra, where this polar lattice can be stabilized by the presence of a large activation barrier.

• Received 24 May 2018
• Revised 17 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.085106