Crystal Structure and Electronic Properties of Bulk and Thin Film Brownmillerite Oxides

TL;DR

This study uses electronic structure calculations to analyze the stability and electronic properties of brownmillerite oxides, revealing how strain and structural factors influence their potential as functional electronic materials.

Contribution

It identifies key structural descriptors affecting stability and electronic properties of brownmillerite oxides, providing insights for designing novel thin film electronic materials.

Findings

Bulk structure is retained under strain but tetrahedral chain orientation can change.

Strain significantly affects the electronic band gap and vacancy alignment.

Intralayer separation of tetrahedral chains influences ground state stability.

Abstract

The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of $B$O$_6$ octahedra and $B$O$_4$ tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr$_2$Fe$_2$O$_5$ and Ca$_2$Fe$_2$O$_5$, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and newly herein proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the `dipoles' generated from rotations of the FeO$_4$ tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of novel functional electronic materials in thin film geometries.