Spin-assisted covalent bond mechanism in “charge-ordering” perovskite oxides

First-principles density functional calculations on the metal-insulator transition (MIT) in perovskite CaFeO${}_3$ point to local ferromagnetic coupling as the microscopic origin for the electronic “charge order” transition. Our atomic, electronic, and magnetic structure analyses reveal that the MIT results from a spin-assisted covalent bonding mechanism between the O 2$p$ and Fe 3$d$ states with anisotropic Fe-O bonds and negligible intersite Fe-Fe charge transfer. We suggest that control of the lattice distortions, which mediate the covalent bond formation, in oxides containing late transition-metal row cations in high valence states provides a platform to tailor electronic transitions.

Figure 1

Monoclinic CaFeO${}_3$ structure (a) with three crystallographically distinct oxygen sites, designated by different shading, coordinate Fe(1) and Fe(2), forming small and large octahedra, respectively. Schematic illustration of the collinear (b) $A$-type and (c) $C$-type AFM spin arrangements.

Figure 2

Orbital and atom-resolved densities of states (DOS) for ferromagnetic CaFeO${}_3$ with $Pbnm$ (a) and $P{2}_1/n$ (b) symmetry. AFM order in $P{2}_1/n$ (c) destroys the gap at $E_F$ (0.0 eV).

Figure 3

Schematic change in band structure without (inset) and with spin polarization in cubic CFO (a). FM order provides half metallicity and enables the FeO${}_6$ breathing distortion to mediate cross-gap covalent bond formation (b) primarily between the majority spin Fe(1) $e_g$ and O $2p$ states (arrowed), opening an electronic gap in the $P{2}_1/n$ structure. The color gradient (red, $t_{2g}$; blue, $e_g$) describes how the character of the states changes across the energy range. Minority $e_g$ states are omitted for clarity.

Figure 4

Majority spin MLWF isosurfaces for (a) $A$-type AFM and (b) FM $P{2}_1/n$ CaFeO${}_3$ on each Fe site reveals the MLWF spread along the equatorial Fe-O bond is greater in (b) than in (a), consistent with enhanced spin-assisted bond covalency.