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Abstract
We show that the NiO6 crystal field energies can be tailored indirectly via heterovalent A cation ordering in layered (La,A)NiO4 Ruddlesden–Popper (RP) oxides, where A = Sr, Ca, or Ba, using density functional calculations. We leverage as a driving force the electrostatic interactions between charged [LaO]1 + and neutral [AO]0 planes to inductively tune the Ni–O bond distortions, without intentional doping or epitaxial strain, altering the correlated d-orbital energies. We use this strategy to design cation ordered LaCaNiO4 and LaBaNiO4 with distortions favoring enhanced Ni eg orbital polarization, and find local electronic structure signatures analogous to those in RP La-cuprates, i.e., parent phases of the high-temperature superconducting oxides.
Highlights
We introduce reduced parameters λ and ξ, which have characteristic lengths that capture the proximity of the [AO]1 + and [A O]0 monoxides providing the electrostatic chemical strain (ECS) effect
The d3z2−r2 orbitals are stabilized relative to the dx2−y2 orbitals ( > 0), and the magnitude of the eg splitting increases with d /d⊥. We suggest that these changes in the electronic structure observed as a function of η could be experimentally detected using polarized Ni K-edge X-ray absorption spectroscopy.[33]
Nickelates, the interlayer structure and Ni crystal field can be tailored without resorting to intentional doping, epitaxial strain, or complex heterostructuring
Summary
(between the monoxide layers) or ξ = 2 for [SrO|NiO2|LaO] (across the perovskite-block). We take ξ to be the minimum value, because the ECS from the nearest-neighbor monoxide layers should be relatively stronger than that across the perovskite block, where screening of dipoles by the NiO2 plane would obfuscate the analysis. Large η is synonymous with maximum electrostatic repulsions from the monoxide layers, producing oxygen ligand displacements (changes in d /d⊥) and directing the crystal field. The electronic configuration of the M cation (here Ni) will influence the structural response and electronic functionality—we explore this contribution explicitly later. Any changes in atomic structure owing to the +U correction are described later; we note here that it weakly modifies the electronic structure of (La,A)NiO4 (A = Sr, Ba), whereas a one- to two-band metal–metal transition is found in
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