Oxygen atoms and molecules at Lanthanum-Strontium Manganite surfaces

TL;DR

This paper develops a localized electronic structure model for oxygen interactions with LaSrMnO3 surfaces, incorporating effects beyond standard approximations, to better understand oxygen binding, vacancy diffusion, and surface chemistry.

Contribution

It introduces a simplified yet effective method to include van-der-Waals interactions and beyond-LDA effects in modeling oxygen-surface interactions on manganite substrates.

Findings

Oxygen vacancies diffuse as neutral species.

Bound molecules avoid vacancies but can fill them by dropping an atom.

Surface interactions involve complex electronic and phonon effects.

Abstract

A localized description, rather than energy bands, is appropriate for the manganite substrate. Empty substrate levels lower in energy than occupied oxygen levels indicate need for further terms beyond the Local Density Approximation. So also does van-der-Waals interaction between the two. Methods to include both are suggested by related, exactly soluble, two-electron problems. The descriptions of the electronic structure of the molecule and a LaSrMnO3 (LSM) substrate are greatly simplified to allow incorporation of these effects and to treat a range of problems involving the interactions between oxygen atoms, or oxygen molecules, and such a substrate. These include elastic impacts, impacts with electronic transitions, and impacts with phonon excitation. They provide for capture of the atoms or molecules by the surface, leaving the neutral molecule strongly bound over a Mn(4+) site. It is found that oxygen vacancies in LSM diffuse as a neutral species, and can appear at the surface. Bound molecules tend to avoid sites next to vacancies but, if there, should drop one atom into the vacancy leaving the remaining triplet oxygen atom bound to the resulting ideal surface, with no need for spin flips nor successive ionization steps.