A Surface Reconstruction with a Fractional Hole: $(\sqrt{5}\times\sqrt{5}) R26.6^\circ$ LaAlO$_3$ (001)

TL;DR

This study reveals the atomic structure and electronic properties of a specific LaAlO3 surface reconstruction, showing a simple lanthanum oxide termination with fractional holes, charge compensation via delocalized holes, and no oxygen vacancies.

Contribution

The paper provides the first detailed structural and electronic characterization of the $(\sqrt{5}\times\sqrt{5}) R26.6^\circ$ LaAlO3 (001) surface reconstruction using transmission electron diffraction and density functional calculations.

Findings

Surface has a lanthanum oxide termination with one lanthanum vacancy per unit cell.

Charge neutrality is achieved through delocalized holes, not oxygen vacancies.

The reconstruction involves cation expulsion and increased covalency between surface atoms.

Abstract

The structure of the $(\sqrt{5}\times\sqrt{5})R26.6^\circ$ reconstruction of LaAlO$_3$ (001) has been determined using transmission electron diffraction combined with direct methods. The structure is relatively simple, consisting of a lanthanum oxide termination with one lanthanum cation vacancy per surface unit cell. The electronic structure is unusual since a fractional number of holes or atomic occupancies per surface unit cell are required to achieve charge neutrality. Density functional calculations indicate that the charge compensation mechanism occurs by means of highly delocalized holes. The surface contains no oxygen vacancies and with a better than 99% confidence level, the holes are not filled with hydrogen. The reconstruction can be understood in terms of expulsion of the more electropositive cation from the surface followed by an increased covalency between the remaining surface lanthanum atoms and adjacent oxygen atoms.