The origin of Sr segregation at La1-xSrxMnO3 surfaces

TL;DR

This paper investigates the electrostatic mechanisms behind Sr segregation at La1-xSrxMnO3 surfaces, revealing that La depletion near the surface reduces surface energy, with implications for understanding surface chemistry in similar materials.

Contribution

It introduces an electrostatic energy model based on exponential segregation profiles, explaining Sr segregation as a charge compensation mechanism at surfaces.

Findings

Segregation reduces surface energy compared to other mechanisms.

Calculated decay length is shorter than experimental observations.

Mechanism may apply to other oxide surface segregations.

Abstract

A uniform distribution of La and Sr in lanthanum-strontium manganites would lead to charged crystal planes, a charged surface, and arbitrarily large surface energy for a bulk crystal. This divergent energy can be eliminated by depleting the La concentration near the surface. Assuming an exponential form for segregation suggested by experiment, the total electrostatic energy is calculated, depending only upon the decay length and on an effective charge Z* associated with the La ion. It is found to be lower in energy than neutralization of the surface by changing Mn charge states, previously expected, and lower than simply readjusting the La concentration in the surface plane. The actual decay length obtained by minimizing this electrostatic energy is shorter than that observed. The extension of this mechanism to segregation near the surface in other systems is discussed.