The reconstruction of Rh(001) upon oxygen adsorption

TL;DR

This study uses density-functional theory to analyze the structural reconstruction of Rh(001) surfaces upon oxygen adsorption, revealing a rhomboid distortion and predicting an order-disorder transition below room temperature.

Contribution

First-principles calculations uncover a specific surface reconstruction pattern and transition mechanism for oxygen-covered Rh(001), challenging previous interpretations from STM images.

Findings

Oxygen atoms sit at the center of black squares in a c(2×2) pattern.

A rhomboid distortion lowers surface energy and predicts an order-disorder transition.

The distortion involves a rotation of the white squares, contrary to some STM-based claims.

Abstract

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density-functional theory. We find that oxygen atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus so formed. We predict that the latter distortion is associated with an order-disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty, white, squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.