Phase diagram of oxygen chemisorbed on nickel (111)

Abstract

The complete phase diagram of oxygen atoms chemisorbed on Ni(111) has been determined. The two-dimensional system is realized for oxygen coverages of 0.5 monolayers and temperatures 500 K. At higher coverages and temperatures oxygen is incorporated into the bulk. The structure of the oxygen overlayer was determined by low-energy electron diffraction and the coverage was monitored by Auger electron spectroscopy. Oxygen forms two structures with long-range order on this surface: a $p(2\ifmmode\times\else\texttimes\fi{}2)$ phase in which the full coverage is 1/4 and a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ structure with full coverage of $\frac{1}{3}$. These phases are separated in coverage by a poorly ordered transition phase. The $p(2\ifmmode\times\else\texttimes\fi{}2)$ phase at full coverage ($\ensuremath{\theta}=0.25$) exhibits a continuous order-disorder transition at ${T}_{c}=440$ K. A tricritical point, corresponding to the intersection of boundaries of a disordered "gas" phase, a single-phase $p(2\ifmmode\times\else\texttimes\fi{}2)$ region, and a mixed-phase region of $p(2\ifmmode\times\else\texttimes\fi{}2)$ islands surrounded by gas, was found at $\ensuremath{\theta}=0.21$, ${T}_{t}=300$ K. The $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ phase was observed at $\ensuremath{\theta}0.29$ and $T300$ K. This phase was characterized by a first-order discontinuous phase boundary. The work function increased smoothly with oxygen coverage.