Phase diagram of oxygen chemisorbed on nickel (111)

The complete phase diagram of oxygen atoms chemisorbed on Ni(111) has been determined. The two-dimensional system is realized for oxygen coverages of $p(2\mathrm{}\times 2)$ phase in which the full coverage is 1/4 and a $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ structure with full coverage of $\frac{1}{3}$. These phases are separated in coverage by a poorly ordered transition phase. The $p(2\mathrm{}\times 2)$ phase at full coverage ($\theta =0.25\mathrm{}$) exhibits a continuous order-disorder transition at $T_c=440\mathrm{}$ K. A tricritical point, corresponding to the intersection of boundaries of a disordered "gas" phase, a single-phase $p(2\mathrm{}\times 2)$ region, and a mixed-phase region of $p(2\mathrm{}\times 2)$ islands surrounded by gas, was found at $\theta =0.21\mathrm{}$, $T_t=300\mathrm{}$ K. The $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ phase was observed at $\theta >0.29\mathrm{}$ and $T<\mathrm{}300\mathrm{}$ K. This phase was characterized by a first-order discontinuous phase boundary. The work function increased smoothly with oxygen coverage.