Low-energy electron diffraction study of submonolayer mixtures of carbon monoxide and nitrogen with argon physisorbed on graphite

Submonolayer mixtures of carbon monoxide (CO) and nitrogen (${\mathrm{N}}_2$) with argon (Ar) are physisorbed on the basal plane of a graphite single crystal. The structures for these mixtures are studied by low-energy electron diffraction (LEED) at temperatures between 10 and 40 K. The phase diagrams of these mixtures are inferred from LEED patterns, theoretical arguments, and comparison with Barrett and Meyer’s bulk phase diagrams of CO-Ar and ${\mathrm{N}}_2$-Ar mixtures. The long-range orientationally and compositionally ordered commensurate CO-Ar pinwheel structure previously reported by You, Fain, Satija, and Passell coexists at low temperatures with a phase-separated incommensurate Ar-rich phase for Ar compositions of more than 25 mol? A short-range commensurate compositional ordering for the ${\mathrm{N}}_2$-Ar mixtures is inferred from the LEED patterns.