Observation of two-dimensional compositional ordering of a carbon monoxide and argon monolayer mixture physisorbed on graphite

Commensurate monolayers of a mixture of 70% carbon monoxide and 30% argon on graphite are studied by neutron and low-energy-electron diffraction. A 2 x 2 superstructure due to 3:1 compositional ordering is observed below 25 K. The compositional ordering is most likely due to molecular-axis ordering of carbon monoxide molecules into a pinwheel pattern with argon atoms at the central sites of each pinwheel. Such a pinwheel structure has been predicted by Harris, Mouritsen, and Berlinsky for planar rotors with anisotropic interactions on a triangular lattice with vacancies.