Orientational Ordering ofN2OMolecules Adsorbed on Graphite (0001): A Novel Commensurate Pinwheel Structure

The structure and orientational ordering of ${\mathrm{N}}_2\mathrm{O}$ molecules physisorbed on graphite (0001) is investigated applying x-ray, neutron, and low-energy electron diffraction techniques. Combining the results of the three techniques, we find that ${\mathrm{N}}_2\mathrm{O}$ forms a highly ordered, hexagonal, commensurate $(\sqrt{21}\times \sqrt{21})R10.89\mathrm{}°$ phase. The unit cell contains seven molecules which are arranged in a seven-sublattice pinwheel structure, unexpected for linear molecules on a hexagonal lattice. Potential energy calculations corroborate these results.