Rotational epitaxy of a nontriangular structure: The $\delta$ phase of oxygen physisorbed on graphite

Low-energy electron diffraction (LEED) observations of molecular oxygen physisorbed on graphite are reported. The structure of the low-coverage $\delta$ phase is shown to be a centered parallelogram and the rotational epitaxy is determined. At temperatures above 22 K where the LEED study was carried out, we find that the rotational epitaxy changes with coverage (the layer exhibits a Novaco-McTague effect). The possibilities of the structure melting anisotropically and of a low-temperature commensurate phase are discussed. Defect-stabilized or metastable structures are also observed.