Long-period commensurate structures near the incommensurate–commensurate phase transition in xenon layers adsorbed on graphite observed by X-ray diffraction

Abstract

By means of X-ray diffraction, long-range periodic structures have been observed in layers of xenon adsorbed on graphite (Papyex). At a coverage of 0.9 and up to ca. 60 K the structure consists of a regular hexagonal superlattice. Within the hexagons the xenon atoms are incommensurate with the surface with a mean xenon–xenon distance of 4.38–4.41 Å, depending on the temperature. The overall structure has the appearance of incommensurate hexagonal islands of xenon separated by ‘light’ domain walls. At higher temperatures this structure changes to a less regular one with a higher density of walls. Below ca. 60 K, as the coverage is increased, there is evidence that the layer changes towards √3 ×√3 commensurate phase through a series of discrete structures. Above 100 K the lattice parameter changes continuously as the coverage is increased.