X-ray study of oxygen monolayers on graphite

Abstract

Oxygen monolayers on graphite have been investigated in the temperature region between 15 and 53 K by X-ray diffraction. The monolayer δ phase with a low 2D density consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The monolayer ζ phase with a higher 2D density forms a simple triangular lattice with the molecular axes perpendicular to the graphite surface; the model of two mutually incommensurate triangular layers for the ζ phase proposed by previous X-ray investigators is inappropriate in monolayer region. The coverages in the phase boundaries proposed by the present study are nearly consistent to those reported by a neutron scattering study, but considerably lower than those reported by previous X-ray and heat-capacity studies. This discrepancy reveals that in those X-ray and heat-capacity experiments a considerable amount of oxygen nucleated as bulk O2 in the cell without being adsorbed on the graphite surface, because a nonequilibrium state occurred during the cooling of the sample. A comparison of the coverages in the phase boundaries with the monolayer capacities calculated from the lattice vectors of the 2D solids suggests that a few percent occupancy of molecules in the second layer is needed during the δ-ζ transition of the first layer of O2 on graphite. The δ phase melted into the fluid I, while the melting of the ζ and δ + ζ coexistence phases occurred in two stages: from the solids into the fluid II and then into the fluid I.