Search for evidence of commensurate argon-on-graphite films near monolayer completion

The structural properties of argon films adsorbed on graphite at 85 K were investigated as a function of coverage using neutron diffraction. This study was prompted by the recent high-resolution heat-capacity measurements of Day et al. which uncovered a series of anomalies near monolayer melting. These features were attributed to the formation of a registered, √3×√3 R30^∘ phase. The diffraction study presented here shows that this commensurate phase does not exist in the portion of the phase diagram suggested by the thermodynamic study. On the contrary, a compressed solid phase with a lattice constant close to that of the close-packed plane of the bulk solid forms. Evidence of second-layer population in the diffraction study suggests that these heat-capacity anomalies might be direct evidence of layer promotion.