The commensurate-incommensurate transition of Kr monolayers on graphite. I. Thermal expansion

The role of anharmonicity on the commensurate (C) phase of Kr and Xe monolayers adsorbed on graphite is analysed. If the usual interatomic interaction parameters are accepted, Kr monolayers at low temperature should be in an incommensurate (I) phase. Thermal expansion stabilises the commensurate phase at temperatures higher than T IC , the temperature at which the energy of creating a wall in the commensurate phase becomes positive. A rough estimate yields T IC approximately=34K. Anharmonicity also explains the positive slope of the experimental chemical potential versus temperature curve at the C-I transition.