Vapor-pressure-isotherm study of monolayer CO films adsorbed on BN

We have measured detailed adsorption isotherms for CO films on BN at 18 temperatures between 50 and 75 K, for coverages in the first layer. The isotherms and isothermal compressibilities are very similar to those reported for CO on graphite (the corresponding features, however, appear approximately 20 K lower on BN). We use the similarity between the characteristics of the thermodynamic features of CO on these two substrates to tentatively identify the phases present, and to propose a phase diagram for CO on BN. Our adsorption results are also compared with data reported for Kr and for ${\mathrm{N}}_2$ films on BN; a possible explanation for why Kr behaves differently from either ${\mathrm{N}}_2$ or CO on BN is discussed.