Thermodynamic study of phase transitions of monolayer N2 on graphite

The experimental techniques used and the results obtained in an extensive thermodynamic study of submonolayer and near-monolayer N2 physisorbed on exfoliated graphite surface are presented. High-resolution ac heat-capacity and in situ isothermal vapor-pressure techniques were employed. For the heat-capacity study, N2 films of 22 different coverages were studied between 18 and 70 K. The ac technique, capable of resolving heat capacity to 0.2%, yields definitive evidence in support of the incipient-triple-point model for the melting transition. A narrow heat-capacity peak due to the orientational ordering transition in the two-dimensional registered solid phase was also observed at 27 K for all submonolayer coverages. The shape of the orientational-ordering heat-capacity peak is found to be consistent with theoretical predictions and a recent computer-simulation study. The shape of the phase diagram of N2 near-monolayer completion at high temperature was probed in the vapor-pressure study. A narrow strip of registered solid-fluid coexistence region at high coverage was found to extend beyond 85 K. The shape of this coexistence region at high temperature was found to be consistent with a two-dimensional three-statePotts-model tricritical-point interpretation with an exponent of 0.55±0.02. Recent theories predict a value of 0.5.