Thermodynamics of first-order and continuous melting of xenon on graphite

We report a high-precision vapor-pressure isotherm study of the melting of near-monolayer xenon adsorbed on Union Carbide's ZYX exfoliated graphite. The data are analyzed in terms of the film isothermal compressibility KT. We identify a sharp peak in KT which occurs in low-temperature data as a first-order melting transition. As the temperature is increased, the amplitude of the peak decreases, reaching zero at a tricritical temperature Tₜ=147±4 K. Above this temperature the melting appears to be continuous. A temperature-independent, weak, and relatively broad contribution to KT underlies the sharp peak at low T and persists above Tₜ; we discuss possible origins of this component.