Phase transitions, thermodynamics and structural analysis of ethane films adsorbed on graphite. A low-energy electron-diffraction study

Abstract

The properties of ethane monolayers and multilayers have been studied using low-energy electron diffraction (LEED) as a probe in the temperature range 63.9–100 K. This work completes previous studies made by volumetry (monolayer and multilayer), neutron diffraction and LEED (monolayer). These latter studies have shown the rich variety of two-dimensional phases: three solids, S₁, S₂ and S₃, a lattice fluid, I₁, a well correlated liquid, I₂, an isotropic liquid, L, and a hypercritical fluid, F. In the present work LEED isotherms have been measured which have allowed us to plot a phase diagram of ln P against T^–1 giving the pressure and temperature domains of stability of the various monolayer and multilayer phases and the isosteric heats at the phase transitions. This phase diagram is in good agreement with previous isotherm measurements at higher temperatures (87 < T/K < 143.5). The multilayer experiments show a type-2 mode of growth with two uniform layers plus bulk for 63.9 < T/K < 86. An LEED pattern analysis around the S₁–I₁, S₂–I₂ and I₂–S₃ transitions has been performed. Particular attention has been paid to the domain-growth kinetics of the S₁ herringbone phase following quenching from high to low temperatures. Finally, the analysis of the S₁ structure has been reconsidered, taking into account recent quasielastic neutron-scattering results.