Simulated behavior of krypton/argon mixtures confined between two graphite slabs: new terrain for familiar systems

We present the results of (N,ρ,T) molecular-dynamics simulations of krypton/argon mixtures confined between two graphite slabs with varying spacing. Structural, thermodynamic and bond-orientational quantities indicate a group of new phases and phase transitions for these already well-explored systems, and they also further delineate the close cooperation of vertical atomic motion and melting in adsorbed systems. For pure argon and systems with a high argon fraction we observe commensurate and rotated phases. Commensurate argon is stabilized over a wide temperature range for certain slab spacings, and high-temperature solid phases exist for all mixture fractions studied. For all systems explored two phenomena are observed: (1) the melting temperature Tm of the system may be controlled to a fairly precise degree within a certain range by only the slab spacing, and (2) competing effects of confinement and heightened room for in-plane atomic fluctuations due to enhanced vertical fluctuations causes Tm to reach a minimum value as the slab spacing is varied. The effects of varying the mixture fraction are also explored and, although emphasis is placed on melting, evidence of confinement-induced and composition-induced phase transitions is given and briefly discussed.