SPECIFIC-HEAT AND PHASE-DIAGRAMS OF H-2 ADSORBED ON D-2-PLATED OR HD-PLATED GRAPHITE

Heat-capacity measurements are used to investigate the phase diagram of H2 adsorbed on exfoliated graphite (gr) plated with approximately one monolayer of either D2 or HD, in the range 3.7<T<13 K. The added H2 dosages are all within the equivalent of one monolayer coverage on the bare graphite. For the system H2/D2/gr a narrow transition at 5.740.03 K seems to indicate triple line melting. This transition shows a very small shift to lower temperatures with increasing coverage. For each coverage within the triple line densities, this transition is followed, at higher temperatures, by another transition culminating at a critical point at 9.440.30 K. The melting line of a H2 compressible solid is traced at coverages above the end of the triple line. For the H2/HD/gr system though, the apparent triple line melting temperature occurs at a higher temperature than for H2/D2/gr and decreases from 6.580.03 K at low densities, to a somewhat constant 6.25 K near monolayer completion. The critical point of the higher temperature phase is at 10.170.2 K. We present a simple model of interlayer isotopic mixing used to explore the origin of the triple line tilting in both systems. © 1995 The American Physical Society.