Structure of monolayer films of hydrogen isotope mixtures

H-2-D-2 mixtures adsorbed on graphite have been studied by neutron diffraction for 2K < T < 18 K and various H-2-D-2 ratios. One commensurate (root 3 x root 3R30 degrees) and one incommensurate (full monolayer) solid coverages were measured. For the incommensurate phase, a single diffraction line at a position intermediate between the (10) lines of pure D-2 and H-2 was observed, showing that the solid is a well-ordered mixed phase. The strong increase of the peak intensity with decreasing temperature for a 50% mixture at monolayer completion is interpreted as a trend towards phase separation at low temperature.