STRUCTURE AND PHASE-TRANSITIONS OF D-2+AR MIXTURES ADSORBED ON GRAPHITE

We report elastic neutron scattering experiments which explore the eff ects of size mismatch and relative composition on the freezing of bina ry two-dimensional mixtures. We coadsorb on graphite at low temperatur e two different species, Ar-36 and D-2. Due to its large quantum motio n, deuterium is highly compressible (more than 50%) when adsorbed as a solid monolayer on graphite. It can therefore span a large range of s ize mismatch with respect to Ar. Results obtained at 20 K for various partial coverages of Ar and D-2 show that three phases coexist on the surface, two Ar-rich hexagonal solids and a D-2-rich phase which itsel f can be disordered or a hexagonal solid, depending on the D-2 partial coverage.