Evidence of a Square Two-Dimensional Solid of Methane Physisorbed on the (100) Surface of Magnesium Oxide

The structure of C${\mathrm{D}}_4$ films adsorbed on a highly homogeneous MgO(100) surface has been probed by neutron diffraction. Both the monolayer and the bilayer crystallize in a commensurate square $c(2\mathrm{}\times 2)$ superstructure. This is the first clear evidence of a two-dimensional condensation of a solid van der Waals film with square symmetry. An order-disorder transition is observed and appears to be a first-order melting.