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

Abstract

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 $c(2\ifmmode\times\else\texttimes\fi{}2)$ 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.