The structure of chloromethane monolayers adsorbed on graphite

Abstract

Two crystalline phases of chloromethane adsorbed on graphite are found, depending upon the coverage. Structures for the two adsorbed layers are proposed, determined from a combination of X-ray and neutron diffraction measurements. The experimentally determined unit cells of both phases are incommensurate and rectangular, with lattice parameters a = 1·014 nm and b = 0·771 nm for the low-coverage phase and a = 0·657 nm and b = 0·426 nm for the high-coverage phase. In the low-coverage phase there are four molecules in the unit cell, arranged to give head to tail zig-zag chains with a p2 space group, and all the molecules lie approximately flat on the surface. The high-coverage phase has two molecules per unit cell, arranged in lines with pg symmetry, elevated from the surface by about 70°. The low-coverage phase melts at a temperature of about 120 K, and the high-coverage phase at around 150 K. There is a coexistence region where the melting point of the high-coverage phase increases with increasing coverage.