Theoretical study of Xe monolayer adsorbed on NaCl(100): Structures and phase diagrams

Calculations based on semiempirical potentials are performed to interpret the structure of the Xe monolayer adsorbed on a clean (100) surface of NaCl. The rare gas atoms are located along Na troughs in which the corrugation remains less than 10 meV, while it can reach 25 meV outside the troughs. The competition between this corrugation and the lateral Xe–Xe interactions favors the occurrence of several (n×2) high order commensurate structures (exhibiting distorted hexagonal geometries) in a very narrow range of potential energy values. The behavior of the free energy with temperature and coverage shows four high order commensurate phase transitions below 80 K with a concomitant discrete variation of the mean lattice parameter aM of the structures from 4.30 to 4.46 Å. Special attention is brought to the study of the lattice parameter behavior between 40 and 80 K which was investigated independently by means of thermal desorption, photoelectron spectroscopy and high resolution electron diffraction experiments. Two sets of high order commensurate phases are determined within this temperature range; one set corresponds to structures with aM=4.365 Å for 31≤T≤55 K while the other gives structures with aM=4.41 Å for 55≤T≤78 K. These results are quite consistent with electron diffraction measurements which show an evolution from a perfect hexagonal phase (aM=4.37 Å) to a distorted hexagonal structure (aM=4.42 Å) around T=55 K.