Tetrafluoromethane monolayer physisorbed on graphite : its orientational walls and its complex phase diagram

Abstract

Neutron diffraction results for a monolayer of CF4 physisorbed on different graphites show the complexity of the system, mainly due to the non spherical shape of the admolecule. From our neutron data and LEED results [6] we recognize at least 6 different phases. These are, with decreasing temperature, liquid, plastic, commensurate solid, incommensurate solid, incommensurate tilted and a surprising triple-peaked incommensurate phase. This low temperature phase is especially studied and compared with different models, of increasing sophistication, based on the concept of small domains with varying degrees of compression which alternate periodically and are limited by different types of walls. These different types of walls are due to the ability of the molecule to have different orientations relative to the substrate. These models give also a tentative explanation to the puzzling fact that diffraction patterns are different according to the graphite substrate used, Papyex or ZYX-Uncar. Finally a tentative phase diagram is given for this complex system.