Symmetry and rotational epitaxy of incommensurate Xe layers on Pt(111)

High-resolution He diffraction studies of the structure and rotational epitaxy of incommensurate Xe layers on Pt(111) are reported. At completion of the commensurate ({}3 ×{} {}3 )R30^∘{} monolayer (C) a continuous phase transition to a striped incommensurate phase (SI) occurs. With increasing incommensurability the striped phase (SI) transforms at a critical misfit of {}6.5% to a hexagonal incommensurate phase (HI) in a first-order transition. Upon further increase of the misfit ({≥}7.2%) the HI phase displays a continuous transition from an aligned to a Novaco-McTague rotated orientation (HIR). Xe on Pt(111) appears to be an ideal model system of two-dimensional phases and their mutual transitions, i.e., it is the first system showing completely the theoretically predicted sequence of C{→}SI{→}HI{→}HIR transitions.