Incommensurate to high-order commensurate phase transition of Kr on Pt(111)

We propose and demonstrate here a criterion, based on thermal expansion, which discriminates unambiguously between ``floating'' and ``locked'' layers. The first observation of a phase transition between an incommensurate ``floating'' and a high-order commensurate ``locked'' phase in two dimensions is reported. High-resolution He-diffraction results show a first-order transition in Kr layers on Pt(111) at coverages of {~}0.8 monolayer with a lattice-parameter jump of {~}0.1 A{}. In the ``locked'' phase every sixth Kr atom is located at a preferred adsorption site (threefold hollow) forming a hexagonal (5×{}5)R0^∘{} superstructure.