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

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Abstract
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 Å. In the ‘‘locked’’ phase every sixth Kr atom is located at a preferred adsorption site (threefold hollow) forming a hexagonal (5×5)R0° superstructure.