Phase diagram of oxygen adsorbed on platinum (111) by first-principles investigation

A complete phase diagram of oxygen atoms adsorbed on a Pt (111) surface with oxygen coverages below half a monolayer has been computed and compared with the surface phase diagrams of related systems. Effective interaction parameters of a lattice model for the triangular lattice of the fcc sites of the Pt (111) surface were determined from first-principles computations. Oxygen forms on the platinum (111) surface two stable ordered phases, which persist up to high temperatures. They are the p(2x2) and p(2x1) phases, having coverages of 1/4 and 1/2 monolayer, respectively. At the coverage of 2/5 monolayer, another stable phase consisting of p(2x1) rows but with every two rows offset by an empty site is predicted by our model, but this phase is stable only below 250 K. All three phases undergo continuous phase transitions to the disordered state upon heating. At coverages lower than 1/4 monolayer and at low temperatures, oxygen atoms cluster into p(2x2) islands, in agreement with observations from a scanning tunneling microscope study. The formation of p(2x2) oxygen islands is a consequence of attractive third-nearest-neighbor interactions, despite the strong repulsion between the first and second nearest neighbors. Two regions separated by first-order phase boundaries are found at coverages between 0.26 monolayer and 0.37 monolayer and coverages between 0.43 monolayer and 0.5 monolayer.