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

Abstract

A complete phase diagram of oxygen atoms adsorbed on a $\mathrm{Pt}\phantom{\rule{0.2em}{0ex}}(111)$ 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 $\mathrm{Pt}\phantom{\rule{0.2em}{0ex}}(111)$ 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 $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $p(2\ifmmode\times\else\texttimes\fi{}1)$ phases, having coverages of $1∕4$ and $1∕2$ monolayer, respectively. At the coverage of $2∕5$ monolayer, another stable phase consisting of $p(2\ifmmode\times\else\texttimes\fi{}1)$ rows but with every two rows offset by an empty site is predicted by our model, but this phase is stable only below $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 $p(2\ifmmode\times\else\texttimes\fi{}2)$ islands, in agreement with observations from a scanning tunneling microscope study. The formation of $p(2\ifmmode\times\else\texttimes\fi{}2)$ 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.