On the formation of Ar-Kr two-dimensional mixtures

Successive adsorption of submonolayer amounts of Kr and Ar onto a Pt(111) surface leads to the formation of separate 2D islands of almost pure Kr and Ar content. Upon annealing, the phase separated Kr and Ar start to mix resulting eventually in a complete stochastic mixture of the two components at about T_mix = 30–36 K, depending on the Ar:Kr mixing ratio. This continuous mixing transition proceeds primarily via the dissolution of Kr atoms into the Ar 2D islands and not vice versa, as might be expected for kinetic reasons. Instead, the intermixing behaviour can be explained by the fact that the binding between an Ar and a Kr atom is stronger than between two Ar atoms but weaker than between two Kr atoms. As a consequence the incorporation of a Kr atom from the edge of an Ar island into its center is energetically favored while the dissolution of Ar into Kr is hindered.