The thermodynamic stability of bimetallic surfaces is generally related to their surface energy and their formation energy. The surface energy, or more correctly, the specific surface energy of a solid ES, is classically defined as the energy required to reversibly increase the surface area A of a given solid, while keeping the mass constant Following this definition, the surface energy determines, e.g., the energetically most preferential shape of a crystal, which can be determined via the Wulff construction or the thermodynamic growth modes during epitaxial growth in an epitaxial system . Correspondingly, the formation energy will determine the stability of a given mixed phase as compared to that of a phase separated system.