Tight-binding and evolutionary search approach for nanoscale CoRh alloys

The dependence of the structural and magnetic properties of Co-Rh alloys at the nanoscale was investigated by exhaustively searching the minimum energy via a symbiotic algorithm on a Gupta potential for particle sizes of 13, 19, and 23 atoms. An unrestricted spd tight-binding Hamiltonian was then used to model the electronic properties. Our results underscore the importance of determining both the geometrical and the chemical configuration. A central result points toward a surface segregation that qualitatively and quantitatively depends on system size, with size effects dominating the surface segregation for small Co-Rh clusters (Rh atoms preferentially occupy surface sites), whereas surface energy dictates the segregation for large Co-Rh nanoparticles (Co segregates to the surface). This might have important consequences for heterogeneous catalysis, where the catalytic activity strongly depends on surface composition.