Theoretical study of the stability of Be_p@Rb_q core-shell clusters

Core-shell structures A_p@B_q can be energetically favored in bimetallic clusters when B has a lower surface energy than A, and p and q are chosen such that a single and complete outer shell of q atoms of B can form around the A_p core. If the difference in the electronegativities of A and B is large, one expects significant charge separation between the A core and the B shell. The combination of a closed atomic shell of B atoms and surface charges (core-shell charge separation) should impart stability on this type of A_p@B_q clusters. We report results of density functional theory calculations, including global optimization of cluster geometries, to characterize the prototype system Be_pRb_q. Our calculations show that Be_pRb_q clusters indeed adopt core-shell structures. However, they do not have particularly large surface charges and are unstable against coalescence. Surprisingly, the Rb_q outer shell carries a net negative charge. This can be interpreted as the analog, for clusters, of electron spill-out at semi-infinite jellium surfaces, and it probably results from the electrons' kinetic pressure. (author)