Cage-forming tendencies in Si_nN_m clusters

We present the results of a computational study of the structure and properties of Si_nN_m clusters, where n+m=20. We use complementary tight-binding and first-principles density functional calculations to determine the optimal arrangement of N atoms on the clusters for m=2,4,6 and 8. The calculations indicate a transition in the basic geometrical structure of the substituted clusters from close-packed structures typical of pure Si clusters for m<4 to cage-like structures for m≥4. We discuss the predicted thermodynamic and chemical stabilities of these novel Si-based cages.