Metal-encapsulated caged clusters of germanium with large gaps and different growth behavior than silicon

Metal ( M)-encapsulated caged clusters of Ge are studied using the ab initio pseudopotential plane-wave method and the generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the M atom, we find Frank-Kasper polyhedral M@Ge(16) for M= Ti, Zr, Hf, and capped decahedral or cubic M@Ge(14) and M@Ge(15) clusters for several M atoms. The growth behavior differs from the one found in M@Si(n) clusters. The highest-occupied-lowest-unoccupied molecular orbital gaps are, however, similarly large or even higher in some cases. Cr@Ge(16) and Fe@Ge(15) are magnetic. The weak interaction between the clusters makes such species attractive for cluster assembled materials.