B80 and Other Medium-Sized Boron Clusters: Core Shell Structures, Not Hollow Cages

Unbiased search on the potential energy surface of medium-sized boron clusters, with B-80, B-74, and B-68 as representatives, was carried out using simulated annealing incorporated with first-principles molecular dynamics. These boron clusters thermodynamically prefer the B-12-centered core shell structures, which resemble the fragment of bulk boron solids. Though these core shell clusters lack a descriptive symmetry and may not be the true global minima, the core-shell B-80 is about 25 meV/atom lower in energy than the widely assumed highly stable "magic" B-80 fullerene. The electronic states and photoelectron spectra of these clusters are closely correlated to the structural motif, which may be helpful for detecting the cluster configurations in experiments.