Peculiar covalent bonds in alpha-rhombohedral boron

A peculiar bonding feature of alpha-rhombohedral boron is found in the elec tron density distribution of the icosahedral B-12 cluster. The electron den sity distribution is obtained by using the maximum entropy method with sync hrotron radiation powder data. It clearly shows the characteristic two- and three-center covalent bond network which threads through atoms on the clus ter surface. Besides, two kinds of covalent bonds in intercluster space are also shown in detail: a three-center trilaterally formed bond among the th ree clusters and a two-center bond with a remarkable feature of a bent bond between the clusters. The obtained covalent bonding features indicate the cluster-based nature of this material. [S0031-9007(99)09272-8].