Electron density distribution of β-rhombohedral boron constructed from icosahedral cluster

Boron forms unusual structures and chemical bonds. Pure boron and many boron-rich borides have local icosahedra in their crystalline structures. The electron density distribution (EDD) in β-rhombohedral boron (β-B105) is visualized by the maximum entropy method (MEM)/Rietveld analysis of powder X-ray diffraction (XRD) data in order to discuss the peculiarity of the chemical bonds. In α-rhombohedral boron (α-B12) and its derivative crystals, peculiar covalent bonds, i.e., intracluster three-center bonds and intercluster bent bonds, have been observed. In β-B105, the intracluster three-center bonds are observed, but the intercluster bent bonds are not. The origin of the bent bond can be explained by the mismatch in the directions of the bond and the lattice vector.