Is the established structure of α-rhombohedral boron correct? Comparative study of IR-active phonons with B6O, B4.3C and β-rhombohedral boron

The established structure of α-rhombohedral boron, based on one B12 icosahedron per unit cell only, is put in question. A careful evaluation of the IR-active phonons in comparison with B6O, B4.3C and β-rhombohedral boron makes it evident that-aside from the B12 icosahedra-the α-rhombohedral boron structure also contains single boron atoms. We assume these single atoms replace the currently assumed inter-icosahedral three-centre-bonds by covalently saturating the outward directing bonds of the equatorial atoms of the three adjacent icosahedra. Indeed, the implied structure formula B12B2 is not supported by previous density measurements. The IR-phonon spectra of the related structures are correlated and merely shifted relative to one another; significant features depend quantitatively on the actual structure, but they can be easily allocated. (paper)