Atomic structure and vibrational properties of icosahedral alpha-boron andB4C boron carbide

The Raman and infrared spectra of a-rhombohedral boron B-12 and of B4C boro n carbide have been determined by accurate first-principles calculations ba sed on density-functional perturbation theory. Our results account for all the features observed experimentally, including the characteristic Raman-ac tive mode around 530 cm(-1), which is attributed to the libration of the ic osahedra. A comparison of the calculated vibrational spectra with experimen tal data allows the first unambiguous determination of the atomic structure of B4C, Analysis of our data shows that the high bulk moduli of cc-rhomboh edral boron and of B4C boron carbide - 220 and 240 GPa, respectively - are mainly determined by the stiff intramolecular bonding within each icosahedr on. This finding is at variance with the current interpretation of recent n eutron diffraction data on B4C in terms of a postulated larger stiffness of the intermolecular bonds in icosahedral solids (inverted molecular compres sibility). Our results show that icosahedral boron-rich solids should be co nsidered as members of a new class of covalently bonded materials. (C) 2000 Elsevier Science B.V. All rights reserved.