Lattice dynamics of α-Boron from ab-initio calculation and Raman scattering under high pressure

The Raman spectrum of icosahedral α-boron presents a very sharp peak at 525cm^-1 that was consistently rejected from the lattice modes but, is still unexplained. New Raman scattering experiments under pressure are compared with ab-initio lattice dynamics calculations. The very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the 525 cm^-1 line which is a highly harmonic librational mode of the icosahedron. This mode is also identified in the Raman spectrum of other icosahedral boron-rich solids.