Lattice Dynamics of Icosahedralα-Boron under Pressure

The Raman spectrum of icosahedral \ensuremath{\alpha}-boron presents a very sharp peak at $525{\mathrm{cm}}^{\ensuremath{-}1}$ 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 $525{\mathrm{cm}}^{\ensuremath{-}1}$ line which is a highly harmonic librational mode of the icosahedron and mainly involves bond bending. This mode is also identified in the Raman spectrum of other icosahedral boron-rich solids.