LATTICE-DYNAMICS OF ALPHA-BORON FROM AB-INITIO CALCULATION AND RAMAN-SCATTERING UNDER HIGH-PRESSURE

The Raman spectrum of icosahedral alpha-boron presents a very sharp pe ak at 525cm(-1) that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under press ure are compared with ab-initio lattice dynamics calculations. The ver y good agreement of the mode frequencies and their pressure coefficien ts yields unambiguous assignment of all observed features, including t he 525 cm(-1) line which is a highly harmonic librational mode of the icosahedron. This mode is also identified in the Raman spectrum of oth er icosahedral boron-rich solids.