LATTICE-DYNAMICS OF ICOSAHEDRAL ALPHA-BORON UNDER PRESSURE

The Raman spectrum of icosahedral alpha-boron presents a very sharp pe ak at 525 cm(-1) that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under pres sure are compared with ab initio lattice dynamics calculations. The ve ry good agreement of the mode frequencies and their pressure coefficie nts yields unambiguous assignment of all observed features, including the 525 cm(-1) line which is a highly harmonic librational mode of the icosahedron and mainly involves bond bending. This mode is also ident ified in the Raman spectrum of other icosahedral boron-rich solids.