The structure of silicon boride, ${\mathrm{SiB}}_3,$ is based on 12-atom, boron-rich icosahedra in which silicon atoms substitute for some boron atoms. Raman bands associated with vibrations of icosahedral atoms in ${\mathrm{SiB}}_3$ are quite broad, reflecting this substitutional disorder. Comparing the Raman spectra of other icosahedral borides with ${\mathrm{SiB}}_3,$ only boron carbides have similarly broad icosahedral Raman bands. The direct correlation of broad icosahedral Raman bands with substitutional disorder supports the proposition that carbon atoms replace icosahedral boron atoms in boron carbides of all compositions.

• Received 8 July 1997

DOI:https://doi.org/10.1103/PhysRevB.57.2675