Density functional theory is demonstrated to reproduce the ${}^{13}\mathrm{C}$ and ${}^{11}\mathrm{B}$ NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. ${\mathrm{B}}_4\mathrm{C}$ can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a ${\mathrm{B}}_{11}\mathrm{C}$ structure with the C atom placed in a polar site, and a few percent have a $\mathrm{B}{}_{12}$ structure or a ${\mathrm{B}}_{10}{\mathrm{C}}_2$ structure with the two C atoms placed in two antipodal polar sites.

• Received 23 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.085506