Atomic Structure and Vibrational Properties of Icosahedral $B_{4}C$ Boron Carbide

The atomic structure of icosahedral ${\mathrm{B}}_4\mathrm{C}$ boron carbide is determined by comparing existing infrared absorption and Raman diffusion measurements with the predictions of accurate ab initio lattice-dynamical calculations performed for different structural models, a task presently beyond x-ray and neutron diffraction ability. By examining the inter- and intraicosahedral contributions to the stiffness we show that, contrary to recent conjectures, intraicosahedral bonds are harder.