Gibbs free-energy calculations based on density functional theory have been used to determine the possible source of failure of boron carbide just above the Hugoniot elastic limit (HEL). A range of ${\mathrm{B}}_4\mathrm{C}$ polytypes is found to be stable at room pressure. The energetic barrier for shock amorphization of boron carbide is by far the lowest for the ${\mathrm{B}}_{12}(\mathrm{CCC})$ polytype, requiring only $6\mathrm{GPa}\approx \mathrm{P}(\mathrm{HEL})$ for collapse under hydrostatic conditions. The results clearly demonstrate that the collapse of the ${\mathrm{B}}_{12}(\mathrm{CCC})$ phase leads to segregation of ${\mathrm{B}}_{12}$ and amorphous carbon in the form of 2–3 nm bands along the (113) lattice direction, in excellent agreement with recent transmission electron microscopy results.

• Received 26 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.97.035502