Boron carbides from first principles

In this work, we focus on the understanding gained from the investigation of the physical properties of boron-carbides with theoretical methods based on density functional theory (DFT). Comparison of computed and experimental vibrational or NMR spectra has shown that the atomic structure of B$_{4}$C consists in C-B-C chains linking mostly B$_{11}$C icosahedra, and a few percent of B$_{10}$C$_{2}$ icosahedra. In particular, C-C-C chains are excluded and can not be responsible for B$_{4}$C amorphization under shockwaves. In this work we find that at lower carbon concentration all models are metastable with respect to B$_{4}$C plus $\alpha $-boron. This could explain actual difficulties in the synthesis of clean samples. Furthermore we discuss effects of temperature and/or pressure on stabilities and properties. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B$_{13}$C$_{2}$.