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). Together with the examination of the DFT total energies of various atomic configurations in the unit cell, comparison with the experiments of the theoretical vibrational or NMR spectra has led to the determination of the atomic structure of B₄C as C-B-C chains linking mostly B₁₁C icosahedra, and a few percents of B₁₀C ₂ icosahedra. In the icosahedron, the carbon atom is found to be in the polar site (B₄C^p). When there are two carbon atoms, they are found to be in antipodal polar positions. At carbon concentrations other than 20%, we find that only four structural models have a negative formation energy with respect to a formation from α-boron + diamond. Moreover, they all have a positive formation energy with respect to B ₄C^p, showing a tendency to decompose into B ₄C^p + α-boron or B₄C^p + diamond. This metastability explains actual difficulties in the synthesis of clean samples, in particular for B₁₃C₂. 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 ₁₃C₂.