The structural evolution of boron carbide via ab initio calculations

The distribution of boron and carbon atoms in boron carbide (B4C), the third hardest naturally occurring material, is a hotly debated subject. In the current work, an ab initio approach is applied to the entire composition range of boron carbide (∼8–20at.%C) to determine what disordering mechanisms are responsible for such a large single-phase region. Based on the correctly predicted crystal structures, enthalpy of formation, and infrared phonon modes, we reveal direct evidence in support of a new defect model for boron carbide, where mixing occurs in the icosahedron for carbon-rich compositions and in the chain for boron-rich compositions.