Configurational order-disorder induced metal-nonmetal transition in B13C2 studied with first-principles superatom-special quasirandom structure method

Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B₁₃C₂ has been a controversial topic in the field of icosahedral boron-rich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while the ordered ground state B₁₃C₂ is metallic, the configurationally disordered B₁₃C₂ , modeled with a superatom-special quasirandom structure method, goes through a metal to nonmetal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chain-end carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a B₁₂ icosahedron bonded to it, giving rise to a B₁₁C^e (BBC) unit. The atomic configuration of the substitutionally disordered B₁₃C₂ thus tends to be dominated by a mixture between B₁₂(CBC) and B₁₁C^e (BBC). Due to splitting of valence states in B₁₁C^e (BBC), the electron deficiency in B₁₂(CBC) is gradually compensated.