Crystal structures and metastability of carbon-boron compounds C3B and C5B

The recent discovery of the diamondlike C3B and C5B compounds has raised hopes of revealing interesting properties and also elicits questions about the stability of such compounds. Using our implementation of the evolutionary global space-group optimization method, we have found ordered structural models for C3B (layered hexagonal) and C5B (diamondlike) with lower energies than previously obtained and revealing unusual layer-stacking sequences. The compounds are less stable than a mixture of freestanding lowest-energy phases of B, C, and C4B, thus C3B and C5B are not ground-state structures. Nevertheless, disordered diamondlike C3B and C5B can be formed exothermically at high temperature in the reaction [graphitelike C3B] + 2C -> [diamondlike C5B] and [graphitelike C3B] -> [diamondlike C3B]. Thus, the disorder on the C and B sites of diamondlike C3B and C5B is responsible for the observed phases. DOI: 10.1103/PhysRevB.87.094103