High-pressure phase transition makes B4.3C boron carbide a wide-gap semiconductor

Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and interrelation of atomic distances, optical properties and Raman-active phonons up to similar to 70 GPa. The anomalous pressure evolution of the gap width to higher energies is striking. This is obtained from observations of transparency, which most rapidly increases around 55 GPa. Full visible optical transparency is approached at pressures of >60 GPa indicating that the band gap reaches similar to 3.5eV; at high pressure, boron carbide is a wide-gap semiconductor. The reason is that the high concentration of structural defects controlling the electronic properties of boron carbide at ambient conditions initially decreases and finally vanishes at high pressures. The structural parameters and Raman-active phonons indicate a pressure-dependent phase transition in single-crystal (B4.3C)-B-nat boron carbide near 40 GPa, likely related to structural changes in the C-B-C chains, while the basic icosahedral structure appears to be less affected.