When carbon impurities trigger the synthesis of alpha boron at high pressure and high temperature

The role of carbon in the formation of α boron at high pressure and high temperature (HPHT) has been investigated by combining HPHT experiments and density functional theory (DFT) calculations. Starting from β rhombohedral or amorphous boron and amorphous carbon at 5 GPa, the α boron phase has been repeatedly observed between 1373 K and 2273 K, at temperatures that are much higher than those reported in the phase diagram of boron. The DFT investigation of the effect of carbon insertion into the α rhombohedral boron atomic structure on the formation enthalpy, volume and optical properties shows that the (B₁₁C) substituted icosahedron defect accounts for the slight volume contraction observed in α boron, is compatible with the observed red colour of α boron crystals, and consistently has a very low formation energy. Our calculations turn out to provide a calibration means to evaluate the concentration of carbon in our samples. Finally, the synthesis temperature is found to be an easy means to control the carbon concentration on demand.