  The role of carbon in the formation of alpha boron at high pressure and high
temperature (HPHT) has been investigated by combining HPHT experiments and
density functional theory (DFT) calculations. Starting from beta rhombohedral
or amorphous boron and amorphous carbon at 5 GPa, the alpha boron phase has
been repeatedly observed between 1473 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 alpha rhombohedral
boron atomic structure on the formation enthalpy, volume and optical properties
shows that only the (B11C) substituted icosahedron defect accounts for the
slight volume contraction observed in {\alpha} boron. It is also compatible
with the observed red colour of {\alpha} boron crystals, and consistently has a
very low formation energy. Our calculations also provide a calibration tool to
evaluate the carbon concentration in the samples. Finally, the synthesis
temperature is found to be an easy means to tune the carbon concentration on
demand.
