Identification of the hexavacancy in silicon with the $B_{80}^4$ optical center

The ring hexavacancy $\left({\mathrm{V}}_6\right)$ has been found by previous theoretical modeling to be a particularly stable defect, but it has not been identified with any observed center to date. Here, we use ab initio calculations to derive the structure and properties of two forms of ${\mathrm{V}}_6{\mathrm{H}}_2$ and identify these defects with the trigonal optical centers $B_{41}$ and $B_{71}^1,$ which are known to contain two hydrogen atoms in equivalent and inequivalent sites, respectively. It follows from the calculations that ${\mathrm{V}}_6$ should also be optically active and we identify it with the $B_{80}^4$ $(J$ line) center. This allows us to place the acceptor level of ${\mathrm{V}}_6$ at $E_c-0.04\hspace{0.5em}\mathrm{eV}.$