The mystery of the missing neutral Si vacancy in 3C-SiC solved

The simple silicon vacancy is an iconic defect in 3C-SiC whose full characterization has thus far proven an inscrutable challenge. The negatively charged v(1-) is well-understood, experiment and theory converging to a high-spin quartet with T_d structure. For v(0), however, standard DFT predicted a high-spin triplet (Torpo, 1999) that, despite a Jahn-Teller degeneracy, remained an undistorted T_d. This conflicted with experiments (e.g., Itoh, 1997; Son 1997) which found no such T_d triplet, nor other unambiguous signature of a v(0). This absence prompted use of more exotic electronic structure approaches (Deak, 1999; Zywietz, 2000), which found a lower energy singlet-spin v(0), in principle invisible to spin-sensitive experiments such as EPR. We find a lower energy yet for a high-spin triplet v(0) distorted from T_d, confirmed in extrapolations from supercells up to large 1000-sites using DFT/GGA. A new manifestation of finite cell error is diagnosed which, when remedied, yields an energy lowering dominated more by an electronic than a structural distortion, prompted by the Jahn-Teller-like degeneracy. This gives a description of the v(0) more consistent with observations (or lack thereof). --- SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525