Bistability of oxygen vacancy in silicon dioxide

I report total-energy electronic-structure calculations within the local density approximation in density functional theory performed for oxygen vacancy V in α-quartz. I find (i) that the O vacancy has a variety of charge states from V++ to V- depending on the location of the electron chemical potential in the energy gap, (ii) that for V0 and V+, the O vacancy shows structural bistability, and (iii) that a set of charge states {V++, V+ V0} is an effective negative-U system which is accompanied by a charge-state dependent structural transformation.