Defects in diamond: The unrelaxed vacancy and substitutional nitrogen

We report self-consistent Green's-function calculations of two defects in diamond. The unrelaxed neutral vacancy and unrelaxed substitutional nitrogen are compared. The vacancy shows a partially empty $T_2$ state at 1.53 eV above the valence edge which should in analogy to silicon drive a Jahn-Teller distortion. Substitutional nitrogen induces a "hyper-deep" $A_1$ state below the diamond valence band and an antibonding $A_1$ state just below the conduction-band edge. The situation is similar to substitutional oxygen in GaP. Our calculations thus contradict the so-far-accepted picture of a $T_2$-state Jahn-Teller instability driving the off-center distortion of nitrogen. We believe that the observed distortion is a result of strong chemical rebonding that cannot be described by a simple Jahn-Teller effect.