Interstitial aggregates and a new model for the I₁/W optical centre in silicon

First principles local-density-functional (LDF) theory is employed to investigate the properties of di-interstitial (I₂), tri-interstitial (I₃) and tetra-interstitial (I₄) structures in silicon. We show that a tri-interstitial defect can account for many of the fundamental properties of the I₁/W-optical centre which is observed in irradiated, annealed silicon.