Spin delocalization of interstitial iron in silicon

The apparent contradiction between a covalently delocalized picture of the Si:Feᵢ⁰ system, suggested among others by the large reduction of the central nucleus hyperfine interaction parameter as compared to the free ion, and the localized picture as has emerged from the analysis of a recent electron-nuclear double resonance (ENDOR) experiment, is resolved by a reinterpretation of the ENDOR data in a linear-combination-of-atomic-orbital treatment, that takes the spin multiplicity and symmetry properties of the paramagnetic (e)² state into account. This reinterpretation is confirmed by the determination of the relative signs of the ²⁹Si hyperfine interaction tensors in an ENDOR experiment under uniaxial stress. The data obtained are consistent with a 25% spin localization on the first six shells of silicon neighbors.