Quadrupole splitting of Mossbauer lines due to defects in CoO

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Abstract
The Fe3+ lines observed in CoO Mossbauer sources may arise from the decay of Co3+ ions associated with cation vacancies in the crystal. These defects produce an electric-field gradient that causes a quadrupole splitting of the resonance line and that can, in principle, distinguish between different types of defects. The calculation of the quadrupole splitting at Fe2+ and Fe3+ sites near various vacancy clusters includes the relaxation of the lattice about the defect. This lattice polarisation and distortion is shown to be extremely important, since simple calculations based on perfect ion positions give very different field gradients at neighbouring sites. The results compared with the experiments available and the quadrupole splittings observed are close to those predicted by a vacancy model.