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PACS 61.80.Ed, 61.82.Ms Dipole defects in gamma-irradiated and thermal treated MgAl 2 O 4 samples have been studied through thermally stimulated depolarisation currents(TSDC) technique and computer modelling methods. The presence of TSDC bands varies from sample to sample and some crystals do not present any band. The origin of these bands has been investigated in several different samples. In the spectra of spinels showing TSDC peaks, three bands at 130K, 160K and 320K are observed. The peaks at 130K and 160K have been attributed to dipole defects. After 1200kGy of gamma irradiation the broad band at 320K dislocates to 290K and increases ten times. Pulsed thermal treatments between 350K and 470K produce a progressive reduction of the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicates the possibility of a superposition of peaks. Gamma irradiation restores the 320K TSDC peak. Taking into account optical absorption(OA) and electron paramagnetic resonance(EPR) results, the thermal reduction of the 320K TSDC band was attributed to V-type centres as a result of hole trapping at tetrahedral and octahedral cation vacancies. Computer modelling methods, based on lattice energy and defect minimisation, were applied to identify dipole defects that occur in these crystals. The calculations were made in normal and inverse spinel structures, doped with Cr, Co, Mn and Fe in order to justify the presence of dipole bands.

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A static computer simulation using the GULP code has been used to study dipole defects in iron doped spinels in normal and inverse structures. The occupation of tetrahedral (Mg2+) and octahedral (At') sites in the normal structure was simulated and the results indicate a preferential replacement of MgZ-by an AI3+ justified by the observed stoichiometric deviation in synthetic spinels. Besides this, A1zO3 excess produces Mg2+ vacancies and favours the hole centre production. Antisite disorder of the form [Mg2+]~:+ and [Al3+IM:+ is one of the possible mechanisms responsible for the existence of dipole defects in these materials. The obtained energy for the interchange between A13+ and Mg2+ was 0.52eV The calculated energies indicate that Fez+ replaces Mgz+ and Felt replaces A13' . The presence of Fe3+ in octahedral sites takes into account the oxidation process of Fez+ in these sites. Computer simulation has also been carried out on the inverse structure where the stability of the lattice was obtained for the Imma space group.