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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This work presents the study of dipole defects in aluminum and magnesium oxide MgAl 2 O 4 using electronic paramagnetic resonance (EPR), optical absorption (OA) and thermally stimulated depolarization currents techniques (TSDC). The R samples TSDC spectrum presented 3 peaks, two of them centered at 160 K and 130 K related to dipolar defects and another peak at 320 K with no dipolar characteristics. Gamma irradiation creates an intense peak at 245 K that is unstable at room temperature and has its maximum dislocated to 290 K when held at this temperature 20 days long. This band is destroyed for thermal treatments above 460 K. First order theoretical models were unable to describe the whole TSDC spectrum behaviour. This result, associated to the TSDC band symmetry and to the modification in the peak position, indicates that the TSDC band at 245 K is formed by the sum of more than one band. Gamma irradiation created a wide OA band at 3 eV associated to V type centers and other bands in the 5 eV region associated to F type centers. The band centered in the 3 eV region is unstable at room temperature, having its maximum reduced when held at this temperature for 12 days long. The 3 eV band is completely destroyed after thermal treatments at 570 K for 5 minutes.