Dipole defects in γ-irradiated and thermally treated samples were studied by using thermally stimulated depolarisation current techniques and computer modelling. The presence of TSDC bands varied from sample to sample, and some crystals had none. In the spectra of spinels exhibiting TSDC peaks, bands at 130K, 160K and 320K were observed. The peaks at 130K and 160K were attributed to dipole defects. After 1200kGy of γ-irradiation, the broad band at 320K changed to 290K and increased 10-fold. Pulsed thermal treatment, between 350K and 470K, produced a progressive reduction in the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicated the possibility of a superposition of peaks. The γ-irradiation restored the 320K TSDC peak. Taking account of optical absorption and electron paramagnetic resonance results, thermal reduction of the 320K TSDC band was attributed to V-type centers; as a result of hole-trapping at tetrahedral and octahedral cation vacancies. Computer modelling, based upon lattice energy and defect minimisation, was used to identify the dipole defects that occurred in the crystals. Calculations were performed for normal and inverse spinel structures, doped with Cr, Co, Mn and Fe, in order to justify the presence of dipole bands.

Characterization of Dipole Defects in MgAl2O4 Spinel. R.P.M.Carvalhaes, M.S.F.da Rocha, S.S.de Souza, A.R.Blak: Physica Status Solidi C, 2005, 2[1], 413-7