Optical absorption, paramagnetic resonance and depolarisation currents in MgAl₂O₄ spinel

Abstract

Optical Absorption (OA), Electron Paramagnetic Resonance (EPR) and Thermally Stimulated Depolarisation Currents (TSDC) techniques were applied to study the effects of irradiation and thermal treatments in the formation, aggregation and destruction processes of dipole defects in MgAl 2 O 4 spinel. Irradiated MgAl 2 O 4 crystals present two OA bands centred at 3.4 eV and 5.1 eV. The 3.4 eV band increases with the irradiation-dose, stabilizes its maximum height for doses near 10 kGy and is completely destroyed for thermal treatments above 500 K. This same band decays when the sample is maintained at room temperature reaching a constant value after a few weeks. The EPR spectrum is composed by two superimposed bands at g = 2.011. The gamma dose dependence, the thermal treatments between 370 K and 500 K and the thermal decay of the two bands at room temperature, show that each band behaves in a different way thus indicating that they are associated with two different defects. Gamma dose of 10 kGy produces a TSDC band at 245 K. When the sample is maintained at room temperature, after the gamma irradiation, for several weeks, a displacement in the peak position to 290 K, is observed. Thermal treatments above 500 K destroy the 290 K band. This band is associated to at least two V-type centres.

Associated Aspects on Structure, Morphology and Photoluminescence of MgAl2O4:x% Gd3+ Nanophosphor Prepared via Citrate Sol–Gel Method

“…The band at 225 nm (5.51eV) is attributed to band-to-band transition and the motive being the fact that the band gap (Eg) of MgAl2O4 can range from 5.1 -5.8 eV. 35,36 The 275 nm excitation is attributed to be due to the host defects for the un-doped sample, 23 which is surely not the case for the Gd 3+ doped samples (only if we assume that the Gd 3+ ground state is located at the edge of the host valence band (VB) as shown in Fig. 10) and this will be explained later in this paper.…”