Characterization of dipole defects in MgAl ₂ O ₄ spinel

Abstract

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.

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

“…In an equimolar synthetic spinel, due to their smallest energy increase in comparison with other calculated point defect energies, 16–20 the predominant intrinsic defects are those resulting from site exchange of cations (so‐called antisite defect pair). Furthermore, the Schottky defect has a smaller energy than either cation or anion Frenkel defects.…”

“…There are actually no reliable data or direct calculations for these formation‐free energies of intrinsic defects relative to the grain boundary. However, as the sign of the space‐charge potential is only necessary in our analysis, we have used the results of the recently calculated formation energies of these defects, relative to the lattice 17–20 . These calculations indicate the preferential replacement of Mg 2+ by Al 3+ in the normal structure (the calculated energy for Al 3+ replacing Mg 2+ is 2.3 eV and that for Mg 2+ replacing Al 3+ is 4.6 eV).…”

“…In the hyperstoichiometric spinel, MgO· n Al 2 O 3 (molar ratio n >1), excess point defects, as substitutional cations, are involved in the interchange of three Mg 2+ and two Al 3+ on tetrahedral sites. One would think that a tetrahedral vacancy should be left, as suggested by defect simulations, 17–20 but it was shown earlier from X‐ray measurements 22 that it is actually an octahedral vacancy that is more likely to form. The question of whether these charge‐compensating cation vacancies reside on octahedral sites, 22,23 tetrahedral sites, 24 or both 25 remains open.…”

“…As we have discussed previously, there are no reliable data or direct calculations for ; thus, to estimate this energy, we have used a method already described in the study of ionic double layers at the surface of Mg‐doped alumina 28,29 . From the theoretical calculations of the Schottky defect energies published by several authors 16–20 , we have determined taking the ratio and to be, respectively, equal to the lattice Al 3+ upon O 2− and Mg 2+ upon O 2− formation energy ratios. Finally, we have retained the lowest obtained value .…”

Optical absorption, paramagnetic resonance and depolarisation currents in MgAl2O4 spinel

“…Thermal treatments above 500 K destroy the 290 K peak and a broad band at 320 K is then observed. The calculated activation energies [7] indicate that the 290 K band results from a single process. All the studied spinel samples show EPR bands centred at g = 2.011.…”