Experimental optical properties of lattice vacancy defects in single crystal α-Al 2 O 3 are reviewed. Specific ultraviolet absorption and luminescence bands are associated with F-type centers. Following extended exposures to epithermal neutron sources, photobleaching and anisotropic properties allowed the association of several of the visible and near infrared bands with F 2 -type centers. Photoconductivity, photostability and charge state considerations provided tentative placement of defect energy levels with respect to the conduction band edge.

Optical properties of lattice defects in α-Al2O3

Comparative study of the photoluminescence performance and photocatalytic activity of CeO2/MgAl2O4 composite materials with an n-n heterojunction prepared by one-step synthesis and two-step synthesis methods

MgAl2O4 with a spinel structure was successfully synthesized using a gamma-ray irradiation assisted polyacrylamide gel method. The samples were synthesized at different calcining temperatures to study the effects on the phase purity, optical, color and fluorescence properties and photocatalytic activity of the as-prepared samples. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis indicates that the calcining temperature has an obvious influence on the crystallinity and the formation of the MgAl2O4 spinel phase. Transmission electron microscopy (TEM) observation shows that the primary crystal of the MgAl2O4 nanoparticles is only 30 nm and exhibits a higher uniformity than those of MgAl2O4 nanoparticles fabricated using the conventional polyacrylamide gel method. Optical properties show that the optical energy gap (Eg) of the MgAl2O4 nanoparticles increases with the increasing calcining temperature, mainly due to the interaction between the N–O and C–O functional groups and the calcining temperature. An abnormal result for the color parameter of the MgAl2O4 xerogel powders calcined at 900 °C indicates the enhancement of the crystallinity and the phase purity of the MgAl2O4 nanoparticles. The fluorescence spectra indicates that the emission intensity of the emission peak at 395 nm decreases with the increasing calcining temperature. The photocatalytic activity of the MgAl2O4 nanoparticles prepared at different calcining temperature were then tested for photocatalytic degradation of the methylene blue (MB) dye. The result indicates that the MgAl2O4 xerogel powders calcined at 800 °C exhibit the highest photocatalytic activity, and that this is relevant to the light absorption ability, band edge position, separation and utilization efficiency of the photogenerated carriers of the as-prepared sample. The fluorescence and photocatalytic mechanisms show that a small number of N–O and C–O functional groups in the presence of the MgAl2O4 nanoparticles can help to improve the fluorescence properties and photocatalytic activity of the MgAl2O4 nanoparticles.

Insight into the optical, color, photoluminescence properties, and photocatalytic activity of the N–O and C–O functional groups decorating spinel type magnesium aluminate

CALCULATIONS OF THE GEOMETRY AND OPTICAL-PROPERTIES OF F-MG CENTERS AND DIMER (F-2-TYPE) CENTERS IN CORUNDUM CRYSTALS

Calculations of the geometry and optical properties of F Mg centers and dimer ( F 2 -type) centers in corundum crystals

The colour centres introduced in sapphire crystals by neutron irradiation and by thermochemical treatment exhibit polarized luminescence. In neutron-irradiated crystals an absorption band at 4.1 eV and luminescence bands at 2.45 and 3.8 eV are due to one type of centres, most probably interstitial aluminium ions. Other donor-type centres give rise to an absorption band at 2.75 eV and a luminescence band at 2.2 eV. In magnesium containing crystals similar centres absorbing at 2.93 eV and emitting at 2.35 eV can be introduced by thermochemical treatment in reducing atmosphere. The preliminary study of the red luminescence indicates its connection with the absorption bands at 3.45 and 3.8 eV.



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Visible Luminescence of Colour Centres in Sapphire

In Mg- and Ca-doped α-Al2O3 crystals irradiated by X-rays at 80 K new paramagnetic colour centres are observed having an absorption band at about 255 nm and an emission band at 310 nm. The hyperfine structure of the EPR spectrum indicates that the unpaired electron spin interacts mainly with two Al27 nuclei. The optical and EPR spectra are consistent with the model of a centre involving one electron trapped at an anion vacancy adjoining the substitutional divalent ion (FMg or FCa). The luminescence band at 290 nm observed in the TSL peak at 225 K is suggested to result from the recombination of the holes with the FMg(FCa) centres.



[Russian Text Ignored]

Impurity-Associated Colour Centres in Mg- and Ca-Doped Al2O3 Single Crystals

The luminescence under X-ray excitation (XL) and the thermostimulated luminescence (TSL) in Al2O3 crystals are studied in the spectral region 2 to 6 eV at temperatures between 80 and 700 K. In crystals treated in reducing atmospheres at high temperatures a luminescence band at 3.0 eV is dominating the XL and TSL spectra. This band is also observed in spectra of neutron-irradiated crystals together with bands at 3.75 and 2.4 eV. In Mg-doped crystals a number of bands are observed peaking at 3.15, 3.75, 4.0, 4.4., and 4.75 eV. The luminescence band at 3.75 eV can be due to F+ centres. The band at 3.0 eV is connected with some donor type defects. In all the crystals studied (except the untreated one) one or both of the main TSL peaks at 225 and 260 K are observed. The first one is ascribed to release of holes, the other to electron release from some donor centres.



[Russian Text Ignored].

Recombination luminescence in single crystal Al2O3

The thermostimulated luminescence (TSL) of vacuum-annealed Al2O3 crystals is studied after selective photostimulation at LNT with light corresponding to the absorption bands of electron or hole centres. The comparison of the results obtained on as-received crystals and crystals irradiated by X-rays at 480 K before photostimulation at LNT leads to the conclusion that the TSL peaks at 100, 260, 325, and 360 K are due to electron recombination with localized holes, and the peaks at 225, 280, 370, and 415 K are due to hole recombination with localized electrons. The luminescence band at 3.0 eV is observed both, in the electron and hole recombination processes. It is suggested that the TSL after photostimulation is mainly of monomolecular character.



[Russian Text Ignored].

On the mechanism of the recombination luminescence of α-Al2O3 crystals with nonstoichiometric excess of aluminium

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Impurity-Associated Colour Centres in Mg- and Ca-Doped Al2O3 Single Crystals

Recombination luminescence in single crystal Al2O3

On the mechanism of the recombination luminescence of α-Al2O3 crystals with nonstoichiometric excess of aluminium