Color-center formation and thermal recovery in X-ray and electron-irradiated magnesium aluminate spinel (original) (raw)
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MRS Proceedings, 1999
High angular resolution electron channeling x-ray spectroscopy (HARECXS) was examined as a practical tool to locate lattice-ions in spinel crystals. The orientation dependent intensity distribution of emitted x-rays obtained by HARECXS is so sensitive to lattice-ion configuration in the illuminated areas that the occupation probabilities on specific positions in the crystal lattice can be determined accurately through comparison with the theoretical rocking curves. HARECXS measurements have revealed partially disordered cation arrangement in MgO·nAl2O3withn= 1.0 and 2.4. Most A13+lattice-ions occupy the octahedral (VI) sites with 6-fold coordination, while Mg2+lattice-ions reside on both the tetrahedral (IV) and the octahedral (VI) sites. The structural vacancies are enriched in the IV-sites. Further evacuation of cations from the IV-sites to the VI-sites is recognized in a disordering process induced by irradiation with 1 MeV Ne+ ions up to 8.9 dpa at 870 K.
Radio-Luminescence of Defects and Impurity Ions in Magnesium Aluminates Spinel Crystals
Solid State Phenomena, 2013
The investigations of radio-luminescence (RL) in magnesium aluminates spinel crystals at variation of the time, intensity of X-irradiation and temperature of sample were provided. There were registered three prominent RL bands related to electron-hole recombination process at anti-site defects, emission of Mn 2+ -and Cr 3+ -ions. The kinetics of the growth of indicated RL emissions show the competing processes of the capture of free charge carriers generated at irradiation by intrinsic defects and impurity ions.
Vibronic photoexcitation spectra of irradiated spinel MgO.nAl2O3(n=2) at low temperatures
2013
A vibronic photoexcitation band at approximately 230 nm was found at 13 K in reactor neutron- and electron-irradiated magnesium aluminate spinel (MgO.nAl2O3). Vibronic structure was found to be temperature dependent and became obscure at over 120 K. Huang Rhys factor S and Debye Temperature D were estimated from the temperature dependence of the 230 nm band by curve-fitting method using Debye approximation. Origin of the vibronic photoexcitation band was suggested as F center.
Radiation-induced luminescence in magnesium aluminate spinel crystals and ceramics
Radioluminescence (RL) and thermoluminescence (TL) in spinel crystals and ceramics were investigated to elucidate the radiationinduced electronic processes in single crystals grown by Verneuil and Czochralski methods as well as transparent and translucent ceramics. Both RL and TL spectra demonstrate a UV-band related to electron-hole recombination luminescence at intrinsic defects; green and red luminescence are identified with emission of Mn 2+-and Cr 3+-ions, respectively. The kinetics of growth of different RL luminescence bands depending on dose at the prolonged X-irradiation shows the competitive character of charge and energy transfer between defects and impurity ions. The dependence of RL intensity on the temperature of the sample was measured in the range of 300-750 K and compared with TL for different emission bands. The variety of maxima in the temperature dependence of RL and in the glow curves of TL measured for different luminescence bands in spinels of different origins and crystalline forms is used to show that charge carrier traps and luminescence centers are not isolated defects but are complexes of defects and impurities. The formation, structure and properties of these complexes depend on the processing conditions.
Optical characteristics of virgin and proton-irradiated ceramics of magnesium aluminate spinel
Optical Materials, 2019
Intrinsic properties of optical ceramics of MgAl 2 O 4 have been studied by means of low-temperature cathodo-, photo-and thermoluminescence methods. Based on the excitation spectra for different emissions measured at 6 K, the UV luminescence at 4.5-5.8 eV is tentatively ascribed to bound excitons (formation energy about 7 eV) near antisite defects (cation in a "wrong" position) and electron-hole recombination in a spinel matrix. The complex UV luminescence band peaked around 5 eV undergoes thermal quenching from 6 to 200 K. The value of energy gap is experimentally estimated as 8.2 eV at 80 K. There is no saturation of the absorption connected with the radiation-induced F-type color centers at the rise of 100-keV proton fluence up to 7 × 10 17 cm −2 , while such irradiation is accompanied by the drastic suppression of cathodoluminescence, especially in UV spectral region. A further study of the origin of this luminescence attenuation still lies ahead.
Optics and Spectroscopy, 2011
Using the density functional method in the pseudopotential approximation, we calculate the probabilities of X ray emission transitions and shapes of the L 2,3 X ray emission bands of Mg in crystals of metallic magnesium and monoxide MgO. We have paid special attention to the study of mechanisms by which these bands are formed; therefore, along with the total intensities, we determine the partial s and d contributions. In addition, the intensities of the L 2,3 bands of Mg have been separated into contributions from direct (intraatomic) transitions and charge transfer cross transitions. We show that an unexpectedly large contribution of partial d transitions in magnesium oxide is caused by cross transitions of electrons of valence states of ligands to the core 2p level of Mg, i.e., charge transfer transitions. In metallic magnesium, the con tribution of these transitions proved to be insignificant. An appreciable contribution of cross transitions has also been revealed in s contributions of X ray emission bands. To estimate the calculation accuracy, we have compared the shapes of theoretical spectra with the shapes of experimental L 2,3 X ray emission bands of Mg.
Journal of Nuclear Materials, 2005
Electron paramagnetic resonance (EPR) and optical absorption spectra were measured during thermal annealing of stoichiometric MgAl 2 O 4 spinel that was previously irradiated in the Materials Open Test Assembly in the Fast Flux Test Facility (FFTF/MOTA) at %680 K to %50 dpa. Both F and F + centres are to persist up to very high temperatures (over 1000 K) suggesting the operation of an annealing mechanism controlled by the thermal stability of extended defects. Using X-ray irradiation following the different annealing steps it was shown that an optical absorption band at 37 000 cm À1 is related to a sharp EPR band at g = 2.0005 and that the defect causing these effects is the F + centre.
Luminescence characteristics of magnesium aluminate spinel crystals of different stoichiometry
IOP Conference Series: Materials Science and Engineering, 2019
Magnesium aluminate spinel single crystals with different stoichiometry, MgAl2O4 (1:1 spinel) and MgO•2.5Al2O3 (1:2.5) were investigated using different optical methods (cathode-, photo-and thermally stimulated luminescence (TSL), optical absorption, "creation spectra" of TSL peaks and phosphorescence by VUV radiation). Low-temperature charge carrier traps and the position of intrinsic UV emission bands depend on the degree of stoichiometry. Antisite defects (ADs), Mg 2+ or Al 3+ located in a "wrong" cation site (Mg|Al or Al|Mg) are the main as-grown structural defects, which serve also as efficient traps for electrons and holes as well as seeds for bound excitons. AD concentration is especially high in 1:2.5 spinel. There are several manifestations of ADs (electronic excitations near ADs) in the spectral region of 77.5 eV, slightly below the energy gap.
Scientific Reports, 2020
MgAl2O4 spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F+) and hole (V) centers in stoichiometric MgAl2O4 spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAl2O4 single crystals and optical polycrystalline ceramics are compared. It is demonstrated that both transparent ceramics and single crystals, as well as different types of irradiation show qualitatively similar kinetics, but the effective migration energy Ea and pre-exponent D0 are strongly correlated. Such correlation is discussed in terms of the so-called Meyer-Neldel rule known in chemical kinetics of condensed matter. The results for the irradiated spinel are compared with those for sapphire, MgO and other radiation-resistant materials.
Charge transfer mechanisms in the thermoluminescence of MgO
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1984
The opt&l and thermal stability of the optical absorption spectrum and the~o~u~n~nce (TL) of as-cleaved MgO samples subjected to ionizing radiation at room temperature has been studied. UV irradiation leads to V-center formation when tight is absorbed in the Fe'+ absorption bands. Under subsequent illumination in the V-band, Fe3+ ions are again formed. X or y irradiation increase the Fe3+ band (4.3 ev) to a saturation value and produce the V band. The same three glow peaks at-93,175 and 220°C are observed in X, y or UV irradiated samples. Two maxima at-4.3 and 5.1 eV are found in the TL excitation spectrum. It is suggested that, as expected, cation vacancies charge compensate for the Fe3+. rons. Charge and excitation transfer between both hole and electron traps and the impurity activators Fe, Cr, Mn and Ni determine the observed TL emission.