Optical characteristics of virgin and proton-irradiated ceramics of magnesium aluminate spinel (original) (raw)
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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.
Effects of Ta doping and irradiation with He+ ions on photoluminescence of MgAl2O4 spinel ceramics
Journal of the European Ceramic Society, 2020
Photoluminescence (PL) properties of pristine and Ta-doped MgAl 2 O 4 spinel ceramics prepared via spark plasma sintering technique and irradiated with He + ions were studied. The results indicate strong influence of the grain boundaries on PL spectra. Ta doping promotes the formation of O and Al vacancies at the grain boundaries leading to an increased number density of F + centres. The ionised irradiation forms antisites preferentially at the grain boundaries, which inhibit excitonic PL and exciton energy transfer while do not affect proper PL of lattice defects. A weak PL excitation band at 7.25±0.25 eV may belong to excitons localised near bulk antisites. In the Ta-doped ceramics, the electronic transitions between 5.75 eV and 7.0 eV belong to an intermediate state situated at the grain boundaries and structurally linked to Ta, which readily transfers energy to F and F + centres; it was assigned to the nucleated Mg 4 Ta 2 O 9 phase.
Radiation induced optical centers in magnesium aluminate spinel ceramics
There were investigated the optical absorption centers formation in magnesium aluminate spinel ceramics under irradiation with UV-light, X-, and gamma-rays. The lithium fluoride doped ceramics were produced by using hotpressing technology. It was revealed that generation by irradiation changes in optical absorption spectra can be used for detection of invisible point defects in prepared ceramics, their distribution through the bulk of spinel disk, and predict the behavior of ceramics in different radiation fields.
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.
Time- and Temperature-Dependent Luminescence of Manganese Ions in Ceramic Magnesium Aluminum Spinels
Materials
Samples of magnesium aluminum spinel ceramics doped with manganese ions were prepared by a high-temperature solid-state reaction method; their potential as red-emitting phosphors was analyzed using a time-resolved luminescence spectroscopy technique, from room temperature to 10 K. It was found that in the red spectral range, the luminescence spectra of manganese ions in the MgAl2O4 spinel showed a narrow band peaking at 651 nm due to the emission of Mn4+ and a broader emission band in the region of 675 ÷ 720 nm; the ratio of intensities for these bands depends on the synthesis conditions. By applying a special multi-step annealing procedure, the MgAl2O4:Mn4+ phosphor containing only tetravalent manganese ions, Mn4+, was synthesized. Broad-band far-red emission observed from MgAl2O4:Mn and Mg1.25Al1.75O3.75F0.25:Mn phosphors, prepared by a conventional method of a solid-state reaction, was interpreted as coming from Mn3+ ions.
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.
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.
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.
Journal of Applied Physics, 2018
We have studied the formation of color centers in magnesium aluminate spinel (MgAl 2 O 4) by X-ray and electron irradiations near room temperature (RT). For this purpose, Cu Kα radiation and three electron energies (1.0, 1.4, and 2.5 MeV) were used for variable fluences (up to 4.2 × 10 18 cm −2). Offline UV-visible absorption spectra were recorded at RT as well as at low temperature down to 27 K after electron irradiation. The dependence of the production rate of F centers (oxygen vacancies) on the electron energy yields a threshold displacement energy of 190 ± 10 eV for oxygen atoms at RT, which is much higher than the determinations by molecular-dynamics simulations. Such a discrepancy is discussed on the basis of available migration data of point defects in spinel. Equipartition of F 0 and F + centers, i.e., the neutral (V O x) and singly ionized (V O .) oxygen vacancies, is reached for high electron fluences. Moreover, the evolution of the width of color-center absorption bands versus temperature is interpreted with the classical theory for F centers (neutral halogen vacancies) in alkali halides. The Stokes shifts are deduced from the temperature dependence of the absorption bandwidths of color centers like for alkali halides and alkaline-earth oxides. Finally, isothermal annealing data for long annealing time show a non-zero asymptotic behavior for both F 0 and F + centers. This uncommon behavior is interpreted by charge exchange processes leading to an equilibrium state between those two color centers.