Color-center formation and thermal recovery in X-ray and electron-irradiated magnesium aluminate spinel (original) (raw)
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Color centers inside crystallic active media
Laser Technology IV: Research Trends, Instrumentation, and Applications in Metrology and Materials Processing, 1995
This paper presents research results on color centers induced by radiation of a xenon lamp in non doped crystals of yttrium aluminum garnet Y 3 Al 5 0 12 (YAG), strontiumlanthanum aluminate SrLaAlO 4 (SLAO), strontium -lanthanum gallate SrLaGa 3 0 7 (SLGO), and in doped crystals: Nd: YAG, Cr,Tm,Ho: YAG (CTH:YAG), Nd: SLAO and Nd: SLGO.
Surface Science, 1999
Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F+ S (H ) (one-electron, paramagnetic) and F S (H ) (two electrons, diamagnetic). F S centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F+ S (H ), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H 2 heterolytic dissociative chemisorption (H 2 H++H−) and consequent H+ stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV-vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg 3 H ), which is built up by an array of three Mg2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.
Journal of Physics: Condensed Matter, 2006
We have used electron paramagnetic resonance to study the thermal annealing of colour centres induced in cubic yttria-stabilized zirconia by swift electron and heavy ionirradiations. Single crystals were irradiated with 1 or 2-MeV electrons, and 200-MeV 127 I, or 200-MeV 197 Au ions. Electron and ion beams produce the same colour centres: namely i) an F + -like centre, ii) the so-called T-centre (Zr 3+ in a trigonal oxygen local environment), and iii) a hole center. Isochronal annealing was performed up to 973 K. Isothermal annealing was performed at various temperatures on samples irradiated with 2-MeV electrons. The stability of paramagnetic centres increases with fluence and with a TCR treatment at 1373 K under vacuum prior to the irradiations.
Microscopic origin of the different colors displayed byMgAl2O4:Cr3+and emerald
Physical Review B, 2008
The difference in color between emerald ͑Be 3 Si 6 Al 2 O 18 :Cr 3+ , green͒ and the Cr 3+-doped spinel MgAl 2 O 4 ͑red͒ is striking, considering that in both systems color is due to CrO 6 9− complexes with a close local symmetry ͑D 3 and the D 3d , respectively͒ and that the measured Cr 3+-O 2− distance is practically the same ͑1.98Ϯ 0.01 and 1.97Ϯ 0.01 Å, respectively͒. By means of density-functional calculations it is shown that this surprising difference can reasonably be explained once the electric field, E R , which all lattice ions lying outside the CrO 6 9− complex exert on localized electrons, is taken into consideration. The origin of the different shape of E R in the two host lattices is analyzed in detail. It is shown that E R raises ͑decreases͒ the 2p͑O͒ levels for Be 3 Si 6 Al 2 O 18 :Cr 3+ ͑MgAl 2 O 4 :Cr 3+ ͒ along the trigonal axis thus favoring a decrease ͑increase͒ of 10Dq. The present work demonstrates the key role played by E R ͑not considered in the traditional ligand field theory͒ for understanding the differences exhibited by the same complex embedded in host lattices which do not have the same crystal structure. Some remarks on the color of Cr 2 O 3 pure compound are also reported.
Surface colour centres on magnesium oxide generated by magnesium and alkali-metal doping
Journal of The Chemical Society, Faraday Transactions, 1994
The addition of small amounts of low ionization energy metals to the surface of magnesium oxide has been studied by EPR spectroscopy. The metals investigated are magnesium and the alkali metals from lithium to rubidium. In the experimental conditions adopted the added metal atoms are ionized and the electrons trapped at the surface in the form of both one-electron (F,+) and two-electron (F,) colour centres. The F,+ paramagnetic centres formed are similar to those centres obtained by y-or UV-irradiation of the solid. The linewidth and saturation properties of the EPR spectra depend on the nature of the added metal, even though the nature of the interaction between the released electron and the parent cation is still unclear. A less intense line present in all spectra is attributed to subsurface F+ centres.
Colour centres in LiF : Mg crystals
Le Journal de Physique Colloques, 1980
Deux types de centres H° et un centre Hont été trouvés par RPE et absorption UV dans le vide, dans des cristaux de LiF : Mg irradiés aux rayons X. Les centres appelés Z 3 et Z 2 dans LiF : Mg ne peuvent pas être identifiés à aucun centre Z, ils sont plutôt en connexion avec l'impureté OH.
Journal of Applied Physical Science International, 2015 , V: 2, No: 4, pp. 137-144
This is a theses presented in the Conference Saratov Fall Meeting which was held 2015, 22-15 Sept, Saratov, Russia. In the original article (Zayko Y.N., Journal of Applied Physical Science International, 2015 , V: 2, No: 4, pp. 137-144) issues related to the introduction of the concept of color temperature used in the colorimetric measurements are briefly discussed. An ambiguity of existing definition of color temperature is stressed. An unambiguous definition of color temperature and the technique of its implementation are suggested. Also a physical interpretation of the RGB color model is sug-gested which introduces the carriers of colors - a complexes of photons. The thermodynamics of these complexes is studied.
Physical Review B, 2004
Single crystals of LiF exposed to swift heavy ions respond by the creation of color centers and defect aggregates. We present a comprehensive study by means of optical absorption spectroscopy using various MeV-GeV ions from 4 He to 238 U and a broad range of fluences. In the single-track regime, the defect characteristics such as the F-center concentration as a function of fluence and energy loss are analyzed. At large fluences, track overlapping occurs and the damage process is dominated by the formation of complex F n centers and defect aggregates. The evolution of F-center and defect clusters is discussed in terms of aggregation as well as recombination of electron and hole centers. Limited efficiency for defect creation with heavy ions is mainly ascribed to annihilation processes of electron and hole centers.
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.