The creation and thermal annealing of three novel paramagnetic defects have been studied in MgO:Al single crystals exposed to fast fission neutrons. According to the analysis of the EPR spectra, these radiation-induced defects with spin S = 1/2 and S = 1 possess several features (the same EPR linewidth, hyperfine structure due to an s-electron interaction with a neighboring 25Mg, signal saturation at very low values of microwave power) typical of the classic F+ center (an electron trapped in an oxygen vacancy) and the similar thermal stability up to about 700 K. At the same time and in contrast to the F+ center, all defects have axial symmetry and involve an additional cation vacancy, vMg. Тhe Fd1+ center contains the F+ and vMg in the neighbor cation site along the <111> axes; in the Fd2+ center, the F+ and vMg in the next-neighbor cation position are located along the <100> axes. The trivacancy F+-dimer center with S = 1 is also oriented along the <100> axes and an vMg is located just between two F+ centers.

Single crystals of MgO and alpha-Al2O3 have been irradiated by either swift heavy ions with varying fluence (70-MeV Ar and 231-MeV Xe ions, from 5 & times; 1011 to 2 & times; 1014 cm(-2)) or fast fission neutrons. The evolution of oxygen-related Frenkel defects (interstitial-vacancy pairs) - their accumulation with irradiation fluence and subsequent isochronal annealing - has been studied by means of the optical absorption, EPR and luminescence methods. The features in the EPR characteristics of the F+ and O-2(-) (involves a charged oxygen interstitial) Frenkel defects (in particular, significant broadening of the relevant EPR lines, signal dependence on microwave power) as well as in the annealing kinetics of the oxygen-vacancy-containing F and F+ centers have been revealed and discussed. The effect of irradiation on the intrinsic excitonic emission (around 7.6 eV), F+-center luminescence and EPR/luminescent characteristics of single Cr3+ centers in corundum have been considered.

The creation and thermal stability of radiation-induced defects in nominally pure Y3Al5O12 single crystals exposed to 230-MeV xenon ions with fluence from 1011 up to 5 & times; 1013 cm-2 have been investigated using optical and EPR methods. The defects responsible for the radiation-induced absorption at 3-5.5 eV clearly demonstrate their radiation origin and are mainly connected with oxygen sublattice. The multistage thermal annealing of oxygen-vacancy-related F and F+ centers occurs at 400-900 K via the mutual recombination with complementary and becoming mobile oxygen interstitials. The annealing of more complex and mainly cation-related defects with the absorption above 5.5 eV (incl. antisite defects) takes place above 1000 K. The manifestations of the as-grown and radiation-induced defects in the EPR and time-resolved cathodoluminescence spectra are considered.

Luminophores based on doped Li2B4O7 have been studied by a variety of methods (photoluminescence, pulsed cathodoluminescence and kinetics thereof, thermoluminescence, and electron paramagnetic resonance). Different impurities exhibit both interplay and competition. The interplay of impurities is expressed in the luminescence sensitization due to effective excitation energy transfer which occurs in composite impurity centers. Such centers consist of two differently incorporated impurities, one at a cationic site, another imbedded in the anionic structure. The impurities can compete for a particular position in the crystal lattice, and the priority of doping affects the result. The luminescence and thermoluminescence properties observed in the samples are connected with the structure of impurity centers. It is shown that the loss of trapped charge carriers due to instant recombination on the luminescence centers and low-temperature thermoluminescence (below 400 K) diminishes the luminescence yield for the high-temperature (above 400 K) thermoluminescence peak used in dosimetry.

A set of Mg1-xZnxAl2O4 spinel powders with varying Mg-Zn composition of the first cation have been fabricated via a facile low-temperature sol-gel citrate route and subsequent annealing at different temperatures, Tann = 800-1200 degrees C. Cubic spinel crystal structure, precise values of the lattice parameters and oxygen coordination have been controlled by the XRD method. The spectra of cathodoluminescence at 1.5-6.1 eV have been measured within short and long time-windows (in particular, of 0-32 ns and 0-2 ms) under the excitation of Mg1-xZnxAl2O4 powders by 100-keV electron pulses at 6 K. The evolution of typical impurity emissions (traces of uncontrolled Cr3+, Mn2+ impurity ions) and intrinsic UV luminescence bands with Mg-Zn composition and additional annealing temperature has been considered. A tentative correlation between fast (decay time about 2 ns) emissions around 3 eV and isotropic EPR signals in the selected samples has been analyzed.

The recovery of radiation damage induced by 231-MeV xenon ions with varying fluence (from 5 × 1011 to 2 × 1014 cm−2) in α-Al2O3 (corundum) single crystals has been studied by means of isochronal thermal annealing of radiation-induced optical absorption (RIOA). The integral of elementary Gaussians (product of RIOA spectrum decomposition) OK has been considered as a concentration measure of relevant oxygen-related Frenkel defects (neutral and charged interstitial-vacancy pairs, F-H, F+-H−). The annealing kinetics of these four ion-induced point lattice defects has been modelled in terms of diffusion-controlled bimolecular recombination reactions and compared with those carried out earlier for the case of corundum irradiation by fast neutrons. The changes in the parameters of interstitial (mobile component in the recombination process) annealing kinetics—activation energy E and pre-exponential factor X—in ion-irradiated crystals are considered.

Basic properties of the simplest radiation defect, the F + center (an oxygen vacancy with a trapped electron), in the simplest oxide material (MgO with a cubic lattice structure) remain for a long time a subject of discussion, in light of highly contradictory literature data. In this paper, new data on the electron paramagnetic resonance (EPR) and optical absorption measurements for the neutron- and energetic-ion-irradiated (70 MeV Ar) MgO single crystals are discussed. Theoretical analysis of the diffusion-controlled F + center isochronal annealing is performed; the obtained migration energies for oxygen interstitials are discussed in light of the available calculations on the mobility of both neutral and charged oxygen interstitials in metal oxides.

Single crystals of MgO have been exposed to 70-MeV argon ions with varying fluence of 4 x 1012-3 x 1014 cm-2. The dependence of radiation-induced optical absorption (RIOA) at 1.7-6.5 eV on irradiation fluence has been analyzed. The EPR signal of the F+ center in ion-irradiated crystal has been detected and via the EPR parameters proved to be the same as in well-studied neutron-irradiated MgO crystals. The precise isothermal annealing of the F + EPR signal has been performed for the first time in a temperature range of 400-1100 K for both a fast neutron irradiated (2.7 x 1018 cm-2) and Ar-irradiated (3 x 1014 cm-2) MgO crystals. In both cases, the F + EPR decay starts only above 700 K and ends at 950-1060 K, depending on irradiation type. Using the same stepwise annealing procedure, the changes in RIOA for ion-irradiated samples have also been analyzed and possible reasons for the discrepancy in the thermal behaviour of optical and EPR F + -absorption have been considered.

Single crystals of alpha-Al2O3 with broad sides oriented perpendicular to the c crystal axis have been irradiated by 231-MeV xenon ions with fluence varying from 5 x 1011 to 1014 ions/cm2. The spectra of radiation-induced optical absorption (absorption of a pristine crystal is subtracted) have been decomposed into Gaussians serving as a measure of oxygen-related Frenkel defects (interstitial-vacancy pairs). The concentration of all structural defects considered - vacancy-type F and F + centers as well as oxygen interstitials - continuously increases with ion fluence. Therefore, radiation-induced origin of elementary absorption bands at 5.6 and 6.6 eV tentatively ascribed earlier to charged and neutral oxygen interstitials has been proved for the first time. The concentrations of charged interstitials (in the form of superoxide ions) have been directly determined by the EPR method. The evolution of cathodoluminescence bands typical of self-trapped excitons (VUV band at 7.6 eV) and F-type defects (bands peaked around 3.0 and 3.8 eV) with the rise of Xe-ion-irradiation fluence has been measured and analyzed.

The recovery of radiation damage induced by 156-& Mcy;eV (132)& KHcy;& iecy; and 2.25-GeV Au-197 ions (fluences between 6.6 x 10(10) and 2 x 10(12) ions/cm(2)) in MgAl2O4 single crystals has been investigated via stepwise (isochronal) thermal annealing procedure. The integral of elementary bands of radiation-induced optical absorption (result of spectra decomposition into Gaussians) has been considered as a concentration measure of relevant structural defects. The general trends of the multistage annealing kinetics of the oxygen-vacancy-related F+ and F centers (430-850 K) as well as complex cation-related defects responsible for absorption bands peaked at 5.9, 6.6 and similar to 7 eV (ends above 1000 K) have been considered. The features of the annealing kinetics of these defects induced by the xenon or gold ions with different values of mean energy loss along the ion trajectories (13.5 and 30.4 keV/nm, respectively) and fluences have been analyzed.

The spectra of steady-state and time -resolved cathodoluminescence have been measured at 5.4 K for alpha-Al2O3 single crystals irradiated by fast neutrons with the same dose of 6.9 x 1018 cm -2 and then preheated to certain temperatures from 295 K to 1250 K. The analysis of these spectra allows to conclude that irradiation decreases the intensity of the 7.6 -eV intrinsic emission related to self -trapped excitons by about 80 times and practically suppresses infrared Cr3+ impurity emission, while the enhancement of luminescence at 2-4.2 eV is connected with the creation of single F -type centers (oxygen vacancy with two or one trapped electrons) and their F2 -type dimers in different charge states. The following evolution of the above -mentioned emission bands with the increase of preheating temperature is determined by the decreasing concentration of the F and F2 defects, partial reabsorption of luminescence as well as varied efficiency of nonradiative processes, in which electron -hole pairs formed by an electron beam are involved. The limitations of the use of specific cathodoluminescence bands for the detection of radiation damage and its further recovery via thermal annealing are considered.

Single crystals of MgAl2O4 have been irradiated by 156-\MeV 132Xe and 2.25-GeV 197Au ions with fluences between 6.6 x 1010 and 2 x 1012 ions/cm2. After isodose ion-irradiation, the spectra of radiation-induced optical absorption (absorption of a pristine sample is subtracted) for the (100), (110) and (111) oriented crystals coincide. The fluence dependence of the concentration of several defects responsible for optical absorption at 4.5-7.1 eV have been analyzed. The integral of all five Gaussians, obtained via spectra decompositions and served as a measure of relevant defects, continuously increases with the fluence without any sign of saturation. Thus, similar to oxygen-vacancy-related F+ and F centers (Gaussians peaked at 4.8 and 5.3 eV, respectively), the defects responsible for elementary bands at 5.9, 6.6 and -7.1 eV are also of radiation-induced origin. The thermal stability of all defects has been determined via stepwise annealing of the irradiated samples. The tentative origin of the 5.9, 6.6 and -7.1 eV bands (their decay occurs by about 200 K higher than that for the Ftype centers) is considered. Nanoindentation measurements show an ion-induced increase of the sample hardness up to 30%. The hardening effect follows the evolution of the energy loss of the Xe and Au ions as a function of sample depth.

Corundum (alpha-Al2O3) possesses a number of unique properties, including high tolerance to harsh radiation environment. Material functionality is strongly affected by radiation-induced structural defects, single interstitial-vacancy Frenkel pairs and their aggregates. Three novel paramagnetic two-electron defects with total spin S = 1 have been revealed in fast neutron irradiated alpha-Al2O3 single crystals by means of the EPR method. Analysis of the EPR characteristics of these defects allows to conclude that the main novel D 1 defect (initial concentration after irradiation about 2.8 x 10 16 cm -3) is oriented along one of three equivalent b crystal axes, that connect two adjacent anion sites of oxygen triangles on the base corundum plane. The D 1 defect is ascribed to the F-2( 2+) center (two adjacent single-charged oxygen vacancies), while the D 2 and D 3 have the same dimer structure with an additional defect in their vicinity. The suggested defect structure is supported by pulse annealing of the EPR signal of relevant defects: defect concentration dependence contains a rising stage at 500-630 K, while complete thermal destruction due to the recombination with complementary oxygen interstitials occurs by 720-750 K. Such behavior qualitatively coincides with earlier studied thermal annealing of the F-2( 2+) center optical absorption in neutron-irradiated corundum.(c) 2022 Elsevier B.V. All rights reserved.

An α-Al2O3 (corundum) possesses unique optical, electrical and mechanical properties, demonstrates high tolerance to heavy irradiation and, in particular, is in a short list of candidates for optical/diagnostics windows in advanced fission and forthcoming fusion (DEMO) reactors. However, material functionality is strongly affected by structural defects induced by radiation of different types. Optical and magnetic characteristics of Frenkel defects (interstitial-vacancy pairs) as well as the processes of their thermal annealing (radiation damage recovery) have been investigated by means of optical absorption and the EPR methods in α-Al2O3 single crystals exposed to fast neutrons with fluence of 6.9 × 1018 n/cm2. For the first time in metal oxides, a single oxygen interstitial, which is not adjacent to any other imperfection, has been detected. The analysis of the EPR spectra/parameters testifies that this interstitial forms with a regular oxygen ion a superoxide ion O2−, stabilized by a trapped hole. The interstitial becomes mobile above 500 K and recombines with a complementary oxygen vacancy that is a part of electronic F+ or F centers. The thermal annealing kinetics of the F and F+ centers has been theoretically analyzed in terms of interrelated diffusion-controlled recombination reactions of radiation defects. The analysis indicates that both, negatively charged and neutral oxygen interstitials (relevant absorption bands at 5.6 and 6.5 eV, respectively) co-exist in similar concentrations.

Corundum (α‐Al2O3) is a technologically important material and, in particular, widely used in optical applications such as luminescent radiation dosimeters, cryogenic scintillators, and is being considered as a promising candidate for windows in future fusion reactors. Its optical and mechanical properties are controlled by the presence of radiation‐induced (in particular, by fast neutrons) defects. Herein, the thermal stability and recombination kinetics of primary anion Frenkel defects—the F and F + electronic centers and oxygen interstitials—in fast‐neutron‐irradiated α‐Al2O3 single crystals are analyzed. Theory is developed considering the formation of both neutral and charged oxygen Frenkel defect pairs; defect migration, interaction and recombination. Based on ab initio calculations and new theoretical kinetics analysis, for the first time, a coexistence is demonstrated, in comparable concentrations, of two interstitial types—neutral O atoms and negatively charged O− ions (with attributed optical absorption band maxima at 6.5 eV and 5.6 eV, respectively); and their diffusion parameters, necessary for the prediction of secondary defect‐induced reactions and defect/material thermal stability, are obtained.

Due to unique optical/mechanical properties and significant resistance to harsh radiation environments, corundum (α-Al 2 O 3 ) is considered as a promising candidate material for windows and diagnostics in forthcoming fusion reactors. However, its properties are affected by radiation-induced (predominantly, by fast neutrons) structural defects. In this paper, we analyze thermal stability and recombination kinetics of primary Frenkel defects in anion sublattice − the F -type electronic centers and complementary oxygen interstitials in fast-neutron-irradiated corundum single crystals. Combining precisely measured thermal annealing kinetics for four types of primary radiation defects (neutral and charged Frenkel pairs) and the advanced model of chemical reactions, we have demonstrated for the first time a co-existence of the two types of interstitial defects – neutral O atoms and negatively charged O - ions (with attributed optical absorption bands peaked at energies of 6.5 eV and 5.6 eV, respectively). From detailed analysis of interrelated kinetics of four oxygen-related defects, we extracted their diffusion parameters (interstitials serve as mobile recombination partners) required for the future prediction of secondary defect-induced reactions and, eventually, material radiation tolerance.

This paper presents the results of the investigation of stress relaxation at the Si-SiO2 interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO2-Si3N4 system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO2 and Si3N4 material. With an appropriate choice of oxidation conditions compressive stresses in SiO2 and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

single radiation-induced superoxide ion O_2^ - has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×10 18 n /cm 2 ) corundum (α-Al 2 O 3 ) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type O_2^ - center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole ( S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the O_2^ - centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F -type centers (mostly with the EPR-active F + centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the O_2^ - center spin S = 1/2, its orientation along the a axis. The O_2^ - center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration.

We have performed the density functional calculations (DFT) on the hole-type defects (V-centres) in magnesium aluminate spinel (MgAl2O4) following the results of recent paramagnetic resonance measurements (EPR) in Nucl. Inst. Methods Phys. Res. B 435 (2018) 31–37. The hybrid B3LYP functional calculations using large supercells of 448 atoms have demonstrated excellent results not only for bulk properties but also properties of the V-centres in MgAl2O4. Three types of V-centres have been considered and confirmed, namely V1, V2 and V22. The DFT calculations have revealed the atomic relaxation pattern and spin density distribution around the hole-type defects that is suggested as an important complement to the experiments. Moreover, the calculated hyperfine coupling constants (HCCs) have been analyzed and compared with those from the measured EPR spectra. A good agreement between the calculated and measured HCC values is observed and discussed.