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This study has been carried out within the framework of the EUROfusion Consortium and has been provided funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The authors are indebted to E.A. Kotomin, A.I. Popov and R. Vila for stimulating discussions. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Calculations have been performed using both the Marconi supercomputer system at the Computational Simulation Centre (Italy) and the Computer Center of St. Petersburg State University. ; In this study we simulate structural, electronic and phonon properties of MgAl2O4 spinel containing a single neutral oxygen interstitial (Oi) per crystalline L4 and L8 supercells, e.g., its dumbbell formed with one of the nearest regular oxygen atoms of the lattice (Oi-Oreg). Due to the splitting of the Wyckoff positions in supercell models of a perfect crystal, five possible Oi positions with different site symmetry have been identified and studied (C1, Cs, C3v D2d and Td). First principles hybrid HSE06 DFT functional calculations on perfect and defective spinel structures have been accompanied by geometry optimization. The calculated properties of spinel crystal (lattice constants, bulk modulus, band gap as well as frequencies of infrared- and Raman-active vibrational modes) are in a good qualitative agreement with the corresponding experimental data. The formation energy of Oi is found to be minimal for the interstitial site of the lowest symmetry (C1). The results obtained are important, in particular, for understanding the radiation and chemical stability as well as other key properties of MgAl2O4 spinel-type oxide crystals. ; EUROfusion Consortium Euratom research and training programme 2014-2018 under grant agreement No 633053; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This work has been performed within the framework of the EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02 "Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion application". The views and opinions expressed herein do not necessarily reflect those of the European Commission. ; 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. ; EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This study has been carried out within the framework of the EURO fusion Consortium and has been provided funding from the Euratom research and training program 2014–2018 under grant agreement No. 633053. The authors are indebted to A.I. Popov, A.C. Lushchik and R. Vila for stimulating discussions. Technical assistance from O. Lisovski is appreciated too. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Calculations have been performed using Marconi supercomputer system based in Italy at CINECA Supercomputing Centre. ; Thermal stability of the primary electronic defects – F‐type centers – in oxide materials is controlled by their recombination with much more mobile complementary defects – interstitial oxygen ions Oi. Thus, the study of interstitial ion migration is of key importance for the prediction of radiation damage in oxides. In this study, several possible migration trajectories for neutral and charged interstitial oxygen ions are calculated in MgAl2O4 spinel using the first principles calculations of atomic and electronic structure. The lowest energy barriers are ≈1.0–1.1 eV and 0.8 eV, respectively. The effective atomic charges, charge redistribution, and lengths of bonds closest to Oi interstitials are analyzed in detail. ; EURO fusion Consortium Euratom research and training program 2014–2018 under grant agreement No. 633053; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This work has been performed within the framework of the EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02 "Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion application". The views and opinions expressed herein do not necessarily reflect those of the European Commission. Research of A.L, E.F.,, V.S and E.S has been partly supported by the Estonian Research Council grant (PUT PRG619); has been also carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. ; 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. ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Luminescent transparent nanoceramics were obtained by thermobaric treatment (TBT) of magnesium aluminium spinel nanopowder. The morphological features were studied by scanning electron microscopy combined with X-ray powder diffraction. Obtained ceramics do not have any agglomerates and pores larger than 100 nm. Crystallites have a high size uniformity. An increase in the lattice constant of nanoceramics compared to the initial powder is observed. Under the TBT, a decrease in the region of coherent scattering due to elastic deformation of crystallites is found. The absence of cracks, large pores, nanosize grains, and high size uniformity reduce light loss in the material, increasing its transparency. Point defects were characterized by photoluminescence and electron spin resonance (ESR) methods. The glow in the 1.8 eV band is caused by the presence of Ti3+ impurity ions. An abnormally wide peak with a maximum at 2.4 eV in the photoluminescence spectrum is recorded. This signal is a superposition of the Mn2+ ions emission bands and oxygen vacancies (F and F+ centres). In the ESR spectrum, signals from impurity ions of iron, titanium, and manganese, as well as an intense signal at g = 2.005 associated with oxygen vacancies in nanoceramics were detected. © 2018 Institute of Physics Publishing. All rights reserved. ; The work was done as a part of the government task (№3.1485.2017/4.6) of the Ministry of Education and Science of the Russian Federation and was carried out in accordance with the scientific and research plans and state assignment of the Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences (AAAA-A16-116122810212-5). ; et al.;NT-MDT Spectrum Instruments;Ostec-ArtTool Ltd.;Promenergolab LLC;Russian Foundation for Basic Research;Taylor and Francis Group

This study was supported by a grant from Latvian Council of Science (agreement No. LZP-2018/1-0214). ; Original results on the EPR and photoluminescence Cr3+ ions in natural magnesium aluminum spinel (MgAl2O4) are presented. The photoluminescence spectra of Cr3+ ions in natural MgAl2O4 have been measured before and after irradiated by fast neutrons. --- / / / --- This is the preprint version of the following article: N. Mironova-Ulmane, A. I. Popov, G. Krieke, A. Antuzevics, V. Skvortsova, E. Elsts, and A. Sarakovskis, Low-temperature studies of Cr3+ ions in natural and neutron-irradiated Mg-Al spinel, Low Temperature Physics,46, 1154 (2020), DOI https://doi.org/10.1063/10.0002467, which has been published in final form at https://aip.scitation.org/doi/10.1063/10.0002467. This article may be used for non-commercial purposes in accordance with American Institute of Physics terms and conditions for Sharing and Self-Archiving. ; Latvian Council of Science (agreement No. LZP-2018/1-0214); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART².

5 pages, 5 figures.-- El pdf del artículo es el manuscrito de autor. ; Isolated tungsten nanoparticles (α-W and β-W phase) were synthesized and epitaxially grown on alumina and spinel particle surfaces with an average tungsten size of ≤20 nm for a low tungsten content (of ≤1.5 vol%). Using tungsten (VI) ethoxide alcoholic solutions, tungsten trioxide hydrated precursors were attached to a ceramic grains surface as a nanoparticle coating. High-resolution transmission electron microscopy (HRTEM) micrographs showed epitaxial interfaces between alumina, spinel and metallic tungsten. This epitaxial growth is assumed to be due to the effect of water vapour on the sublimation of ortho-tungstic acid during the reduction process in a hydrogen atmosphere. The planes involved in the epitaxy were found to be (220)Al2O3||(121)W and (311)MgAl2O4||(110)W. ; This research was supported by the European Union under the IP-NANOKER project (FP6-515784-2), by the Spanish Ministry of Education and Science under the project MAT2006-10249-C02-01, and by the Autonomous Region of Madrid (CAM) under project 200660M011-NanoCeraMetal. S Lopez-Esteban was supported by the Spanish Ministry of Education and Science under the Ramón y Cajal Programme. ; Peer reviewed

We are grateful to Drs E. Vasil'chenko and A. Maaroos for the help with experiments and useful discussions. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. In addition, the research leading to these results has received funding from the Estonian Research Council  Institutional Research Funding IUT02-26. ; 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), Mg2+ or Al3+ located in a "wrong" cation site (Mg|A1 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 7-7.5 eV, slightly below the energy gap. ; Eesti Teadusagentuur IUT02-26; H2020 Euratom 633053; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This work aims to elaborate the benefits of transparent ceramics in ballistic protection systemswhich are employed by the military in their vehicles. The main focus lies on improving bothoptical and mechanical properties as well as reducing the weight of the system as a whole.Substituting the standard multilayered front glass panel with a polycrystalline ceramic, suchas the spinel MgAl2O4 used in this work, considerably improves its ballistic performance, whilesimultaneously reducing its mass and volume. In order to achieve these improved properties,which most commonly result from a fine-grained microstructure, a very fine grained andreactive powder is used. However, the use of such powders raises additional difficulties inshaping and homogeneous sintering, all of which have a negative effect on the transparency ofthe ceramic. The first focus of this study is the selection of an appropriate powder, carried outby comparison of multiple commercially available powders. This following step highlightedthe rheological and morphological particularities of the examined powders, necessary for theshaping of ceramics, and influencing the sintering and thus the final transparency, such asgranular rearrangement, specific surface area or permeability. Once the spinel powder hadbeen chosen, a processing study was carried out on it, exploiting its agglomeration to obtain avisibility transparent ceramic with a fine microstructure. Since some nanopowders presentedwith difficulties in shaping, further optimization of different treatment methods (use of afreeze dryer, an oven or a spray dryer) in combination with appropriate sintering approacheswere explored. Ultimately, by combining a spray dryer and a PEG additive, the study resultedin the production of a transparent spinel ceramic with a fine microstructure (grain size of4-5 µm) and an in-line transmission of 71% at 650 nm. A parallel study on strengtheningthe transparent spinel by introducing various additives, such as nanodiamond, alumina, andzirconia was also carried out. The ...

This work aims to elaborate the benefits of transparent ceramics in ballistic protection systemswhich are employed by the military in their vehicles. The main focus lies on improving bothoptical and mechanical properties as well as reducing the weight of the system as a whole.Substituting the standard multilayered front glass panel with a polycrystalline ceramic, suchas the spinel MgAl2O4 used in this work, considerably improves its ballistic performance, whilesimultaneously reducing its mass and volume. In order to achieve these improved properties,which most commonly result from a fine-grained microstructure, a very fine grained andreactive powder is used. However, the use of such powders raises additional difficulties inshaping and homogeneous sintering, all of which have a negative effect on the transparency ofthe ceramic. The first focus of this study is the selection of an appropriate powder, carried outby comparison of multiple commercially available powders. This following step highlightedthe rheological and morphological particularities of the examined powders, necessary for theshaping of ceramics, and influencing the sintering and thus the final transparency, such asgranular rearrangement, specific surface area or permeability. Once the spinel powder hadbeen chosen, a processing study was carried out on it, exploiting its agglomeration to obtain avisibility transparent ceramic with a fine microstructure. Since some nanopowders presentedwith difficulties in shaping, further optimization of different treatment methods (use of afreeze dryer, an oven or a spray dryer) in combination with appropriate sintering approacheswere explored. Ultimately, by combining a spray dryer and a PEG additive, the study resultedin the production of a transparent spinel ceramic with a fine microstructure (grain size of4-5 µm) and an in-line transmission of 71% at 650 nm. A parallel study on strengtheningthe transparent spinel by introducing various additives, such as nanodiamond, alumina, andzirconia was also carried out. The primary objective was to improve the mechanical propertiesof the spinel, while keeping its optical properties untouched. The addition of a secondaryphase as reinforcement raised many difficulties, mainly due to the size disparity, differentrefractive index of the spinel and potential unwanted reactions that can occur during sintering.Lastly, the final study focused on complex shaping of spinel ceramics for ballistic protection.As a prerequisite, a complete examination of the spinel's suspension and its behaviour duringshaping had to be performed. The new findings validated liquid shaping as a viable solutionto achieve transparent 3D profiles, while the classic uniaxial pressing route did not yield anyconclusive results. Each of the studies was based on the morphology of the powder and itsevolution during shaping and sintering. The results were obtained through microstructuraland rheological studies as well as the evaluation of optical and mechanical properties. ; Ce travail d'élaboration de céramique transparente pour la protection balistique s'inscrit dansun contexte d'allégement de structure et d'amélioration des propriétés optiques et mécaniques.Le remplacement de la face avant en verre par une céramique polycristalline, telle que lespinelle MgAl2O4 permet d'optimiser considérablement les performances balistiques, touten diminuant la masse et le volume de la protection. Le principal objectif de ce travail a étél'obtention d'une céramique transparente type spinelle avec une microstructure à grains finspermettant d'obtenir de bonnes propriétés mécaniques et balistiques. Afin de contrôler lamicrostructure de la céramique, il est nécessaire d'adopter des conditions de frittage douces,c'est pourquoi l'utilisation d'une poudre très fine et réactive a été envisagée. Mais l'utilisationd'une telle poudre soulève souvent des problèmes de mise en forme et de frittage homogène,qui sont un frein à la transparence de la céramique. Cette étude s'est tout d'abord focaliséesur la poudre de spinelle en comparant quatre poudres de spinelle commerciales. Cette partiea permis de mettre en avant les particularités rhéologiques ou morphologiques nécessaires àla mise en forme des céramiques, influençant ainsi le frittage et donc la transparence finale,comme par exemple le réarrangement granulaire, la surface spécifique ou la perméabilité. Unefois la poudre de spinelle choisie, une étude de traitement a été menée sur cette dernièreen exploitant son agglomération afin d'obtenir une céramique transparente dans le visibleavec une microstructure fine. Le challenge a été d'optimiser sa mise en forme par différentstraitements et son frittage. Cette étude s'est soldée par l'obtention de céramique transparenteà fine microstructure (taille de grain de 4-5_m) et avec une transmission en ligne de 71%à 650nm, grâce à l'utilisation d'un spray dryer et d'un additif PEG. Une autre étude s'estconcentrée sur la possibilité de renforcer ce spinelle transparent par l'ajout de matériau dedifférentes natures (nanodiamant, alumine, zircone). L'objectif a été d'améliorer les propriétésmécaniques du spinelle, tout en gardant intactes ses propriétés optiques. L'ajout de phasesecondaire en tant que renfort soulève de nombreuses difficultés de par leurs tailles, l'indicede réfraction différent du spinelle et les éventuelles réactions qui peuvent se produire lors dufrittage. Une dernière étude a été menée sur la mise en forme de forme complexe, permettantainsi d'autres applications du spinelle en tant que protection. La voie classique par pressageuniaxial s'étant avéré peu concluante, une mise en forme par voie liquide s'est montrée êtreune solution envisageable. Elle a nécessité une étude complète de la mise en suspension duspinelle et son comportement lors de sa mise en forme. Chacune des études s'est appuyée surla morphologie de la poudre et son évolution lors de la mise en forme et le frittage, par lebiais d'études microstructurales et rhéologiques ainsi que l'évaluation des propriétés optiqueset mécaniques.