Analytical scanning and transmission electron microscopy were used to study the microstructure of Ce,Er-doped Na0.5La0.5MoO4 laser crystals. Crystals were grown by the Czochralski method from the melts with a nominal composition of Na0.5La0.5−xCexEr0.005MoO4, where x = 0.125 and 0.15, then annealed at 700 and 1000 °C in the oxidizing atmosphere. We found the secondary phase precipitation of Ce2O3 oxide in as-grown crystals, while after high-temperature annealing the CeO2 precipitated crystals are always observed. Impurity ions Ce3+ occupy the La sites, and approximately 20% of the nominal Ce content is involved in the formation of Ce oxide secondary phase precipitates. The length of CeO2 precipitated crystals ranged between 100 nm and 550 nm (average length was 200 nm) and their width was 30–70 nm. The mechanism of CeO2 formation is discussed. The orientation relationships of Na0.5La0.5−xCexEr0.005MoO4/CeO2, the degree of coherence of the interface, and the preferential directions of their growth in the matrix were established. CeO2 crystals precipitated in the matrix cause light scattering with a wavelength comparable to the size of the precipitates and lead to deterioration of optical transparency of the material.

Cation-deficient Na 2–3 x Gd x MoO 4 solid solutions in the NaGd(MoO 4 ) 2 –Gd 2 (MoO 4 ) 3 system have been simulated by the interatomic potentials method. The parameters and volume of unit cell, as well as density, bulk modulus, enthalpy, vibrational entropy, and heat capacity in dependence of the composition are determined. Temperature dependences of the heat capacity and vibrational entropy are plotted. The local structure of the solid solutions has been studied. It is shown that the vacancy–oxygen distances are on average 5.0% larger than the Na–O distances and 11.8% larger than the Gd–O distances. The sizes of these coordination polyhedra slightly increase with an increase in the gadolinium content, which is accompanied by an increase in the unit-cell size. The parameter c increases with a higher rate as compared to a , which is indicative of distortion of the unit cell and polyhedra.

Atomistic simulation of powellite crystals has been carried out. A system of parameters of interatomic potentials is proposed for the calculation of point defects in powellite. The formation energies of intrinsic defects and their clusters, which are spatially related combinations of defects, are estimated. The most favorable localization of interstitial calcium and oxygen ions was determined. The formation energies of the Frenkel and Schottky defects are calculated. It was shown that the most energetically preferred in stoichiometric powellite are the oxygen Frenkel defects. Defects arising from deviations from stoichiometry are considered. It is established that the clustering of defects allows to reduce the energy of their formation by 15-20%. Keywords: powellite, tungstates and molybdates, atomistic simulation, intrinsic defects.

Проведено моделирование твердых растворов CaMoO 4 -NaGd(MoO 4 ) 2 методом межатомных потенциалов. Показано, что твердые растворы существуют во всем диапазоне составов и близки к идеальным. Получены зависимости от состава параметров и объема элементарной ячейки, плотности, модуля объемной упругости, энтальпии, колебательной энтропии и теплоемкости. Построены температурные зависимости теплоемкости и колебательной энтропии. Анализ локальной структуры и ее изменения в зависимости от состава твердого раствора Ca 1-x Na x/2 Gd x/2 MoO 4 показал, что в твердом растворе межатомные расстояния Gd-O в среднем на 2.62% меньше, а Na-O на 3.15% больше расстояний Ca-O. В целом это приводит к увеличению параметров и объема элементарной ячейки при образовании твердых растворов Ca 1-x Na x/2 Gd x/2 MoO 4 по сравнению с CaMoO 4 . Ключевые слова: моделирование, молибдаты, твердые растворы, локальная структура.

Simulation of CaMoO 4 -NaGd(MoO 4 ) 2 solid solutions by the method of interatomic potentials is carried out. It is shown that solid solutions exist in the entire range of compositions and are close to ideal. Dependences on the composition of the lattice parameters and volume of the unit cell, density, bulk modulus, enthalpy, vibrational entropy and heat capacity are obtained. Temperature dependences of heat capacity and vibrational entropy are constructed. Analysis of the local structure and its changes depending on the composition of the Ca 1-x Na x/2 Gd x/2 MoO 4 solid solution showed that in solid solution the interatomic distances of Gd-O are on average 2.62% less, and Na-O is 3.15% greater than the distances of Ca-O. In general, this leads to an increase in the lattice parameters and volume of the unit cell during the formation of Ca 1-x Na x/2 Gd x/2 MoO 4 solid solutions compared to CaMoO 4 . Keywords: simulation, molybdates, the solid solutions, local structure.

The content of all matrix elements, including oxygen, was determined by X-ray spectral microanalysis in series of single crystals of sodium-gadolinium molybdates (NGM) grown from melt by Czochralski technique. All grown NGM crystals, including those grown from stoichiometric charge, are non-stoichiometric and contain a molybdenum deficiency of about 3% in average. The NGM congruently melting composition has been determined. For crystals grown from melt of equimolar (stoichiometric) composition, the content of cationic vacancies in the (Gd + Na) sublattice is close to zero, while the composition of crystal differs from the initial melt. As the Gd excess increases, the content of cationic vacancies in the (Gd + Na) sublattice increases up to 10%. It has been found that cation-deficient NGM crystals are also anion-deficient. The concentration of oxygen vacancies varies from 0.5% to 5% of the amount of anion sites. Possible mechanisms responsible for the formation of cationic and anionic vacancies in NGM crystals are discussed. (c) 2021 Elsevier B.V. All rights reserved.

Ytterbium and ytterbium -niobium doped sheelite single crystals have been grown by the Czochralski method in air and in protective atmospheres and subjected to additional annealing in air, in CO/CO 2 atmosphereand in forevacuum. Comparative investigations of optical absorption and photoluminescence spectra of the crystals in the visible and near-IR spectral regions upon UV excitation were performed. A consistent pattern is proposed, explaining the nature of the donor centers involved into the process of down-conversion population of 2 F 5/2 excited state of Yb 3 + ions in the crystals.

Yb:CaWO4 and Yb, Nb:CaWO4 single crystals have been grown by the Czochralski method in air and in protective atmospheres and subjected to additional annealing in air, in CO/CO2 atmosphere, and in forevacuum. The optical absorption spectra in the range from 250 to 1500 nm and the luminescence spectra of these crystals in the visible and near-IR spectral regions upon UV excitation have been investigated. It is shown that additional introduction of Nb5+ ions into Yb:CaWO4 crystal increases by an order of magnitude (almost to unity) the Yb3+ distribution coefficient between CaWO4 crystal and melt. It is found that optical excitation of the crystals in the range of 260–355 nm induces down-conversion luminescence of Yb3+ ions from the 2F5/2 level in the vicinity of 1 µm. An increase in the oxidative potential of synthesis atmosphere, as well as the introduction of niobium into the crystal composition, weakens this luminescence. A consistent pattern explaining the nature of the donor centers involved in down-conversion population of excited state 2F5/2 of Yb3+ in the crystals is proposed. Within this pattern, Yb2+ ions play the role of these donor centers. Another (much less efficient) mechanism of population of the 2F5/2 level is intracenter relaxation from the higher-lying charge transfer excited state within Yb3+ ions. At the same time, it is confirmed that color centers based on oxygen vacancies and partially reduced tungsten ions, as well as self-trapped excitons on tungstate complexes, are not involved in population of the excited state 2F5/2 of Yb3+ ions, and the optical centers formed in Yb:CaWO4 crystals as a result of vacuum annealing suppress ytterbium luminescence.

Continuous wave and pulse X-band electron paramagnetic resonance spectroscopy are used to determine structure, magnetic and relaxation properties of paramagnetic centers formed by impurity ytterbium ions in forsterite (Mg2SiO4) single crystals. It is found that Yb3+ ions substitute Mg2+ ions both as single ions and as dimeric associates with nearby magnesium vacancy. For all ytterbium centers, magnetic properties are characterized by the strong easy plane anisotropy. Measurements in temperature range 5–15 K showed that spin–lattice relaxation of the Yb3+ ions in Mg2SiO4 is due to joint action of direct one-phonon process, Raman two-phonon process and resonance two-phonon Aminov–Orbach process.

A newly developed experimental technique based on 169 Tm-containing cryogenic bolometer detector was employed in order to perform the search for solar axions. The inclusion of target material into the active detector volume allowed for significant increase in sensitivity to axion parameters. A short 6.6 days measurement campaign with 8.18 g detector crystal yielded the following limits on axion couplings: | g A γ ( g A N 0 + g A N 3 ) ≤ 1.44 × 10 − 14 GeV − 1 and | g A e ( g A N 0 + g A N 3 ) ≤ 2.81 × 10 − 16 . The achieved results demonstrate high scalability potential of presented experimental approach.

Based on the principle of minimum production of entropy, dependence is obtained for determining the size of particles resistant to crushing. The size of particles resistant to crushing is proportional to the surface energy of the particles to the 3/5 degree and inversely proportional to the specific mixing power to the 2/5 degree. Experimental studies have been carried out on the crushing of aluminum oxide particles by varying the process parameters: the grinding time, the size of the grinding balls, the mass ratio of the grinding balls and Al2O3 powder. The specific mixing power is represented by regression dependence on the parameters of the grinding process mode. Using the obtained dependences to determine the size of particles during grinding, the conditions of the process were found that ensure the production of particles no larger than 1.5 μm. 

Samples of a SiC composite with an oxide additive of eutectic composition produced from the organomagne-siumoxane-yttriumoxane-alumoxane oligomer (OMYA) and multi-walled carbon nanotubes (MWCNTs) were prepared by the spark plasma sintering (SPS). It is shown that the combined introduction of an oxide additive derived from OMYA together with MWCNTs significantly increases the fracture toughness and strength of the composite.

AbstractA search for resonant absorption of solar axions by $$^{169}\mathrm {Tm}$$169Tm nuclei was carried out. A newly developed approach involving low-background cryogenic bolometer based on $$\mathrm {Tm}_{3} \mathrm {Al}_{5} \mathrm {O}_{12}$$Tm3Al5O12 crystal was used that allowed for significant improvement of sensitivity in comparison with previous $$^{169}\mathrm {Tm}$$169Tm based experiments. The measurements performed with 8.18 g crystal during 6.6 days exposure yielded the following limits on axion couplings: $$|g_{A\gamma } (g_{AN}^0 + g_{AN}^3) \le 1.44 \times 10^{-14}\,{{\mathrm{GeV}}^{-1}}$$|gAγ(gAN0+gAN3)≤1.44×10-14GeV-1 and $$|g_{Ae} (g_{AN}^0 + g_{AN}^3) \le 2.81 \times 10^{-16}$$|gAe(gAN0+gAN3)≤2.81×10-16.

A concentration series of sodium–gadolinium molybdate single crystals have been grown by the Czochralski method from melts of stoichiometric and some nonstoichiometric compositions in atmospheres with different oxygen contents. The actual compositions, microhardness, and crack resistance of the grown crystals have been investigated. All crystals are characterized by a significant sodium deficit with respect to stoichiometry and a large number of vacancies in the (Na + Gd) sublattice. The range of congruent melting is determined for this compound, and its homogeneity range is estimated to extend (on the scale of atomic concentration ratios Gd/Na) at least from 1.10 to 1.75. It is found that the microhardness of sodium– gadolinium molybdate crystals is significantly anisotropic, whereas the degree of crack resistance anisotropy does not exceed the measurement error. At the same time, neither microhardness nor crack resistance exhibit any significant dependence on the growth charge composition and the synthesis conditions in the investigated range of variation in these parameters.

Atomistic modeling of europium-containing scheelite-like molybdates promising for use in solid-state lighting is performed. The analysis of local structure of the nearest environment of europium ion in the powellite-based solid solutions, namely Ca-2(MoO4)(2) - Eu-2(MoO4)(3) and Ca-2(MoO4)(2) - NaEu(MoO4)(2), has been accomplished. The similar analysis was also performed for solid solutions based on double molybdates of both stoichiometric NaGd(MoO4)(2) - NaEu(MoO4)(2) and cation-deficient Na2Gd4(MoO4)(7) - Na2Eu4(MoO4)(7) compositions. The comparison of the local structures has been implemented at equal content of europium replaced similar to 5% of "calcium" sites. It is shown that the symmetry of the cation site occupied by europium in all the studied solid solutions decreases compared to the symmetry of this site in the scheelite structure. The nearest distance between the europium ions is 3.90-4.01 angstrom. It is shown that the multivariate environment of the activator ion in the double molybdates causes a strong dispersion of interatomic distances Eu-O. It is found that these changes lead to a strong distortion of the oxygen polyhedra EuO8 holding the relative proximity of their average size. The latter circumstance may explain the observed by spectroscopic studies the existence of single center of europium in these complex systems.

The $^{169}$Tm nuclide has first nuclear level at 8.41 keV with magnetic type transition to the ground state and, therefore, can be used as a target nucleus for the search of resonant absorption of solar axions. We plan to use a Tm-containing crystal of a garnet family Tm$_3$Al$_5$O$_{12}$ as a bolometric detector in order to search for the excitation of the first nuclear level of $^{169}$Tm via the resonant absorption of solar axions. With this perspective in mind, a sample of the Tm$_3$Al$_5$O$_{12}$ crystal was grown and tested for its bolometric and optical properties. Measurements of chemical and/or radioactive contaminations were performed as well. In this paper we present the test results and estimate the requirements for a future low-background experimental setup.

The concentration series of sodium‐gadolinium molybdate (NGM) crystals have been grown by Czochralski technique from stoichiometric Na0,5Gd0,5MoO4 melt, and from seven different nonstoichiometric melts in various atmospheres. The actual cation compositions of the crystals are measured by microprobe analysis. The location of the congruently melting composition is found for this compound, and the homogeneity field is evaluated. The unit cell parameters of the crystals have been calculated from X‐ray diffraction (XRD) analysis results. Hydrostatic densities of the crystals are measured. Based on these results, the actual molar weights of the samples are calculated that allows more precise determination of the crystals compositions. The studied crystals are shown to contain substantial sodium deficiency and excess of gadolinium compared to stoichiometry. The amount of cation vacancies occurs in both sublattices along with increase of Gd2O3/Na2O molar ratio in the crystals.

Different versions of quantum cutting mechanisms in Yb doped Scheelite-like molybdate and tungstate single crystals were studied, with participation of various schemes of donor-acceptor interaction (various kinds of donor centres: self-trapped excitons at molybdate/tungstate complexes; high-lying excited states of Yb3+ ion; Yb2+ ion; traps based on cation and oxygen vacancies; accidental impurities.). All the versions were rejected.