A series of co-doped LiNbO3:Mg:Er crystals were grown in a single technological cycle and under the same technological conditions by Czochralski. In each subsequent step of the growth cycle, the content of Mg and Er dopants decreased. The initial concentration of dopants in the melt was [Mg] = 4.0 mol% and [Er] = 0.78 mol%. The melt was obtained from a homogeneously doped batch. The batch included the Nb2O5:Mg:Er precursor synthesized by the liquid-phase method. The physicochemical features of crystallization were studied. The optical properties of the crystals were investigated using laser conoscopy and photoinduced light scattering. Macro- and microdefect structures were studied by optical microscopy. Quantitative phase analysis was performed for single-crystal samples. The defect structures of powdered LiNbO3:Mg:Er samples were determined by refining XRD patterns by Rietveld. The optical quality of doubly doped crystals corresponds to that of singly doped LiNbO3:Er crystals. Mg significantly reduces the transparency of LiNbO3:Mg:Er crystals in the ultraviolet and violet spectral ranges. The optimal dopant concentration in the melt was [Er] = 0.63 mol% and [Mg] = 3.0 mol%, and [Er] = 0.47 mol% and [Mg] = 3.07 mol% in crystal. The optical properties of LiNbO3:Mg:Er crystals make them promising active nonlinear optical materials for generating and converting laser radiation.

Features of the defect structure of a nominally pure LiNbO3stoich crystal and double-doped LiNbO3:Zn:Mg (3.45:1.41 mol

A technological scheme has been developed for producing a uniform single-phase LiNbO3:Gd:B charge of a given composition. The charge is necessary to grow using the Czochralski method structurally and optically uniform lithium niobate (LN, LiNbO3) crystals co-doped simultaneously with gadolinium and boron. Studies of the optical and photorefractive properties of the resulting single crystal have been carried out by express assessment methods based on the number of scattering centers in the crystal volume photorefractive light scattering and laser conoscopy. Research indicates high optical quality of the crystal and its promising use as a nonlinear optical material.

The paper studies ceramic samples prepared using traditional ceramic technology also knows as conventional sintering. Samples were prepared from Eu3+ activated fine powders of gadolinium niobate-tantalates GdNb1−yTayO4 (y = 0, 0.7, 1); Eu3+ concentration was x = 0.01, 0.2 and 0.4. Powders were synthesized using nitrate solutions Gd and Eu and niobium and tantalum hydroxides. The hydroxides were coprecipitated from hydrofluoric acid solutions. Full-profile analysis (Rietveld refinement) of XRD (X-ray diffraction) patterns of polycrystals determined the phase composition and refined the parameters of the structure of solid solution phases depending on the Eu concentration. Morphological features of the microstructure have been studied for gadolinium niobate-tantalate ceramics Gd1−xEuxNb1−yTayO4 where y = 0, 0.7, 1 and x = 0.01, 0.2, 0.4. Their mechanical properties have been studied; strength characteristics (Young’s modulus, microhardness) and critical stress intensity factor for mode I KIC have been estimated. The compositions and modes for producing ceramics with optimal mechanical properties have been determined. The photoluminescence (PL) has been studied in ceramic solid solutions in the visible range upon excitation by near UV (376 nm). The features of energy transfer between the host of ceramic GdNb1−yTayO4:Eu solid solution (y = 0; 0.7; 1) and 4fn-4fn levels of Eu in concentrations x = 0.01, 0.2 and 0.4 have been established. We have established that concentration quenching of PL in single-phase ceramics Gd1−xEuxNbO4 occurs above Eu concentration x > 0.01 and in ceramics Gd1−xEuxTaO4—at Eu concentration x > 0.2. The sample with the composition Gd0.99Eu0.01NbO4 demonstrated the maximal PL intensity among all other studied samples. The least PL intensity is observed in Gd0.99Eu0.01Nb0.3Ta0.7O4 relative to individual compounds of gadolinium niobates and tantalates, activated with Eu in the same concentration x = 0.01. The significant difference in Stark splitting of levels has been shown for solid solutions based on niobium (Gd1−xEuxNbO4) and tantalum (Gd1−xEuxTaO4). The compositions of ceramic GdNb1−yTayO4:Eu solid solution have been established which have maximum luminance. We have determined that the luminance of some of the studied samples exceeded the luminance of an industrial phosphor based on Y2O3:Eu3+.

Series of gadolinium tantalum niobates activated with Eu3+ and Tb3+ ((Gd1-y-zEuyTbz)NbxTa1-xO4 (x = 0, 0.3, 1; y = 0.6; z = 0.06; 0.15; 0.3)) were obtained for the first time. The formation of substitutional solid solutions was demonstrated. The obtained ceramic solid solutions had a monoclinic structure with I2/a space group. Electron probe microanalysis, photoluminescence (PL) and cathodoluminescence (CL) were used for studies of gadolinium tantalates and tantalum niobates series activated with Eu3++Tb3+. CL and PL spectra, concentration dependences of CL intensity and decay times, as well as excitation spectra for 5D0-7F2 Eu3+ and 5D4-7F5 Tb3+ bands were analyzed. It was shown for the first time that both energy transfer from Tb3+ to Eu3+ ions, and a reverse process - energy transfer from Eu3+ to Tb3+ ions are observed. This reverse process led to a decrease in CL intensity and decay times of europium bands in the presence of terbium 3+ in these materials. Presumably this phenomenon is influenced by anti-Stokes luminescence. This is confirmed by the dependences of anti-Stokes luminescence probability on temperature and excitation density.

Fine powders of yttrium orthoniobate activated by Eu, Sm, Tb, Er have been synthesized by the sol-gel. The synthesized powders have been calcined at different temperatures (1000, 1100 and 1180 degrees C). Luminescent ceramics (Y0.96 & IEcy;u0.01Sm0.01Tb0.01Er0.01)NbO4 were prepared from fine powders by uniaxial hot pressing (UHP) via various technological modes. For the first time, the crystal structure, microstructure, spectral composition, photoluminescence properties of (Y0.96 & IEcy;u0.01Sm0.01Tb0.01Er0.01)NbO4 ceramics have been studied depending on the temperature conditions of the synthesis of powders and UHP conditions during sintering of ceramics. The mechanical properties of these ceramics (Young's modulus, microhardness and critical stress intensity factor for the mode I KIC) have been evaluated.

A comparative analysis of the photoluminescent properties, concentration of OH--groups and optical quality of double-doped crystals obtained from charges of different genesis has been performed. In a LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal obtained by solid-phase doping, the content of OH--groups is higher than in a LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal obtained by homogeneous doping. These changes occur as a result of the simultaneous formation of two types of complex defects in the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal structure: Zn-Nb(3-)-OH and VLi- OH . It has been established that photoluminescence in the visible region is caused by radiative transitions of Er3+ without the manifestation of the host's own luminescence in the studied crystals. For the LiNbO3:Er:Zn crystal obtained by solid-phase doping, the luminescence intensity is 77% higher than in the crystal obtained by homogeneous doping. This may be due to the participation of OH--groups in the energy transfer between the host and the Er3+ ions.

Comparative studies of double-doped LiNbO3:Er:Zn crystals of different genesis have been carried out using infrared absorption spectroscopy (in the region of OH-- group stretching vibrations) and Raman spectroscopy. A LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal obtained by solid-phase doping and a LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal obtained by homogeneous doping were used in the study. No significant changes have been recorded in the infrared absorption spectra and Raman spectra of crystals obtained using different technologies. Minor changes in the main parameters of the absorption bands with frequencies of 3483 and 3492 cm-1 have been detected in the infrared absorption spectra. This may be due to the higher concentration of zinc dopant in the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal. Measuring the half-width parameter of the band with a frequency of 271 cm-1 in the Raman spectra of the studied crystals helped to establish that the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal has a higher ordering of the structural units of the cation sublattice compared to the LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal.

For the first time, terbium-activated gadolinium tantalo-niobates were synthesized ((Gd1-xTbx)NbyTa1-yO4). The synthesis was carried out by the liquid-phase method. All studied materials had the main crystalline modification M GdNbO4. The content of impurity phases did not exceed 2-5%. The photo- and cathodoluminescence spectra were studied. Concentration quenching of the terbium ion luminescence for a series of solid solutions containing tantalum ((Gd1-xTbx)Nb0.9Ta0.1O4) occurs earlier than for pure gadolinium niobates ((Gd1-xTbx)NbO4). The obtained materials are promising for use as scintillators. Keywords: gadolinium tantalum-niobates, liquid-phase synthesis, Tb3+, luminescence, cathodoluminescence.

Obtaining optical materials based on lithium niobate with controlled optical properties is an important task of modern materials science. To date the problem of obtaining heavily doped LiNbO3 crystals of optical quality with a macroscopically uniform impurity distribution has not been solved. In this work a comparative analysis of studies of the acoustic, optical and photorefractive properties of LiNbO3:Zn lithium niobate single crystals obtained by direct and homogeneous doping from melts with concentrations of 5,38-9,0 mol% Zn has been carried out. The following methods have been used: piezoacoustics, photoinduced light scattering and laser conoscopy. The values of the static piezoelectric modulus have been determined. According to these values, all crystals are single-domain. Studies of crystals have confirmed the absence of a photorefractive effect in them. It was established that LiNbO3:Zn crystals grown on the basis of the method of homogeneous doping with concentrations in the melt of 6,8-9,0 mol% Zn are characterized by the highest structural and optical uniformity. This is of interest for the technology of growing large zinc-doped lithium niobate crystals by the Czochralski method.

На основе метода гомогенного легирования из прекурсора Nb 2 O 5 :4.5 мол. % Zn и карбоната лития синтезирована однофазная шихта LiNbO 3 :4.1 мол. % Zn, из которой методом Чохральского выращены кристаллы ниобата лития. На основании результатов экспресс-оценки оптического качества кристаллов и рассчитанных значений пьезомодуля d 333 установлено, что исследуемые кристаллы характеризуются высоким оптическим качеством. Исследования кристаллов гомогенного и прямого легирования в области концентраций примеси 4.02–5.38 мол. % Zn методами фотоиндуцированного рассеяния света и лазерной коноскопии подтвердили их высокую оптическую однородность и стойкость к лазерному повреждению, фоторефрактивный отклик в кристаллах полностью отсутствовал. Полученные результаты могут быть использованы при выращивании крупногабаритных кристаллов LiNbO 3 :Zn, характеризующихся низкими значениями коэрцитивного поля и представляющих особый интерес для создания преобразователей лазерного излучения на периодически поляризованных структурах.

Complex oxides of Y3Al5O12, Gd(Nb,Ta)O4, (Zr,Hf,Y)O2, (Zr,Y)O2, activated with trivalent rare earth ions Nd3+, Eu3+ and Tb3+ have been studied in terms of their luminescent properties. These materials are promising scintillators with high radiation, chemical, and mechanical stability. The aim of the work was to develop ceramic radiation-resistant scintillators based on oxides activated by rare-earth ions. The study included: an evaluation of changes in the intensity and kinetics of luminescence decay from single crystal samples to ceramics; a study of the complete or partial replacement of light elements by heavier ones effect on the luminescent properties; a study of the possibility of using a sensitizer to increase the luminescence yield when luminescence is excited by an electron beam of medium energies.

The results of studies on the synthesis of niobium pentoxide (Nb2O5:Mg:Y) simultaneously doped with magnesium and yttrium and a charge of lithium niobate (LiNbO3:Mg:Y) obtained on its basis have been presented. A technological scheme has been developed and optimal conditions have been determined at each stage of the process. In accordance with them, a monophase charge of a given composition has been synthesized. The charge is intended for growing lithium niobate crystals of high optical quality.

We have established that relatively simple calculations of the Coulomb interaction in the lattice of doped lithium niobate (LN, LiNbO3) can confirm the physical properties of real crystals. We have developed a method for the double adjustment of real XRD data for calculations of Coulomb interaction in a LN cluster. The study considers two crystals doped with boron (LN:B); LN:B(1) has been grown from a charge with 0.02 mol% B2O3, boron has been introduced by homogeneous doping, LN:B(2) has been grown from a charge with 0.547 mol% B2O3, and boron has been introduced by direct solid-state doping. XRD and Rietveld method data have been obtained for these crystals. The obtained data have been used to build a model of the LN cluster; the cluster in the calculations consists of six oxygen octahedra of the LN structure. The cluster configuration has been chosen in such a way that the structure contains two tetrahedral voids. We have studied 10 variants of filling a cluster with intrinsic cations (Li, Nb), defects, and vacancies. There are 10 of them because, in addition to the basic cations in their positions, defects are present in the structure. In terms of the defects used (Nb-Li, Nb-V), we have used only those that Rietveld found for these exact LN:B crystals, and the vacancy in the niobium octahedron (V-Nb) compensates for these defects, according to the models known for LN. The energy of the Coulomb interaction between the cluster structure of a real crystal and the boron cation localized in it in different positions has been calculated for each of the configurations. Calculations have demonstrated that B is more likely to be embedded near a defect than in a regular structure. This means that boron positively influences the local substructure of doped LN crystals, not only structures the melt during crystal growth. Calculations have shown that the type and location of structural defects affect the position of boron in the structure of a LN crystal. Calculations have also shown that LN:B(1) has a more stable structure, including optical damage resistance. The photoinduced light scattering (PILS) patterns and conoscopic patterns confirm this conclusion for the studied LN:B crystals. The information obtained in this study may be useful for interpreting the defective structure of LN crystals co-doped with boron and metals (Mg, Zn, etc.). This will supplement the knowledge available in the literature regarding models that describe the structure of complexly doped LN crystals.

Ceramic samples of polycomponent solid solution (Y0.96Eu0.01Sm0.01Tb0.01Er0.01)Nb0.7Ta0.3O4 have been prepared by sol–gel synthesis from fine powders obtained using nitrate solutions of rare earth elements REE and coprecipitated hydroxides of niobium and tantalum. The structural state of the initial powders’ crystal lattice has been investigated. The morphological features of the microstructure of the ceramics samples have been studied in dependence of temperature regimes of their preparation. The ceramics’ strength characteristics (Young’s modulus) and the critical stress intensity factor of the mode I KIC have been estimated. Cathode- and photoluminescent properties of ceramic solid solutions (Y0.96Eu0.01Sm0.01Tb0.01Er0.01)Nb0.7Ta0.3O4 have been studied.

A series of six LiNbO3:Tb ([Tb] = 0.1 — 2.89 wt%) crystals and seven LiNbO3:Er ([Er] = 0.08÷2.71 wt%) crystals were grown according to a single methodology. Optical uniformity and optical resistance were compared in LiNbO3:Tb and LiNbO3:Er of various chemical composition. The periods of the crystal lattice were determined by the methods of a full -profile analysis of the XRD patterns of polycrystals; models of the atomic structure of LiNbO3:Tb and LiNbO3:Er crystals were analyzed with a change in the dopant concentration. In a series of LiNbO3:Tb crystals, a concentration threshold near the concentration of terbium ~ 2.2 — 2.3 wt% was discovered for the first time. A concentration threshold near the concentration of erbium ~ 2.4 — 2.5 wt% was discovered in LiNbO3:Er. In the area of the concentration threshold, pronounced anomalies of physicochemical, optical and structural characteristics are observed. Growth irregular and regular domain microstructures were revealed in as-grown LiNbO3:Tb and LiNbO3:Er crystals by optical and atomic force microscopy. Structural characteristics and threshold effects are studied in LiNbO3:Gd and LiNbO3:Gd,Cu crystals by Raman spectroscopy, PILS, laser conoscopy and optical microscopy. In LiNbO3:Gd crystals the photorefractive effect is suppressed at as low concentration as [Gd] = 0.05 wt%. For LiNbO3:Gd,Cu crystals a distinct photorefractive response is observed, it increases with increasing Cu concentration. The effect of the association of defects (carriers) is experimentally confirmed and thermodynamically justified by the example of oxygen-octahedron structures such as perovskite and pseudoi:lmental during ion conduction in a certain temperature interval. The discovered phenomenon is extremely important for creating and evaluating the temperature range of operability of ion current sources.

In the work, a fine powder of GdNb0.9Ta0.1O4-mixed gadolinium tantalum niobate activated with rare earth (REE) cations (Sm3+, Eu3+, Tb3+ and Er3+) was obtained by sol–gel synthesis. The evolution of the powder from amorphous to crystalline form was also studied in the work. The evolution was studied by synchronous thermal and X-ray analysis. The phase composition and structure of the resulting Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 powders were analyzed in detail. Ceramic samples of the Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 polycomponent solid solution were prepared from the sol–gel synthesized powder using traditional ceramic technology. The phase composition and characteristics of the structure of individual phases of the Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 ceramic solid solution was determined by full-profile analysis of XRD patterns of polycrystals. We established that incorporation of REE (Tb, Er, Eu, Sm) into the gadolinium site in GdNb0.9Ta0.1O4 solid solution leads to various distortions of the corresponding polyhedra. Note that the distortion degree in this case is much greater than the distortion of the initial GdNbO4 structure. The photoluminescent (PL) properties of the Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 solid solution were studied in the visible wavelength range. Analysis of literature and our own data revealed: electronic relaxation pathways in Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 ceramics can be different depending on the energy of the exciting radiation. Excitation by the 376 nm line leads to internal energy conversion over 4fn–4fn levels of REE cations (Sm3+, Eu3+, Tb3+ and Er3+). The energy transfer between the Nb4+–O−–Ta4+–O− groups and REE is maximal in this case, while the radiation of the matrix from Nb4+–O−–Ta4+–O− emission centers is minimal. Upon excitation in the near-UV range (376 nm), Gd3+ cations do not participate in the energy transfer between the matrix and 4fn–4fn levels of REE dopants. The maximum PL of Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 ceramics is observed in the green–red region of the spectrum from 5D0–7F2 and 4G5/2–6H7/2 transitions of Eu3+ and Sm3+. The emission is maximal at 612 nm; it corresponds to the 5D0–7F2 electric dipole transition of the Eu3+ cation. We established that the efficiency of energy transfer between the matrix and doping REE cations for Gd0.96Eu0.01Sm0.01Tb0.01Er0.01Nb0.9Ta0.1O4 ceramics strongly depends on the energy of the exciting radiation.