Yb3+-doped magnesium monomolybdate crystal (Yb:MgMoO4) is grown by the Czochralski method. The crystal structure, thermal expansion, vibronic properties, polarized room- and low-temperature spectroscopy of this new compound were studied. According to the Rietveld refinement, Yb:MgMoO4 belongs to the monoclinic class (sp. gr. C2/m, lattice constants: a = 10.276(9)& Aring;, b = 9.289(6)& Aring;, c = 7.027(6)& Aring; and beta = 106.903(8)degrees). The highest-energy Raman mode of Yb:MgMoO4 is found at 969 cm(-1) and assigned to symmetric stretching vibrations of [MoO4](2-) tetrahedra. Yb3+ ions in MgMoO4 exhibit intense and broad absorption at 978 nm, relatively broad and smooth emission bands above 1 mu m (stimulated-emission cross-section: sigma(SE) = 0.76 x 10(-20) cm(2) at 1031 nm for light polarization E || c) and a long luminescence lifetime (0.63 ms). The low-temperature (12 K) spectroscopic studies evidenced a multi-site behavior for Yb3+ ions in MgMoO4 revealing at least 8 non-equivalent optical centers which are assigned to different charge compensation mechanisms. This induces a strong inhomogeneous spectral line broadening. For envisioned laser applications, the problem of low Yb3+ segregation coefficient (K-Yb similar to 0.029) needs to be solved.

MgWO 4 single crystals were grown by top seed solution growth method. Their thermal conductivity, mechanical strength properties, as well as dispersion and thermal coefficients of the refraction indices were measured.

The single crystals of the concentration series of $\mathrm{CaMoO}_{4}-\mathrm{Na}_{0.5} \mathrm{Gd}_{0.5} \mathbf{M o O}_{4}$ scheelite-like solid solutions was grown and investigated. According to most of the properties studied, this solid solution shows some deviations from additivity.

Spectral and mechanical strength properties of Yb 3+ ,Li + :ZnWO 4 crystals were investigated, diode-pumped ZnWO 4 laser with an output power of 2,41 W was demonstrated.

Monoclinic $\mathrm{Eu}^{3+}$-doped MgWO 4 crystal is grown from the flux using Na 2 WO 4 as a solvent and its polarized emission properties in the visible are studied revealing a multisite behavior of this promising laser material.

$\mathrm{Tm}^{3+}$-doped MgMoO 4 crystal was grown by Czochralski. Thermal properties of crystal were determined. Spectroscopic studies were shown broad emission band in 1.6 $2.15 \mu \mathrm{m}$ region (the ${ }^{3} \mathrm{~F}_{4} \rightarrow{ }^{3} \mathrm{H}_{6}$ transition) and a long ${ }^{3} \mathrm{~F}_{4}$ excited-state lifetime that makes this crystal promising for lasers.

In the Y2SiO5 single crystal doped with iron, transitions of three triclinic Fe3+ centers localized in silicon positions were detected near g = 4.3. The positions of these transitions weakly depend on the orientation of the magnetic field. The orientation behavior of other transitions of these centers has been studied. The parameters of the constructed spin Hamiltonians of two of them in the main axes satisfy the conditions: b(2)(0)- D is close to b(2)(2)- 3E while b(2)(0) >> g beta B. These centers can be attributed to Fe3+ ions localized in silicon positions and compensated for both locally and non-locally by oxygen vacancies.

${T m}^{3+}, \mathbf{L i}^{+}: \mathbf{Z n W O}_{4}$ single-crystal was grown by the Czochralski (Cz) method. The actual concentrations of the dopants were measured. Dispersions and temperature coefficients of the refractive indices (RI), as well as the mechanical strength characteristics of the crystal were measured. Spectroscopic studies of the crystal were carried out. The first laser operation above 2 $\mu \mathrm{m}$ at this crystal was obtained.

The influence of the charge treatment by ultrahigh dilution (UHD) technology on oxide single crystals grown by the Czochralski technique was studied for monoclinic MgMoO4 crystals doped by 1 at. % of Nd3+ ions. The series of 10 Nd:MgMoO4 crystals was grown from the charges that were subjected to UHD treatment, as well as from the charges treated with two types of control or with no special treatment at all. The grown crystals were studied by X-ray powder diffraction analysis, inductively coupled plasma atomic emission spectroscopy, mass-spectrometry, optical absorption, emission spectroscopy and luminescence kinetic analysis. We found that: (i) wetting of MgO + MoO3 mixture by a water-ethanol solution before calcining leads to some enrichment of the mixture with MoO3, whereas the wetting of the charge after the calcining leads to some enrichment of it with MgO; (ii) congruent melting composition of MgMoO4 crystal is in the field of some MoO3 excess; (iii) the solid-phase solubility of the excess MoO3 in MgMoO4 probably does not depend on temperature, whereas the solid-phase solubility of the excess MgO in MgMoO4 crystal depends on temperature. We suggest that the corresponding solidus line passes through the range of retrograde solubility; (iv) the crystals grown within this range are characterized by the enhanced Nd3+ segregation coefficient between the crystal and the melt (approximately 0.006 versus 0.004); (v) unit cell parameters of MgMoO4 crystal with the excess of MoO3 are larger than those of the crystal of the stoichiometric composition and of the crystal with the excess of MgO; (vi) the shapes of the optical absorption and luminescence spectra of Nd:MgMoO4 crystal do not depend on the charge treatment; (vii) luminescence decay kinetics are single-exponential for all the studied crystals, the luminescence decay time being different for the crystals grown from the charges that underwent different types of treatment; (viii) the luminescence intensity of Nd:MgMoO4 crystal grown from the charge that underwent UHD treatment before calcining (solid-phase synthesis) is reduced by an order of magnitude in comparison with the other studied crystals.

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.

Thulium-doped magnesium molybdate single-crystal (Tm3+:MgMoO4) was grown by the Czochralski method (melting point: 1322 degrees C). The actual Tm3+ doping level was measured to be 0.1 at.% (the segregation coefficient was only 0.02). Tm:MgMoO4 belongs to the monoclinic class (sp. gr. C2/m, lattice constants: a = 10.2769(2) angstrom, b = 9.2898(5) angstrom, c = 7.0269(4) angstrom, beta = 106.898(6)degrees). The thermal expansion tensor of this crystal is determined. Its thermal conductivity is also measured at 50-300 K (at room temperature, it amounts to 2.64 Wm- 1K-1). Polarized Raman spectra are presented and the strongest mode is found at 957 cm-1. Optical spectroscopy of Tm3+ ions is studied indicating broad emission bands extending beyond 2 mu m (the 3F4 -> 3H6 transition) and a long lifetime of the 3F4 excited-state (1.972 ms). This makes Tm:MgMoO4 promising for broadly tunable and mode-locked lasers, when the technological problem of fabricating crystals with higher Tm3+ concentrations will be solved.

Special techniques for deep purification of ZnO and WO3 have been developed in this work. A ZnWO4 single crystal has been grown by the Czochralski method using purified ZnO and WO3 chemicals, along with the ZnWO4 crystal-etalon, which has been grown at the same conditions using commercially available 5N ZnO and WO3 chemicals. The actual accidental impurities compositions of both the initial chemicals and the grown crystals have been measured by inductively coupled plasma mass-spectrometry. A complex of comparative spectroscopic studies of the crystals has been performed, including optical absorption spectra, photo-, X-ray-, and cathodoluminescence spectra and decay kinetics, as well as the photoluminescence excitation spectra. The revealed differences in the measured properties of the crystals have been analyzed in terms of influence of the accidental impurities on these properties.

The effect of the self-separation of a single ultrashort light pulse of a parametric Raman CaMoO4 laser with birefringence phase matching under intracavity synchronous pumping by a mode-locked 1064 nm Nd:YAG laser is experimentally investigated and theoretically simulated. The conditions for the self-separation effect for the single ultrashort pulse at an anti-Stokes wavelength of 973 nm with the pulse duration of 9 ps and the pulse energy of up to 9 μJ are defined.

The generation of a crystalline anti-Stokes parametric Raman laser with collinear phase matching under intracavity synchronous pumping by a 1064-nm passively Q-switched longitudinally mode-locked YAG:Nd3+ laser has been studied. A CaMoO4 crystal is chosen as an active medium. This crystal has optimal birefringence for matching Stokes–anti-Stokes parametric Raman four-wave interaction at a phase-matching angle of 71° and an angular phase-matching width of 4°, which is insensitive to angular phase mismatch. The conditions for generating a single ultrashort anti-Stokes pulse at a wavelength of 973 nm with a duration of 9 ps and energy of up to 9 μJ are determined.

ZnWO4 single crystals both undoped and doped with various concentrations of Yb3+ and Li+ ions have been grown. The actual concentrations of dopants had been measured by atomic emission spectroscopy with inductively coupled plasma, as well as by polarized optical absorption spectroscopy. The segregation coefficients of the dopants between the crystal and melt have been evaluated. Hardness and fracture toughness of the crystals were measured by indentation method. The dependencies of the measured values on the dopants concentrations were analyzed. The optimal dopants concentrations, providing the best mechanical strength of Yb,Li:ZnWO4 crystal have been found.

Magnesium tungstate, MgWO 4 , is a promising laser host crystal. MgWO 4 is monoclinic and optically biaxial. We have studied the dispersion of its principal refractive indices, np < nm < n g , in the spectral range of 0.53 – 1.06 µm. MgWO 4 possesses high refractive index and a strong natural birefringence. np = 1.981, nm = 2.016 and ng = 2.167 at~ 1.06µm.

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.

The series of Neodymium-doped magnesium molybdate crystals (Nd:MgMoO 4 ) belonging to the monoclinic class (sp. gr. C2/m) was grown by the Czochralski method from the charges underwent the special physical treatments. Complex of polarized optical spectroscopic studies was performed. The actual Nd concentrations in the crystals, as well as the unit cell parameters were measured. The peculiarities of the crystals growth process and of the measured properties in connection with the kind of the special charge treatment are discussed.

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.