We report on the first laser operation of the disordered Tm3+,Ho3+-codoped NaGd(MoO4)2 tetragonal double molybdate crystal delivering 549 mW at 2.05 μm with a slope efficiency of 32.4% and a laser threshold of 119 mW.

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 results of laser site selective and Zeeman spectroscopy studies of ZnWO4 single crystal doped with Er3+ ions are reported. Three types of Er3+ sites have been discovered. The energies of the levels of the (4)I(15/)2 and I-4(13/2) multiplets and the g-factors of several states were determined for three orientations of the crystal relative to the magnetic field. The possible structure of three types of sites and further research techniques are discussed.

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.

Yb3+,Li+-codoped monoclinic zinc tungstate (ZnWO4) crystals with optimized Li+ content providing efficient local charge compensation were grown by the Czochralski method. Heavy Li+ codoping makes the Yb3+-doped ZnWO4 crystals less prone to cracking, improves the Yb3+ segregation, reduces the melting point and induces inhomogeneous spectral line broadening. The polarized absorption and stimulated-emission cross-sections of Yb3+ in ZnWO4 were determined. The maximum stimulated-emission cross-section sigma(SE) is 2.94x10(-20) cm(2) at 1055.5 nm corresponding to an emission bandwidth of 12.2 nm for light polarization E parallel to N-p. The formation of Yb3+ optical centers in singly Yb3+-doped and Yb3+,Li+-codoped zinc monotungstate crystals is revealed by low-temperature spectroscopy. The Yb, Li:ZnWO4 laser pumped by a commercial 976-nm Yb-fiber laser generated 2.41 W at similar to 1.06 mu m with a slope efficiency of 76.4%, a laser threshold of 143 mW and linear polarization.

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.

We report on the crystal growth, polarized spectroscopy, and laser operation of monoclinic Tm3+, Na+(Li+) codoped zinc monotungstate (ZnWO4) crystals. The Li+ codoping with an optimized Tm/Li ratio enables almost complete local charge compensation leading to better crystal quality, higher Tm segregation coefficient, and longer luminescence lifetime and ultimately the demonstration of laser operation. The modified Judd-Ofelt theory was employed to calculate the Tm3+ transition probabilities yielding a radiative lifetime of the F-3(4) state of 2.59 ms. The corresponding intensity parameters are Omega(2) = 5.194, Omega(4) = 0.658, Omega(6) = 0.471 [10(-20) cm(2)] and alpha = 0.110 [10(-4) cm]. Tm,Li:ZnWO4 features strongly polarized emission spectra extending beyond 2 mu m owing to a large total Stark splitting of the ground-state, Delta E (H-3(6)) = 644 cm(-1). The stimulated-emission cross-section in this spectral range reaches 0.47 x 10(-20) cm(2) at 2015 nm for light polarization E || N-p. The continuous-wave Tm,Li:ZnWO4 laser generated 282 mW at 1.98 mu m with a slope efficiency of 14.7 %, and laser emission at 2.03 mu m was also achieved.

$\mathrm{Tm}^{3+}$-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.

Luminescence characteristics of prospective downconversion material $\mathrm{NaLa}\left(\mathrm{MoO}_{4}\right)_{2}$: Yb were investigated by means of selective laser spectroscopy. The luminescence quantum yield for $\mathbf{Y b}^{3+}$ ions was measured as $123 \%$. Photodynamic processes and energy transfer mechanisms responsible for efficient energy transfer are discussed.

Stoichiometric tetragonal disordered $\mathrm{NaEu}\left(\mathrm{WO}_{4}\right)_{2}$ laser crystal is grown by the Czochralski method and its polarized spectroscopic properties are studied. The stimulate-demission cross-section for the deep-red ${ }^{5} \mathrm{D}_{0} \rightarrow{ }^{7} \mathrm{~F}_{4}$ transition is $2.53 \times 10^{-20} \mathrm{~cm}^{2}$ at $\sim 702 \mathrm{~nm}$ for $\pi$-polarization.

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.

The EPR spectra of Czochralski grown Y2SiO5:Cr and Y2SiO5:53Cr crystals have been studied. In addition to the previously known spectra of Cr3+, the spectrum of the Cr4+ ion was detected and studied for the first time. The fine structure parameters of tetravalent chromium in Y2SiO5 host were obtained based on the analysis of the angular and frequency-field dependences of the EPR spectra. The conclusion about tetrahedral coordination of Cr4+ ion in this host was made. The concentrations ratio of c(Cr3+)/c(Cr4+) ions in the crystal has been evaluated.

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.

The study of grown Sc 2 SiO 5 : Gd and Sc 2 SiO 5 : Fe crystals was carried out by the paramagnetic resonance method. It has been established that Fe 3+ ions replace Sc 3+ in both crystallographic positions, while Gd 3+ ions exhibit a single center localized in a larger position with a coordination number of 7. Measurement of the orientational behavior of the positions of transitions of Fe 3+ and Gd 3+ centers in two orthogonal planes made it possible to determine the parameters of their triclinic spin Hamiltonians. To determine the localization of impurity ions Cr 3+ , Fe 3+ and Gd 3+ in scandium and yttrium silicates, the orientation of the main Z axes of the fine structure tensors of the second rank was used. Keywords: scandium and yttrium silicates, impurity ions, paramagnetic resonance.

The study of grown Sc 2 SiO 5 :Gd and Sc 2 SiO 5 :Fe crystals was carried out by the paramagnetic resonance method. It has been established that Fe 3+ ions replace Sc 3+ in both crystallographic positions, while Gd 3+ ions exhibit a single center localized in a larger position with a coordination number of 7. Measurement of the orientational behavior of the positions of transitions of Fe 3+ and Gd 3+ centers in two orthogonal planes made it possible to determine the parameters of their triclinic spin Hamiltonians. To determine the localization of impurity ions Cr 3+ , Fe 3+ , and Gd 3+ in scandium and yttrium silicates, the orientation of the main Z axes of the fine structure tensors of the second rank was used.