The radial selectivity of in-situ core-doped crystal rods grown by the double die EFG method

The radial selectivity of in-situ core-doped Cr, Nd: LiNbO₃ crystal rods, grown by the double die EFG (edge-defined film fed growth) method, has been investigated. Two crucibles are combined with an outer and inner die for ascending of different doped melts. The critical growth parameters affected by the diffusion depended spreading effect of the dopant within the melt meniscus have been estimated. The first experimental tests show a good agreement with the theoretical prediction. Single crystalline LiNbO₃ rods with a length of about 100 mm and an outer diameter of 5 mm consisting of sharp separated inner Cr- or Nd-doped core region with diameters between 1.5 and 4 mm were grown successfully.     

Enhancement of the Automatic Diameter Control System for the Czochralski Growth of Nd:GdVO4 Crystals

The automatic diameter control system was enhanced for the growth of Nd-doped GdVO₄ single crystals by the Czochralski method. It was estimated that excessive amount of feedback in the control system is the effective way to improve the shape of rare-earth vanadate single crystals. The spectral analysis for crystal weight and generator power signals was carried out in real time during growth process for determination of oscillations in the control system.     

Peculiarities of cascade photon emission and energy storage in M1−xPrxF2+x (M=Ca,Sr, Ba,x≈0.35) crystals

Peculiarities of cascade photon emission (CPE) and energy storage in M_1?xPr_xF_2+x (M=Ca, Sr, Ba, x≈0.35) crystals were studied. The investigation of lattice parameters revealed that these solid solutions belong to the fluorite structure type with the lattice constant noticeably different from that of MF₂ crystals. Absorption, emission and excitation spectra of M_0.65Pr_0.35F_2.35 were measured at LHeT and RT. As it turned out the typical for 4f²→4f² transition in Pr³⁺ emission lines are broadened as compared with the PrF₃ crystal. The analysis of the excitation spectra broadening does not allow bringing out the type of the superlattice, which is inherent to the material, but it indicates clearly the simultaneous presence of different types of the Pr centers in mixed crystals. Yet another specific feature is the higher radiation sensitivity of these fluorides relatively PrF₃, MF₂ and Pr-doped MF₂ crystals. Coloration efficiency enhances in direction Ca→Sr→Ba, and the positions of induced absorption band depend on composition of the solid solution. Colorization, thermo-stimulated luminescence and afterglow of the M_1?xPr_xF_2+x crystals denote high radiation sensitivity as compared with M_1?xCe_xF_2+x.     

Radiation stability of M1 − xPrxF2 + x (M = Ca,Sr, Ba) crystals

The radiation stability of the mixed crystals M_1 ? xR_xF_2 + x (M = Ca, Sr, Ba) depends on types of the alkaline-earth and rare-earth ions. Different to Eu- and Ce-containing systems, M_1 ? xPr_xF_2 + x solid solutions have a low radiation resistance, which may be associated with hole trapping on praseodymium ion according to the reaction Pr³⁺ → Pr⁴⁺ which is typical for praseodymium. The coloration efficiency of M_1 ? xPr_xF_2 + x crystals grows in the row Ca → Sr → Ba, which is explained satisfactorily within the model of rare-earth clusters, the structure of which is determined by the ratio of the base alkaline-earth cation to the praseodymium ion radii.     

Temperature dependence of impurity quenched luminescence in Tm:LiLuF4

The study of the optical properties of a LiLuF₄ crystal doped with Tm³⁺ yielded the discovery of a strong temperature dependence of the Tm-Tm diffusion coefficient. Spectroscopic characteristics have been investigated as a function of the sample temperature, with particular regard to the luminescence decay following pulsed excitation. An appreciable excitation of the lifetime of the ³F₄ manifold is observed over the temperature range 8.9-298 K. The Judd-Ofelt calculations point out a radiative lifetime considerably longer than the experimental one. These facts suggest a theoretical interpretation based on the presence of impurities that quench the manifold and on a temperature-dependent energy migration between Tm³⁺ ions. A one-parameter best fit of the experimental measurements strongly confirms this hypothesis. Weak OH⁻ ion concentration is detected by means of IR and UV spectra, thus supporting the theoretical interpretation.     

Viscosity Measurement of La3Ga5SiO14 Melt

The viscosity of La₃Ga₅SiO₁₄ melt was measured by a fixed-crucible rotor technique in the range between 1520 and 1596 °C. The melt was found to be Newtonian. The viscosity data ranged between 73 and 97 mPa. s, decreasing with increasing temperature. The activation energy of the viscous flow and the volume of a viscous flow unit estimated from the measured data were 90 kJ/mol and 3 × 10⁻³ nm³, respectively.     

Growth and Characterization of Na2CaGe6O14 Single Crystals

Transparent, crack-free Na₂CaGe₆O₁₄ (NCG) single crystals were successfully grown using the Czcchralski technique. The largest crystal had 30 mm in total length and 18 mm in maximal diameter. Best crystal quality was achieved under low temperature gradient arrangement. The crystal structure of NCG has been refined from single crystal X-ray diffraction data. Some piezoelectric properties of NCG are reported.     

Enhanced Automatic Diameter Control System of Czochralski-grown Crystals

The automatic diameter control system with two ways of the crystal diameter determination is described. The correction of temperature oscillations during the growth process is applied in the describing system. Also, it allows to estimate the crystal quality already during the growth process. The control system is intended for the growth of crystals with different physical properties.     

Growth and optical characteristics of Ce-doped and Ce : Na-codoped BaLiF3 single crystals

Ce-doped and Ce : Na-codoped BaLiF₃ single crystals were grown by the Czochralski technique under reactive atmosphere. Na⁺-ions, used as a charge-compensating impurity, compete with Ce³⁺-ions for Ba²⁺ sites in the host, reducing the Ce³⁺ incorporation and degrading the mechanical integrity of crystals. Ce-doped BaLiF₃ showed potential for UV tunable lasers.     

Magnetooptical studies of defects and recombination luminescence in LiBaF3

Optically detected EPR investigations have been performed on the recombination luminescence (RL-EPR) of LiBaF₃ crystal, X-irradiated at T=4.2 K. RL-EPR lines of V_K- centres were found, as well as further lines of a defect with and an axial g-tensor with its main axis along a [1 0 0] direction of the crystal.

Measurements of the magnetic circular dichroism of the absorption (MCDA) have been performed on LiBaF₃ crystals X-irradiated at two temperatures (4.2 K and RT). After irradiation at T=4.2 K, the main MCDA bands peak at 453 and 500 nm, but after irradiation at T=300 K, the main bands peak at 444 and 390 nm, there is a change of the sign between the peaks in both cases. The MCDA-detected EPR (MCDA-EPR) consists of one broad EPR line in both cases and belong to electron trap centres. Analysis of half-widths of MCDA-EPR lines showed that both defects should have g-tensors with their axes along the [1 0 0] direction. This symmetry has to be expected for F-type centres in LiBaF₃ crystals. The low-temperature electron centre has a more perturbed ground state as the RT centre.