Synthesis of powellite-rich glasses for high level waste immobilization

Increasing amounts of MoO₃ were added to SiO₂-B₂O₃-Na₂O-CaO-Al₂O₃ glasses in order to trap molybdenum as powellite (nominally CaMoO₄). Different heat treatments were performed to study their influences on powellite crystallization by X-ray diffraction and EPR. The glass compositions studied in this work lead to glass-ceramics rich in CaMoO₄, up to [MoO₃] = 5 mol% no poorly durable Na₂MoO₄ phase was identified by XRD. Trivalent actinides surrogates (Gd³⁺) were observed to incorporate into CaMoO₄ crystals.     

Structure-property relationship for complex-substituted molybdates and tungstates of the scheelite family

A high-precision X-ray diffraction study of crystals with the nominal compositions (Na_0.5Y_0.495Er_0.005)WO₄ and (Na_0.5Gd_0.25La_0.25)MoO₄ and the subsequent refinement and analysis of their compositions showed that an increased number of vacancies at the tungsten sites contributes to the cracking of crystals. It was also found that an associate—a color center (a vacancy at the molybdenum site and an electron localized near the vacancy)—gives the crystals a yellow color. The formation of regularly oriented isostructural domains with coherent boundaries as a bulk defect was observed for the first time for crystals of the scheelite family.     

White light emission materials of self‐assembled rare earth molybdates NaRe(MoO4)2 micro‐particles: the controllable synthesis,growth mechanism and luminescent properties

The tetragonally double alkaline rare earth molybdates NaRe(MoO₄)₂ (Re = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) micro‐particles in aqueous solution of Re(NO₃)₃ and Na₂MoO₄ were prepared hydrothermally by a facile and effective EDTA‐assisted method, and investigated by XRD, SEM, FE‐SEM, photoluminescence (PL) excitation and emission spectra. The results showed that the radii of Re ions, pH and the molar ratio of Re(NO₃)₃/Na₂MoO₄ in the synthesis solution play an important role in controlling sizes, morphologies and luminescent properties of the final products. And the possible formation mechanism for the bipyramid‐like structure and role of EDTA were discussed in detail. The microstructure and PL property of NaY_xLa_1‐x(MoO₄)₂:10%Eu³⁺ was investigated, and it showed that NaY_0.5La_0.5(MoO₄)₂ exhibited regular sphere‐like morphology and emitted the stronger red emission with better color purity than other products. When Tm³⁺, Dy³⁺ and Eu³⁺ with appropriate concentrations were used as dopants to modify this bipyramid NaY(MoO₄)₂ material, the white light emission material of NaY(MoO₄)₂:1%Tm³⁺‐11%Dy³⁺‐0.6%Eu³⁺ was successfully synthesized and its CIE chromaticity coordinates is (0.33, 0.33), which is the standard white light point. These novel white‐light‐emitting NaY(MoO₄)₂:1%Tm³⁺‐11%Dy³⁺‐0.6%Eu³⁺ with single phase would be a promising material in the WLED field.     

Continuous-wave laser operation of disordered double tungstate and molybdate crystals doped with ytterbium

Several ytterbium doped disordered doubles tungstate and molybdate crystals, NaGd(WO₄)₂, NaLa(WO₄)₂, NaLa(MoO₄)₂ and LiGd(MoO₄)₂, were grown with sufficient Yb-doping and optical quality. The crystal field splitting of the Yb³⁺ multiplets and the lifetimes are similar to the biaxial monoclinic double tungstates but the peak absorption cross sections are roughly one order of magnitude lower. Room temperature cw laser operation was achieved under Ti:sapphire and diode laser pumping. The polarization dependence of the laser emission and possible power scaling were studied. The slope efficiency achieved with the best sample, Yb:LiGd(MoO₄)₂, was 64.5%. The maximum output power obtained was 900 mW for Yb:NaLa(MoO₄)₂ pumped in the absorption maximum. The laser tunability was studied with an intracavity Lyot filter and the tuning ranges (FWHM up to 33 nm) achieved indicate interesting potential for mode-locked femtosecond operation.     

Structure of Gd<Subscript>0.95</Subscript>Bi<Subscript>0.05</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals at 293 and 90 K

The structure of GdFe₃(BO₃)₄ single crystals has been studied by X-ray diffraction at 293 and 90 K. The crystals are grown from a flux in the Bi₂Mo₃O₁₂–B₂O₃–Li₂MoO₄–Gd₂O₃–Fe₂O₃ system. The results of chemical analysis and structural study show that these crystals contain bismuth as an impurity. It is found that bismuth atoms are located at gadolinium sites in the structure. A decrease in the temperature is accompanied by a lowering of the symmetry from sp. gr. R32 (at 293 K) to sp. gr. P3₁21 (at 90 K). The presence of two types of iron chains with different geometries at 90 K promotes a change in the magnetic properties of these crystals with a decrease in the temperature.     

Effect of neodymium oxide on the solubility of MoO3 in an aluminoborosilicate glass

High molybdenum and rare earth concentrations in soda-lime aluminoborosilicate glasses may lead to the crystallization of molybdenum-rich phases such as alkali and alkaline-earth molybdates (Na₂MoO₄, CaMoO₄) and also rare earth (RE)-rich phases such as apatite (Ca₂RE₈(SiO₄)₆O₂) during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy wasteforms. To understand the effect of neodymium addition (from 0 to 16 wt.% Nd₂O₃) on the phase separation and the crystallization tendency of molydbate phases and on the structure of a Mo-bearing nuclear glass belonging to the SiO₂-B₂O₃-Al₂O₃-Na₂O-CaO-MoO₃ system, crystallization and structural studies have been performed by X-ray diffraction, scanning electron microscopy, electron microprobe analysis, Raman and optical absorption spectroscopies. The results obtained show that the addition of an increasing amount of Nd₂O₃ induces a significant increase of the solubility of molybdenum in the glass, characterized by a decrease of the phase separation and of the crystallization tendency of molybdate phases. The increase of chemical disorder in the structure of Mo-bearing glasses when Nd₂O₃ is added - and more precisely in the depolymerized regions where Nd³⁺ cations and [MoO₄]²⁻ entities are located - could be at the origin of the evolution of the molybdenum solubility in the glass.     

Coupled substitution of gallium by magnesium and zirconium in single crystals of gadolinium gallium garnet

Single crystals of gadolinium gallium garnet in which the gallium ions on the octahedral sites were partially substituted by coupled substitution of magnesium and zirconium have been grown using the Czochralski technique. Single crystals of 36 mm in diameter and 100 mm in length corresponding to the formula Gd₃Ga_5-x-yMg_xZr_yO₁₂ have been obtained from melt compositions in which 0.1 ? x = y ? 0.7. The dislocation and inclusion densities of the single crystals are below 5 cm^-2. The lattice parameters increase linearly from 12.382 Å for gadolinium gallium garnet to 12.489 Å for x = y ≈ 0.54. The distribution coefficient increases in this concentration range from K_eff = 0.58 to K_eff = 0.89. Boules grown from melt compositions in which x = y ? 0.7 appear cloudy through precipitation of Gd₂Zr₂O₇ as a second phase.     

Infrared spectra and structure of superionic conducting glasses in the system AgIAg2OMoO3

Thin blown films of glasses with the mole ratio Ag₂O/MoO₃ = 1 in the system AgIAg₂OMoO₃ (or the pseudobinary system AgIAg₂MoO₄) show three absorption bands in the range 4000-200 cm^?1; 875 cm^?1 (w), 780 cm^?1 (s), and 320 cm^?1 (m, b), which are characteristic of tetrahedral MoO₄^2? ions. The glasses with the ratio Ag₂O/MoO₃ < 1 have two additional bands at 600 cm^?1 (w) and 450 cm^?1 (vw), which are characteristic of condensed ions of MoO₄ tetrahedra, probably Mo₂ O₇^2? ions. These glasses are thus composed of Ag⁺, I^?, MoO₄^2?, and probably Mo₂O₇^2? ions, and classified as “ionic” glasses containing one type of cations. The presence of partial covalency in the Ag⁺… ^?OMo link and the influence of ion exchange of Ag⁺ with K⁺ on IR spectra are discussed. The molar volume of the glasses with the ratio Ag₂O/MoO₃ = 1 is primarily determined by a fairly dense packing of the constituent anions, I^? and MoO₄^2?.     

Second harmonic generation in Na3Gd2(BO3)3 crystals

Na₃Gd₂(BO₃)₃ crystals with dimensions up to 22 × 20 × 5 mm³has been grown from NaBO₂ flux by the top‐seeded solution growth (TSSG) method for the first time. Differential scanning calorimetry (DSC) result shows that Na₃Gd₂(BO₃)₃ melts incongruently. The infrared spectrum indicates that Na₃Gd₂(BO₃)₃ contains characteristic triangular [BO₃]^3– groups responsible for the nonlinear optical effect. For the as‐grown crystal, the transmittance exceeds 80% in the wavelength range of 315 nm to 2670 nm, and the UV cutoff wavelength is 207 nm. The damage threshold is 0.47 GW cm^–2 at 1064 nm. Moreover, Na₃Gd₂(BO₃)₃ crystal exhibits an optical second harmonic generation effect which is 1.3 times as large as that of KH₂PO₄ (KDP). (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of ferroelectric‐ferroelastic Tb2(MoO4)3 crystals

Transparent and nearly colorless ferroelectric‐ferroelastic β′‐Tb₂(MoO₄)₃ (TMO) single crystals have been grown by the Czochralski (CZ) method. The single crystal structure was investigated by X‐ray powder diffraction and was shown to be a single phase with the structure similar as the β′‐Gd₂(MoO₄)₃ crystal. The optical transparency of the TMO crystal has been measured and the crystal is almost transparent in the visible and near infrared regions. The defects of TMO crystal were evaluated by etching technique and the ferroelectric domain structures were observed by an optical microscope. A high‐resolution X‐ray diffraction analysis demonstrates that the as‐grown TMO crystal possesses relatively high optical quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Laser-induced crystallization of β′-RE2(MoO4)3 ferroelectrics (RE: Sm, Gd, Dy) in glasses and their surface morphologies

The glasses with the compositions of 21.25RE₂O₃-63.75MoO₃-15B₂O₃ (RE: Sm, Gd, Dy) were prepared and the formation of β′-RE₂(MoO₄)₃ ferroelectrics was confirmed in the crystallized glasses obtained through a conventional crystallization in an electric furnace. The features of the glass structure and crystallization behavior were clarified from measurements of Raman scattering spectra. Continuous-wave Nd:YAG laser with a wavelength of 1064 nm (laser power: 0.6-0.9 W, laser scanning speed: S = 1-16 μm/s) was irradiated to 10.625Sm₂O₃-10.625Gd₂O₃ (or Dy₂O₃)-63.75MoO₃-15B₂O₃ glasses, and the structural modification was induced at the glass surface. At the scanning speed of S = 10 μm/s, crystal lines consisting of β′-Gd_2−xSm_x(MoO₄)₃ or β′-Dy_2−xSm_x(MoO₄)₃ crystals were patterned on the glass surface. It was found that those crystal lines have the surface morphology with periodic bumps. At S = 1 μm/s, it was found that crystal lines consist of the mixture of paraelectric α-Gd_2−xSm_x(MoO₄)₃ and ferroelectric β′-Gd_2−xSm_x(MoO₄)₃ crystals, indicating the phase transformation from the β′ phase to the α phase during laser irradiation. Homogeneous crystal lines with β′-RE₂(MoO₄)₃ ferroelectrics have not been written in this study, but further research is continuing.     

Local study on the MoO4 units in AgI-doped silver molybdate glasses

The short-range order around molybdenum has been investigated in AgI-doped silver molybdate glasses (AgI)_x(Ag₂MoO₄)_1?x (with x = 0.67 and 0.75) by Mo–K edge EXAFS measurements as a function of temperature. The difference from crystalline Ag₂MoO₄ is weak. A softening of the Mo–O nearest-neighbours bond has been detected, but the MoO₄ units still exhibit high rigidity. Above the T_g temperature, no meaningful evidence of local structural and dynamical modifications has been observed around molybdenum.     

Growth of LGSO: Ce crystals by the Czochralski method

Single crystals of Lu_2x Gd_{2 ? 2x} SiO₅: Ce (0 < x < 1) compounds with different atomic ratios Lu/(Lu + Gd) have been grown by the Czochralski method. It has been shown that a change in the spatial symmetry from P2₁/c to C2/c in the course of substitution of lutetium for gadolinium occurs at the ratio Lu/(Lu + Gd) = 0.1. The lattice thus formed with symmetry C2/c in the structure of Lu_2x Gd_{2 ? 2x} SiO₅: Ce crystals favors the maximum possible incorporation of Ce³⁺ ions into the sevenfold-coordinated position with respect to oxygen. This explains the substantial improvement of the scintillation characteristics of the grown crystals.     

Optical properties of trigonal single crystals (Yb,Tm)Al3(BO3)4 grown from fluxes based on the bismuth and lithium molybdates

The conditions for synthesis of Yb _x Tm_{1 − x} Al₃(BO₃)₄ (x = 0, 0.1, 0.2, 1.0) single crystals from fluxes based on bismuth trimolybdate Bi₂Mo₃O₁₂ and lithium molybdate Li₂MoO₄ are investigated. It is proposed to grow them by the group method on seeds. The polarized optical absorption spectra are measured for two mutually orthogonal linear polarizations at temperatures of 100 and 300 K.     

Recrystallization of natural chrysoberyl in multicomponent melts

Chrysoberyl and alexandrite crystals have been grown from solutions in melts based on the Li₂CO₃-MoO₃, Bi₂O₃-MoO₃, PbO-V₂O₅, Na₂B₄O₇, and K₂MoO₄-MoO₃ systems using natural alexandrite and chrysoberyl debris as the initial BeAl₂O₄ compound. An analysis of the morphology and homogeneity of the crystals grown has revealed the Bi₂O₃-MoO₃ solvent to be the most appropriate. The optimal color characteristics (??quality?? of alexandrite effect) manifest themselves when adding about 5 mol % Cr₂O₃. The largest crystals (up to 10 mm in size) were obtained from a solution in melt based on PbO-V₂O₅ at a ratio of the crystal-forming component to the solvent of 9: 91 wt %; These characteristics, along with a relatively low operating temperature (970°C), give grounds to consider this type of solvent promising.     

Hydrothermal synthesis and the crystal structure of borate cancrinite (Na,Ca)2[Na6(AlSiO4)6](BO3) · 2H2O

Transparent prismatic single crystals of borate cancrinite (Na,Ca)₂[Na₆(AlSiO₄)₆](BO₃) · 2H₂O are prepared through hydrothermal crystallization. The parameters of the hexagonal unit cell and intensities of 10806 reflections are measured on an Enraf—Nonius CAD4 automated diffractometer. The compound crystallizes in the hexagonal crystal system with the unit cell parameters a = 12.745(4) Å, c = 5.180(2) Å, V = 728.6(4) ų, and space group P6₃. The structure is determined by direct methods and refined using the full-matrix least-squares procedure in the anisotropic approximation for the non-hydrogen atoms. The refinement of the structure is performed to the final discrepancy factor R ₁ = 0.027 for 2889 unique reflections with I > 2 σ (I). In the structure of the borate cancrinite, the AlO₄ and SiO₄ tetrahedra form a zeolite-like framework in which twelve-membered hexagonal channels are occupied by sodium atoms and BO₃ groups, whereas six-membered channels are filled with sodium and calcium atoms and water molecules. The mean interatomic distances are found to be as follows: (Si-O)_mean = 1.614 Å and (Al-O)_mean = 1.741 Å in the AlO₄ and SiO₄ tetrahedra, (Na-O)_mean = 2.542 Å in the seven-vertex sodium polyhedra, and [(Na,Ca)-O]_mean = 2.589 Å in the ditrigonal bipyramids.     

Ionic conductivity of KMgCr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> molybdate

The ionic conductivity σ of KMgCr(MoO₄)₃ crystal has been investigated in a temperature range of 575–932 K by impedance spectroscopy in the frequency range of (5–5) × 10⁵ Hz. Ternary molybdate was obtained from the initial MgMoO₄ and KCr(MoO₄)₂ reagents by solid-phase technique in air at 923–973 K for 200 h. The temperature dependence σ(T) of a ceramic sample exhibits a jump of σ by a factor of about 4 at 833 ± 5 K, which is caused by the first-order phase transition. The σ value above the phase-transition temperature reaches 6 × 10^–4 S/cm (932 K) at an ion-transport activation enthalpy of 0.84 ± 0.05 eV. The most likely carriers in KMgCr(MoO₄)₃ are K⁺ cations.     

<Emphasis Type="Italic">Trans</Emphasis>-dibromodioxodimethylformamide molybdenum(VI) MoO<Subscript>2</Subscript>Br<Subscript>2</Subscript>(OCHNMe<Subscript>2</Subscript>)<Subscript>2</Subscript>

The molecular structure of trans-dibromodioxodimethylformamide molybdenum(VI), MoO₂Br₂(OCHNMe₂)₂ has been determined. Crystal data: Triclinic, P-1, a = 12.3005(2), b = 15.8763(4), c = 21.1653(6) ?, α = 71.992(1)°, β = 88.966(2)°, γ = 89.999(1)°, V = 3930.1(2) ?, Z = 12. Trans-dibromodioxodimethylformamide molybdenum(VI) was obtained by the reaction of sodium molybdate, HBr and dimethylformamide and was characterized by IR, and ¹H NMR spectroscopy and elemental analysis.     

Growth of Er2(SO4)3 ⋅ 8H2O crystals and study of their structure and properties

New crystals of the composition Er₂(SO₄)₃ ? 8H₂O have been synthesized by the method similar to that used for synthesis of (CH₃)₂NH₂Al(SO₄)₂ ? 6H₂O. The synthesized crystals were studied by the X-ray diffraction method. The crystals are monoclinic C2/c) and contain no (CH₃)₂NH₂ ions. It is established that, contrary to DMAAS crystals, Er₂(SO₄)₃ ? 8H₂O crystals undergo no phase transitions and possess neither ferroelectric nor ferroelastic properties.