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1.
A new class of tungstate fluorophosphate glasses was identified in the NaPO3-BaF2-WO3 ternary system. The variation of several physical properties was determined with respect to chemical composition. Characteristic temperatures, density and refractive index increase as tungsten oxide content increases. The optical transmission range and specially the energy bandgap depend of the WO3 amount. No crystallization could be observed for the most WO3 concentrated vitreous samples (?20% molar). Color and optical properties of the glasses depend of the melting time because of the presence of reduced tungsten species like W5+ and W4+. In addition, photodarkening is observed in tungsten rich glass samples under UV laser illumination and this phenomenon can be reversible by heat treatment near the glass transition temperature.  相似文献   

2.
Potassium bismuth tungstate [KBi(WO4)2] single crystals have been grown by the top‐seeded solution growth technique. Bulk crystal with dimensions up to several centimeters is obtained for the first time. Several self‐flux systems have been used for the growth from the solution and the experiments using K2W2O7 as a solvent are detailed. Powder and single crystal X‐ray diffraction of this crystal are reported. The structure refinement shows that KBi(WO4)2 crystallizes in the monoclinic space group C2/c, with a=10.837(3), b=10.586(3), c=7.622(2)Å, β=130.860(3)°, V=661.4(3)Å3, and Z=4. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
Bismuth tungstate catalysts with Bi/W ratios 2/3, 1/1, 2/1, and 6/1 have been investigated after various heat treatments by room temperature X-ray analysis. The activity and selectivity of Bi2WO6 catalyst are suggested to be connected with the presence of corner sharing W—O octhedra in the bulk of the solid that give rise to tetragonal pyramidal W—O sites at the surface. The very low activity of Bi2(WO4)3 is attributed to edge-shared octahedral coordination of W while the same of Bi6WO12 is due to tetrahedral coordination of W. The inactivity of Bi2W2O9 may be associated with its typically low oxidation power. The promoting influence of Bi2O3 is to enforce a structural situation on W—O so that W may be present in a tetragonal pyramidal 5-configuration.  相似文献   

4.
Potassium bismuth tungstate [KBi(WO4)2] single crystals with dimensions up to 20 mm × 15 mm × 15 mm have been successfully grown by using the top‐seeded solution growth technique and K2W2O7 as solvent. Experiments show that this crystal is unstable in a strong acid or alkali environment and has a blue fluorescence emission. The density, hardness, melting point, absorption edge, transparency range, prominent Raman shift frequency are 7.57 g/cm3, 238 kg/mm2, 800 °C, 380 nm, 400–5450 nm, 868 cm–1 respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
Raman and luminescence spectroscopy were used to determine the structure of alkali borate tungstate glasses: M2O(B2O3)2·xWO3, M = Li or Na (0 < x < 1). Raman scattering results showed the dominant tungstate species in these photochromic glasses to be tetrahedral WO4=. At high concentrations of WO3, WO3·H2O, and W2O7= are also present. Luminescence measurements provided evidence for an octahedral WO3 structure not identified by the Raman results. The results also revealed a possible change in the structure of the glasses similar to that observed in alkali borate glasses and associated with the “borate anomaly”. In addition, preliminary measurements are reported on the variation of the band gap, density, index of refraction, and the elastic coefficient C11 determined by Brillouin scattering with composition.  相似文献   

6.
A series of Li2O–Al2O3–ZrO2–SiO2 glasses doped with different concentrations of WO3 (0 to 5.0 mol.%) have been synthesized. Differential thermal analysis of the samples indicated increasing glass forming ability with the increasing concentration of WO3 in the glass matrix. A variety of spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties (over a range of frequency and temperature) of these glasses have been investigated. The optical absorption and ESR spectral studies have pointed out that a part of tungsten ions do exist in W5+ state in addition to W6+ state especially in the samples containing low concentration of WO3. The IR and Raman spectral studies have suggested that there is a decreasing degree of disorder in the glass network with increase in the concentration of WO3. The values of dielectric parameters viz., dielectric constant, loss and ac conductivity at any frequency and temperature are observed to decrease as the concentration of WO3 is increased. Such changes have been attributed to decrease of redox ratio or decreasing proportions of W5+ ions that act as modifiers in the glass network. The quantitative analysis of the results of ac conductivity and dielectric properties have indicated an increase in the insulating character of the glasses with the concentration of WO3; this is attributed to the presence of tungsten ions largely in W6+ ions that participate in the glass network forming with WO4 structural units.  相似文献   

7.
《Journal of Crystal Growth》1999,196(1):181-184
High-temperature solutions of the systems K2W2O7–KGd1−xLnx(WO4)2 (x=0.05, 0.10, 0.20 and 0.50; Ln=Er, Eu and Ho) were investigated. The solubility curves were plotted for each x value and each dopant and the concentration and temperature regions of crystallization of the low-temperature monoclinic phase were determined. The distribution coefficients were determined for gadolinium and for the dopants. The data obtained were compared with the distribution coefficients in the case of double doping of KGd(WO4)2 with erbium and neodymium.  相似文献   

8.
《Journal of Non》2006,352(23-25):2371-2375
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.  相似文献   

9.
Using oyster shell, single-crystalline Na2Ti6O13 and Na2W4O13 one-dimensional nanostructures (ODNS) were simply synthesized from granular TiO2 and WO3 particles by thermal heating at 700 °C. The oyster shell was used as a source of both sodium and carbon dioxide, which have an effect on unidirectional growth of single-crystalline Na2Ti6O13 and Na2W4O13 from TiO2 and WO3. The growth of sodium metal oxide ODNS from metal oxide particles was reproduced by replacing oyster shell with chemical reagents (NaCl, CaCO3) or gas (CO2), allowing identification of the main factors for the growth of ODNS. The characteristics of biological material (i.e., oyster shell) lend certain advantages in the fabrication of sodium metal oxide ODNS: (a) low processing temperature with high yield; (b) an eco-friendly method; and (c) no requirement of additional processes such as separation or washing. This work suggests a new approach for using biological material for the crystal growth of nanostructured materials.  相似文献   

10.
《Journal of Non》2006,352(52-54):5618-5632
A continuous network model of xWO3–(1  x)TeO2 glasses is developed, based on quantum-chemical calculation and Raman spectra analysis, in order to relate the structural and vibrational properties with glass composition. The tungstate–tellurite glass network is shown to be formed mainly by structural units of three types, TeO4 trigonal bipyramids, OTeO2 trigonal pyramids, and WO6 octahedra with OW double bonds. Most of the W atoms are found to be incorporated into single OWO5 octahedra, with no more than several percents of these atoms occurring in 2[OWO5] paired tungstate centers. The structural and vibrational properties of tungstate–tellurite glasses of several compositions are analyzed by application of the model and a novel interpretation of the Raman spectra is suggested.  相似文献   

11.
《Journal of Non》2006,352(52-54):5564-5571
We systematically added WO3 (up to 10 mol%) and P2O5 (up to 16 mol%) in TeO2–BaO–SrO–Nb2O5 (TBSN) glass system and studied thermal and optical properties of the resultant glasses. The dependences of the additive concentration on glass transition (Tg) and crystallization (Tx) temperatures are presented. The TBSN glass added with ⩾4 mol% WO3 and P2O5 showed high stability against crystallization. The changes in optical band gap energy due to WO3 and P2O5 addition was studied using UV–VIS–NIR absorption spectrometry. The WO3 addition shifted the optical band gap to longer wavelengths, whereas P2O5 addition shifted that to shorter wavelengths. Effects of the WO3 and P2O5 addition on the Raman spectra of TBSN glass are clarified. New Raman bands due to WO4 and PO4 tetrahedra formed in the resultant glasses broadened their Raman spectra. Present glasses are characterized by higher thermal stability and wider Raman spectra, therefore, they are promising candidates for fiber Raman amplifiers in photonics systems.  相似文献   

12.
Shaaban M. Salem 《Journal of Non》2012,358(11):1410-1416
Homogeneous (50P2O5–(30 ? x)PbO–20NaF–xWO3 where x = 0.0, 5, 10 and 15 mol%) glasses were synthesized using a melt-quenching method. The short range structures of the phosphate samples were examined by Fourier transform infrared spectroscopy. The infrared spectral studies have pointed out the existence of conventional PO4, WO4 and WO6 structural units in the glass network, the number of WO4 tetrahedra decreases as WO3 concentration increases. The optical transmittance and reflectance spectrum of the glasses have been recorded in the wavelength range of 190–1100 nm. The values of the optical band gap Eop for all types of electronic transitions and refractive index have been determined and discussed. The real and imaginary parts ε1 and ε2 of the dielectric constant have been determined. The type of electronic transitions in the present glass system is indirectly allowed and the high values for the refractive index and dispersion are recorded due to the high polarizability of tungsten ions. The results of refractive indices as determined reveal the homogeneity of samples and were found to depend on the glass composition. The electrical properties of the glasses were investigated by ac conductivity from 0.12 to 100 kHz for temperatures ranging from room temperature to 600 K. The study of dielectric properties suggested increase in the insulating character of the glass system with increase in the content of WO3. The ac conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from W4+ to W5+ ions.  相似文献   

13.
The Tungsten oxides WO3, W20O59, W18O49 and WO2 have been transported by TeCl4 in monocrystalline Form. W3O is not a equilibrium phase of the system, and therefore it is impossible to obtain this phase by CTR. The value of the lower phase boundary of tungsten trioxide is 2.96.  相似文献   

14.
The glass formation region in the ternary ZnO―Bi2O3―WO3 system is determined by melt quenching technique (cooling rates 101-102 K/s). New original glasses are obtained in a narrow concentration range with high WO3 content (60-75 mol%). Homogeneous glasses of the composition (100 − x)[0.2ZnO·0.3Bi2O3·0.5WO3]xMoO3, were obtained between 20 and 60 mol% MoO3. Characterization of the amorphous samples was made by x-ray diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR). The thermal stability of glasses decreases with the increasing of MoO3 content. The glass transition temperature, Tg, varies between 340-480 °C, while the crystallization temperature, Tx, varies between 388-531 °C. The tungstate glasses possess higher crystallization temperature (Tx over 500 °C) in comparison with the other vanadate and molybdate non-traditional glasses. The glass network is realized by transformation of three-dimensional structure of WO3 into a layered one, consisting mainly of WO6 units. We supposed that the network of quaternary glasses is built up by MoO4, MoO6 and WO6. At low concentration ZnO and Bi2O3 facilitate the disorder in the supercooled melts, while at high concentration stimulate crystallization processes. These oxides belong to the intermediate ones.  相似文献   

15.
The results of a study of KGd(WO4)2 single crystals by Brillouin scattering method are presented. The Brillouin spectra for the acoustic phonons propagating in the [100], [010], [001], [110] and [101] directions, taken in 90° and 180° scattering geometry at room temperature have been recorded. Using the Brillouin spectroscopy the refractive indices for some directions of KGd(WO4)2 crystals were determined. The refractive indices were also calculated for the same directions by the rotation transformation of the principal optical axes of the optical indicatrix to the crystallographic directions of KGd(WO4)2 crystals. Moreover, some anomalies of the acoustic phonons propagating connected with birefringence of KGd(WO4)2 crystals were discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
The crystallisation kinetics of strontium tungstate from unstirred saturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallisation temperatures T0 = 1000° to 800°C to room temperature at cooling rates RT = 0.67° to 3.3°C min−1. The main crystal growth was diffusion rate-controlled; the final crystal growth was rate-controlled by the development rate of excess solute concentration. The estimated diffusion rate constant (kD) values increased with cooling rates and initial crystallisation temperatures. They are higher than the rate constants for diffusion-controlled growth of calcium tungstate from sodium tungstate melts, but very much smaller than those for strontium tungstate from lithium chloride melts.  相似文献   

17.
N. Maury  F. Cambier 《Journal of Non》2011,357(3):1079-1084
This paper shows that glass-ceramics containing highly surface and bulk preferentially oriented fresnoite Sr2TiSi2O8 crystals can be synthesised by a simple isothermal heat treatment of suitable glass compositions in the Sr-Ti-Si-K-B-O system. For all tested compositions, crystallisation starts from the free surfaces of the specimens and propagates to bulk with time. If most of these compositions lead to (00l) preferential orientation at the specimens' surfaces, bulk crystal texture is very composition dependent. The effects of variation in K2O and B2O3 contents on the crystallisation have been studied. It is shown that low K2O and high B2O3 contents are required to keep the (00l) orientation from the surface into the bulk. This result seems to be explained by the viscosities of the initial and residual glasses at the temperature of crystallisation: a low viscosity leads to a fine and homogeneous microstructure with small and strongly (00l) bulk oriented crystals.  相似文献   

18.
Single crystals of NaBi(WO4)2 (NBW) have been grown by Czochralski technique, employing differently synthesized starting charge and different axial temperature gradients. The causes behind inherent problems related to compositional changes and associated lowering in crystallization temperature have been probed by analyzing XRD and DTA patterns of post growth residual charge. Presence of low melting phases viz. Na2WO4 and hitherto unreported compound Na5Bi(WO4)4 is thought to be responsible for the observed decrease in crystallization temperature. This problem was tackled by segregating pure phase material through re‐crystallization, under high axial temperature gradient. The use of re‐crystallized charge enabled transformation of almost the entire charge into a single crystal of high transparency. The effect of starting charge synthesis and temperature gradient on the optical transmission characteristics of NBW crystal has also been investigated. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
The reflection spectra in the fundamental-absorption region, 5–25 eV (250–40 nm), of optically active crystals with cubic symmetry (NaClO3, NaBrO3) and uniaxial optically active crystal (LiIO3) have been investigated. It is shown that the reflection spectra of cubic crystals have a similar structure, which is determined by the electronic transitions in the XO3 group. The comparison of these spectra with the corresponding spectrum of lithium iodide made it possible to determine the type of transition in the spectra of cubic crystals. Using the projection operator method, it was shown that the sign of optical rotation of cubic crystals with symmetry T is independent of the screw axis sign. Possible reasons for the unprecedentedy large optical rotation of lithium iodide crystal in the optical axis direction are considered.  相似文献   

20.
The growth and scintillation properties of the Na2W2O7 crystal are reported. The solid reaction between Na2CO3 and WO3 is used to synthesise the Na2W2O7 material. The Na2W2O7 single crystal has been grown by the Bridgman method. And the Na2W2O7 single crystal with sizes 14×7×6 mm3 has been achieved. The transmission spectra, the Ultraviolet fluorescence spectra and the X-ray excited luminescence spectra of the Na2W2O7 crystal are measured. The measurement results show that the Na2W2O7 crystal is a promising intrinsic scintillator.  相似文献   

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