Local motifs in GeS2–Ga2S3 glasses Pethes, I.; Nazabal, V.; Chahal, R. ...
Journal of alloys and compounds,
07/2016, Letnik:
673
Journal Article
Recenzirano
Odprti dostop
The structure of (GeS2)0.75(Ga2S3)0.25 and (GeS2)0.83(Ga2S3)0.17 glasses was investigated by Raman scattering, high energy X-ray diffraction and extended X-ray absorption fine structure (EXAFS) ...measurements at the Ga and Ge K-edges. The reverse Monte Carlo simulation technique (RMC) was used to obtain structural models compatible with diffraction and EXAFS datasets. It was found that the coordination number of Ga is close to four. While Ge atoms have only S neighbors, Ga binds to S as well as to Ga atoms showing a violation of chemical ordering in GeS2–Ga2S3 glasses. Analysis of the corner- and edge-sharing between GeS4/2 units revealed that about 30% of germanium atoms participate in the edge-shared tetrahedra.
•Structural models of GeS2–Ga2S3 glasses consistent with XRD + EXAFS data are created.•Chemical order is respected but Ga–Ga bonds are present caused by S-deficiency.•The coordination number of Ga is 3.7 ± 0.3.•The frequency and geometry of corner/edge-sharing GeS4/2 units were determined.
•We discuss the chemical mechanism of chromium conversion coatings (CCC).•We identify the functions of each chemical species composing the coating.•We identify the role and the influence of each step ...of the process of CCC.•We evaluate the corrosion properties of each compound of the coating formed during the deposition process.
Magnesium and its alloys present several advantages such as a high strength/weight ratio and a low density. These properties allow them to be used for many aeronautical applications but they are very sensitive to corrosion. To solve this problem, conversion coatings are deposited on the surface before a protective top coat application. Several kinds of coatings exist but the best protective is chromium conversion coating (CCC). This process is now limited by several environmental laws due to the high toxicity of hexavalent chromium. However, in order to reduce hazardous impact onto the environment and to find alternative coatings, the chemical mechanisms of CCC deposition and protection on magnesium alloy are detailed for the first time in this work. The studied process includes 4 pre-treatments steps and a conversion immersion bath. The pre-treatment steps clean and prepare the surface for improving the coating deposition. The coating properties and its composition were characterized by voltammetry and XPS technics. A final layer of chromium(III) oxide and magnesium hydroxide composed the coating giving it its protective properties. Trapped orthorhombic potassium chromate has also been identified and gives to the coating its self healing property.
Dy3+ and Pr3+ doped Ga5Ge20Sb10S(Se)65 glasses provide good emission efficiencies in the mid-infrared (mid-IR). The complex glassy network of these chalcogenide glasses were characterized by Raman ...scattering and extended X-ray absorption fine structure, to put in evidence a structure combining tetrahedra of Germanium and Gallium and trigonal pyramids of Antimony. The arrangement of these structural units permits the introduction of rare earth ions thanks, in particular, to a charge compensation generated by the Ga in a tetrahedral site. Consequently, Pr3+ and Dy3+ ions inserted in such low phonon energy glasses emit efficiently in mid-IR, between 3.5 and 5 μm. Finally, Dy3+ and Pr3+ doped Ga5Ge20Sb10S(Se)65 fibers were obtained from bulk glass preform. Efficient emission in mid-IR was obtained by pumping Dy3+ doped Ga5Ge20Sb10S65 and Pr3+ doped Ga5Ge20Sb10S(Se)65 fibers at 920 nm and 2 μm, respectively.
Carbonate-rich silicate and carbonate melts play a crucial role in deep Earth magmatic processes and their melt structure is a key parameter, as it controls physical and transport properties. ...Carbon-rich melts can be strongly enriched in trace elements, but the structural incorporation mechanisms of these elements are difficult to study because such melts generally cannot be quenched to glasses. In this contribution we investigate the influence of CO2 on the local environments of trace elements contained in silicate glasses with variable CO2 concentrations and in silicate and carbonate melts. The melts were studied in-situ at high pressure and temperature conditions using the Paris-Edinburgh press (2.2 to 2.6GPa and 1200 to 1500°C). The compositions studied include sodium-rich peralkaline silicate melts and glasses and carbonate melts similar to those occurring naturally at Oldoinyo Lengai volcano. The local environments of yttrium (Y), lanthanum (La) and strontium (Sr) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Main findings of the study suggest: (1) In peralkaline silicate glasses the local structure of Y is unaffected by the CO2 content. Contrary, a slight increase of oxygen bond lengths of Sr and La is inferred with increasing CO2 content in peralkaline glasses, while they remain constant in glasses of even higher peralkalinity independent of the CO2 content. (2) In silicate melts of different CO2 contents Y-O bond lengths are constant, while a slight increase within carbonate melt compositions is deduced. On the other hand, a steady bond lengths increase over the whole compositional range is inferred for La-O and Sr-O. This may well be explained by distinct preferences of these elements for specific local environments. Based on these new data, we suggest potential mechanisms for the structural incorporation of these elements, a key step towards understanding their partitioning behavior in natural magmatic systems.
The structures of nickel sulfoselenide Chevrel phases have been determined. All the richest nickel phases crystallize in rhombohedral symmetry. A single-crystal structural study makes it possible to ...identify the special nickel cation locations inside the host network for both pure sulfide and selenide Chevrel phases. We also show that these nickel atoms can form some kind of cluster in the cavities of the host or Ni–Mo bondings.
The evolution of Ni2+ environment has been systematically investigated using optical and in situ X-ray absorption spectroscopy (XAS) to determine the influence of nucleating agents (TiO2 and/or ZrO2) ...during the formation of spinel in magnesium aluminosilicate glass–ceramics. The results were complemented by in situ X-ray diffraction data. According to XAS and optical spectroscopy, the nature of nucleating agents does not modify significantly the Ni environment in initial glasses. However, it has a relatively strong influence in the observed crystallization sequence. Ni2+ ions do not enter the Zr-containing crystalline phase of ZrO2 or ZrTiO4 but a Ni2+ coordination change from the fivefold coordinated sites, with a small amount of tetrahedral sites in parent glasses, to 6Ni2+ and 4Ni2+ sites in spinel (in glasses nucleated by ZrO2 and/or TiO2) or in β-quartz solid solutions (in glasses nucleated by ZrO2) has been found.
PDF
