Borosilicate glass structure: An investigation of high resolution B K-edge XANES
The Alkali-borosilicate glasses in the systems Na2O-B2O3-SiO2 and K2O- B2O3-SiO2 have been prepared by melting/quenching in air and studied using synchrotron radiation B K-edge XANES to estimate the evolution of boron coordination as a function of composition. The ratio of alkali/B2O3 (R) and SiO2/B2O3 (K) in the glasses are respectively between 0.5 to 2.0 and 0.5 to 7.0. The edge features of trigonal B (B) and tetrahedral B (B) in B K-edge XANES spectra have been interpreted carefully from B standards such as (B2O3 and BPO4), as well as, a wide range of borate minerals. We find that the proportion of tetrahedral B in glass is increasing as a function of both R and K, similar to previous studies. Contributions of the B and B features to the B K-edge XANES is complex with 6-7 individual transitions contributing to the overall spectral envelope. Many of these transitions are common to both B coordination states making extraction of quantitative B numbers difficult. However, we can calculate the proportion of tetrahedral B accurately by appropriate curve- fitting.
The Effects of Composition, Pressure and Temperature on the Structure of Potassium Silicate Glasses
Potassium Silicate glasses of composition xSiO2 _ (100-x)K2O (x = 85, 80, 75, 70, 67 mol%) were analysed with an X-ray Photoelectron Spectrometer (XPS). The extent to which bridging oxygen and metal bridging oxygen atoms react to form non-bridging oxygen atoms was documented. Compositional data derived from XPS analysis and values calculated via stoichiometry were compared. It was found that an excess of metal bridging oxygen atoms remained in the glass network suggesting that the reaction 2K2O + SiO2 -> K4SiO4 does not go to completion. Stated otherwise, not all potassium atoms give rise to non-bridging oxygen atoms. An equilibrium constant of k = 90 was calculated to fit the distribution of bridging oxygen atoms obtained from XPS analysis. Two high pressure experiments were performed on glasses of composition 70mol% SiO2 _ 30mol% K2O at pressures of 2.0 GPa and 4.0 GPa at ambient temperature. Another two experiments were conducted at 4.0 GPa and 1200°C. The bridging oxygen abundance was found to increase with pressure. The high pressure - temperature experiments showed a variation in the abundances of bridging oxygen atoms relative to the samples analysed under atmospheric conditions. An implication is that increases in pressure affect the bridging to non-bridging oxygen concentration by driving the reaction left. The results presented are preliminary and are in the process of being investigated in greater detail.
Network Rigidity in GeSe2 Glass at High Pressure
Gradual structural transitions in glasses, which occur via continuous changes in density, are inherently difficult to identify due to their disordered nature. Acoustic measurements using synchrotron radiation have been performed on glassy GeSe2 up to pressures of 9.6 GPa. A minimum observed in the shear-wave velocity, associated anomalous behaviour in Poisson's ratio, and discontinuities in elastic moduli at 4 GPa are indicative of a gradual structural transition in the glass. This is attributed to a network rigidity minimum originating from a competition between two densification mechanisms. At pressures up to 3 GPa, a conversion from edge-to-corner sharing tetrahedra results in a more flexible network. This is contrasted by a gradual increase in coordination number with pressure, which leads to an overall stiffening of the glass.
A First Step Toward Understanding Nucleation Processes: in situ High-Temperature X-ray Diffraction and Absorption Investigations
Nucleation is the first step of the transition between the amorphous and crystalline states and thus plays a key role in Earth and Materials sciences whenever crystallization takes place. In spite of its considerable importance in igneous petrology and industrial applications (ceramics, glass-ceramics, etc.), nucleation remains known poorly because of the difficulties of investigating the structural rearrangements that take place at a nm scale when an ordered atomic packing begins to develop in a melt. In addition, the structure of amorphous phases is not only difficult to determine, but the wealth of information available for glasses is not necessarily applicable to nucleation because of the existence of temperature-induced structural changes in melts. In view of the basic geological and industrial importance of the SiO2-Al2O3-CaO system, we have investigated a calcium aluminosilicate whose crystallization has already been studied. And because elements such as Ti or Zr can promote rapid nucleation, information can be gained about the structural changes they induce by probing specifically their own environment. In this work we have thus performed a high-temperature study of the very first steps of crystallization in a calcium aluminosilicate with 7 mol percent ZrO2 by X-ray absorption measurements at the Zr K-edge et 1873 K on the homogenous melt and 1173 K on a nucleating supercooled liquid. To complement these results with information on medium range order (MRO) X-Ray diffraction experiments have also been performed under the same conditions. As a reference, the glass has been investigated by both techniques at room temperature.
Al Speciation in Silicate Melts: AlV a new Network Former?
The first human glasses were made 3500 BC. It was essentially sodo-lime silicate glass. To improve the chemical resistance, the thermal properties and increase the viscosity it is interesting to add aluminum in these silicates. But what is the speciation of the aluminum and how it varies according to the chemical composition and to the temperature? The aluminum appears essentially in four or five fold coordination in glasses and melts melted. The proportion of Al varies according to the alkaline or to the earth-alkaline content and to the temperature. We shall present in a first part the influence of the network-modifier on the proportion of Al and then we shall present some new results of absorption of high-temperature using NMR and XANES spectroscopy at the Al K-edge. Finally, from glass transition temperature measurements we propose to explain that Al can be a new network former.
Are Obsidians some kind of quenched vitroceramics ?
X-ray Absorption spectroscopy (XANES) and Optical Absorption Spectroscopy (OAS) have been used to determine the local environment around iron in calco-alkaline rhyolitic glasses (obsidians) of various localities. The OAS measurements were performed at ambient and low temperatures (up to 10K). XANES measurement show that obsidians present variable Fe2+/Fe3+ ratio depending on formation conditions and provenance. OAS measurements show that isolated Fe2+ is located in a regular octahedral site , a local environment never seen in other types of iron bearing glasses, whereas isolated Fe3+ is found in tetrahedral coordination, in the glassy matrix. Specific large bands in the spectra are related to Fe2+ - Fe3+ and Fe2+ - Ti4+ intervalence charge transfers showing a increasing intensity with the decrease of temperature, from ambient to 10K. The specific coordination of Fe2+ together with the two charge-transfers may be related to the existence of iron oxide clusters (5nm), in the glass. These clusters, showing a local re-arrangement around iron in the glass, may be precursors of small crystalline iron oxide phases (titano-magnetite and magnetite) either amorphous or crystalline, evidenced using TEM (10nm). The existence of these clusters and their nature seem to be related to the conditions of formation of the investigated obsidians.
Structural control of the stability of nuclear waste glasses
Vitrification of liquid high-level radioactive waste in borosilicate glasses has received a great attention in several countries. The fundamental properties of the waste forms are their chemical and mechanical durability. We present an overview of the local structure of inactive analogs of the French nuclear glass, using structural information obtained by a combination of X-ray absorption Fine Structure (XAFS) and Wide Angle X-ray Scattering (WAXS). We will first contrast several classes of elements, such as Zr, Mo or Zn, which give nuclear glasses peculiar positive or adverse properties for the industrial process: enhanced chemical stability, phase separation, crystal nucleation and separation. These properties may be rationalized using Pauling rules, familiar to Mineralogists, as other properties are correctly modelled in simplified glass compositions using molecular dynamics. We will also point out the importance of the melt-to-glass transition and the consequence of the glass structural properties on the resistance of glassy matrices to irradiation. Glass alteration affects the long-term stability of the glass. It is characterized by an amorphous (glass)-amorphous (gel) transformation. Depending on alteration conditions, alteration layers may have or not a protective character, which will influence radionuclide retention over time. We will present the structural modification of the surface chemistry of the glass monoliths during short-term experiments and the evolution towards a gel, which forms progressively at the expense of the glass. The protective character of the gel, observed during glass leaching under near-saturated conditions, will be rationalized by its structural properties.
Tetrahedrally Coordinated Fe3+ in Silicate Glasses: A Mossbauer, Iron K-edge XANES and Raman Spectroscopies Study
In natural or industrial glasses, iron is the most abundant transition metal. A good knowledge of its redox equilibrium is important to better understand the chemical and structural evolution of magmas (crystallization, viscosity), and also to optimize vitrification processes and properties of iron-bearing glasses. To study the role of iron in silicate glasses and melts, we have used in a consistent manner the Mössbauer, iron K-edge XANES and Raman spectroscopies to investigate several series of silicate glasses as a function of redox state. The samples were selected to cover a wide composition range and to investigate the interactions of iron with two network forming cations, namely, Al3+ and B3+. The glasses investigated were synthesized at high temperature under various conditions of oxygen fugacity to achieve different redox ratios for each composition. Therefore, the iron redox state was varied from the most oxidized to the most reduced. Iron redox ratios were first determined by wet chemical analysis and in some cases by room temperature Mossbauer spectroscopy. This experimental method was also used to determine the local structure of iron of some of the investigated glasses. These results where compared to iron K-edge XANES/EXAFS spectroscopy results, which lead to the iron redox state and indicate that Fe2+ is in octahedral coordination whereas Fe3+ is in tetrahedral coordination. In addition, Raman spectroscopy gave us information on the network polymerization of glasses. Clearly changes in Raman spectra are visible with the evolution of iron redox ratio. For a given composition, we observed systematically, in the 800-1200 cm-1 envelope, which is sensitive to the environment of tetrahedrally coordinated cations, the growth of a band with the iron content and the oxidation state of the sample. The peak area of this band, which we attribute to vibrational modes involving tetrahedrally coordinated Fe3+, increases with the oxidation of the sample. This evolution leads us to establish a calibration procedure for a given composition. Calibration curves can be followed to investigate in situ kinetics of redox reactions. We present here results on the role of iron and its interactions with the silicate network for several compositions as pyroxene based glasses and iron bearing alkali alumino-borosilicate glasses.
M-Edge XANES Study of Ce3+/Ce4+ in Synthetic Andesitic Glasses
Rare earth elements (REEs) have provided useful information in igneous, sedimentary and metamorphic petrology. The REEs have similar chemical and physical properties due to the fact that they all form stable 3+ ions with similar ionic radii, however, a small number of REEs can also exist in other oxidation states. The speciation of the REEs europium (Eu) and cerium (Ce) in a melt is primarily controlled by the oxygen fugacity (fO2) of the system. Under the fO2 conditions observed in terrestrial magmas Eu can exist as 2+ or 3+ and Ce as 3+ or 4+. The speciation of these elements has significant effect on partitioning between phases in igneous systems. Glasses are amorphous solids that exhibit the glass transition [c.f. 1] and are widely used in a number of commercial and industrial applications. Glasses can be used as proxies for silicate melts and, thus, can be used to investigate the chemical and physical behaviour of magmas and igneous processes. To investigate the change in Ce speciation with fO2 in igneous systems a series of doped Fe-free andesitic glasses were synthesized using a piston cylinder at 1GPa and 1300°C. Oxygen fugacity was internally buffered using noble metal oxides for high fO2s and graphite to achieve low fO2s. Cerium M-edge XANES spectra were measured using synchrotron radiation on the SGM beamline at the Canadian Light Source. The spectra show dramatic changes in the 0.902 and 0.907 keV regions of the M4 peak. A total of seven curves were fit to the M4 peak giving R2 values greater than 0.995. The center of the fourth peak (∼0.902 keV) shows systematic shift to lower energy with decreasing fO2 which is a direct result of changes in the relative proportions of Ce3+ and Ce4+ between samples. 1. Henderson, G.S. 2005. The structure of silicate melts: A glass perspective. Canadian Mineralogist, 43, 1921- 1958.