Investigating cerium redox changes between aluminosilicate glass and melt: A multispectroscopic approach.

Redox control of glasses is paramount both to their fusion process and to obtaining the desired properties of high technological glasses. However, the link between melting parameters, such as temperature, furnace atmosphere, or quenching rate, and the redox state of the final products is poorly understood. In this work, in situ x-ray absorption near-edge structure (XANES) data at Ce L3-edge data were acquired at high temperatures on cerium-containing sodium aluminosilicate glasses, allowing the determination of thermodynamic constants necessary to predict the cerium redox state over a wide temperature range (900-1500 °C). The results obtained were compared to the Raman spectra of samples quenched at different temperatures. Our findings demonstrate that the quench performed was fast enough to block the cerium oxidation state, meaning the redox measured at room temperature is representative of a high temperature state. This was further verified by room temperature Raman spectroscopy, where a relationship was found between the spectra and melting conditions. Wet chemical analysis, XANES at Ce L3-edge, Raman spectroscopy, and optical absorption spectroscopy were successfully used to determine the redox state of cerium in aluminosilicates.

Structure of diopside, enstatite, and magnesium aluminosilicate glasses: A joint approach using neutron and x-ray diffraction and solid-state NMR.

Neutron diffraction with magnesium isotope substitution, high energy x-ray diffraction, and 29Si, 27Al, and 25Mg solid-state nuclear magnetic resonance (NMR) spectroscopy were used to measure the structure of glassy diopside (CaMgSi2O6), enstatite (MgSiO3), and four (MgO)x(Al2O3)y(SiO2)1-x-y glasses, with x = 0.375 or 0.25 along the 50 mol. % silica tie-line (1 - x - y = 0.5) or with x = 0.3 or 0.2 along the 60 mol. % silica tie-line (1 - x - y = 0.6). The bound coherent neutron scattering length of the isotope 25Mg was remeasured, and the value of 3.720(12) fm was obtained from a Rietveld refinement of the powder diffraction patterns measured for crystalline 25MgO. The diffraction results for the glasses show a broad asymmetric distribution of Mg-O nearest-neighbors with a coordination number of 4.40(4) and 4.46(4) for the diopside and enstatite glasses, respectively. As magnesia is replaced by alumina along a tie-line with 50 or 60 mol. % silica, the Mg-O coordination number increases with the weighted bond distance as less Mg2+ ions adopt a network-modifying role and more of these ions adopt a predominantly charge-compensating role. 25Mg magic angle spinning (MAS) NMR results could not resolve the different coordination environments of Mg2+ under the employed field strength (14.1 T) and spinning rate (20 kHz). The results emphasize the power of neutron diffraction with isotope substitution to provide unambiguous site-specific information on the coordination environment of magnesium in disordered materials.

Lithium Borates from the Glass to the Melt: A Temperature-Induced Structural Transformation Viewed from the Boron and Oxygen Atoms.

A multiedge study of the local structure of lithium borate glasses and melts has been carried out using X-ray Raman scattering (XRS) as a function of temperature. Thanks to a wide range of compositions, from pure B2O3 up to the metaborate composition, we are able to finely interpret the modifications of the local environment of both the boron and oxygen atoms in terms of boron coordination number, formation of nonbridging oxygens (NBOs), and polymerization degree of the borate framework as a function of temperature and composition. A temperature-induced [4]B to [3]B conversion is observed above the glass transition temperature (Tg) from the glass to the melt from the triborate composition up to the metaborate composition. Two distinct melt structures are reported: a well-polymerized borate network-with few NBOs-below the triborate composition and a depolymerized borate network above the diborate composition with a rapid increase of the number of NBOs when Li2O is added. These two structurally distinct melts allow explaining the two dynamic regimes observed for lithium ion diffusion.

Quantitative determination of the phosphorus environment in lithium aluminosilicate glasses using solid-state NMR techniques.

We investigated using solid-state NMR spectroscopy the short-range structural features in lithium aluminosilicate glasses with the addition of P2O5 and considering various Al2O3/Li2O ratios. The phosphorus environment is determined quantitatively using 31P Magic Angle Spinning NMR constrained by results obtained from 31P-27Al Multiple-Quantum Coherence-based NMR techniques. Phosphorus is mainly located as orthophosphate and pyrophosphate species in glasses with a low amount of Al2O3. These depolymerized units disappear with increasing Al2O3 content and a strong affinity of PO4 tetrahedra for aluminum is revealed, which reduces phase separation. The local environments of framework (Si and Al) and charge-balancing (Li) cations are also studied through NMR experiments to assess the influence of P2O5 addition. The Si environment is mostly modified by the presence of P2O5 in glasses containing a low amount of Al2O3, with an increase of Q4Si species in relation to phase separation phenomena observed in these compositions. Conversely, P2O5 addition does not have a significant influence on the 27Al NMR response. 7Li NMR spectra reflect a change in the structural role of Li when P2O5 or Al2O3 is added. The observed structural changes can be rationalized to improve our knowledge of the structure-property relationships, focusing, in particular, on phase separation and nucleation/crystallization processes that are strongly affected by the presence of P and the evolution of its local environment with composition.

Speciation Change of Uranyl in Lithium Borate Glasses.

Determining the uranyl(VI) UO22+ reactivity in crystalline and amorphous oxides is necessary to control its mobility. The intrinsic versatility of borate structural units containing both triangular BO3 and tetrahedral BO4 makes them original and rich hosts for uranyl. As part of the effort to determine the uranium stability in borate oxides, we have determined the speciation of uranium(VI) in two lithium borate glasses containing, respectively, 10 mol % and 30 mol % Li2O using a combined structural and spectroscopic approach based on X-ray absorption spectroscopy (XAS). M4- and L3-edge high-resolution XAS demonstrates the speciation of U(VI) as uranyl in both glasses. Comparison of uranyl bond distances obtained by EXAFS with distances found in borate crystals reveals that in the low alkali borate glass, uranyl is present as hexagonal bipyramids with six equatorial oxygen ligands. This local environment was never observed in any other oxide glass. We show that the increase of the lithium content induces the decrease of the equatorial coordination number. The associated uranyl bond elongation suggests the influence of the alkali cations in relation with drastic changes in the structure of the borate network. The spectroscopic evidence of this speciation change is discussed in terms of change in the uranyl electronic structure and covalency.

Short and medium range structures of 80GeSe2-20Ga2Se3 chalcogenide glasses.

The short and medium range structures of 80GeSe2-20Ga2Se3 (or Ge23.5Ga11.8Se64.7) chalcogenide glasses have been studied by combining ab initio molecular dynamics (AIMD) simulations and experimental neutron diffraction studies. The structure factor and total correlation function were calculated from glass structures generated from AIMD simulations and compared with neutron diffraction experiments showing reasonable agreement. The atomic structures of ternary chalcogenide glasses were analyzed in detail, and it was found that gallium atoms are four-fold coordinated by selenium (Se) and form [GaSe4] tetrahedra. Germanium atoms on average also have four-fold coordination, among which Se is 3.5 with the remaining being Ge-Ge homo-nuclear bonds. Ga and Ge tetrahedra link together mainly through corner-sharing and some edge-sharing of Se. No homo-nuclear bonds were observed among Ga atoms or between Ge and Ga. In addition, Se-Se homo-nuclear bonds and Se chains with various lengths were observed. A small fraction of Se atom triclusters that bond to three cations of Ge and Ga were also observed, confirming earlier proposals from 77Se solid state nuclear magnetic resonance studies. Furthermore, the electronic structures of ternary chalcogenide glasses were studied in terms of atomic charge and electronic density of states in order to gain insights into the chemical bonding and electronic properties, as well as to provide an explanation of the observed atomic structures in these ternary chalcogenide glasses.

Structural study of Na2O-B2O3-SiO2 glasses from molecular simulations using a polarizable force field.

Sodium borosilicate glasses Na2O-B2O3-SiO2 (NBS) are complex systems from a structural point of view. Three main building units are present: tetrahedral SiO4 and BO4 (BIV) and triangular BO3 (BIII). One of the salient features of these compounds is the change of the BIII/BIV ratio with the alkali concentration, which is very difficult to capture in force fields-based molecular dynamics simulations. In this work, we develop a polarizable force field that is able to reproduce the boron coordination and more generally the structure of several NBS systems in the glass and in the melt. The parameters of the potential are fitted from density functional theory calculations only, in contrast with the existing empirical potentials for NBS systems. This ensures a strong improvement on the transferability of the parameters from one composition to another. Using this new force field, the structure of NBS systems is validated against neutron diffraction and nuclear magnetic resonance experiments. A special focus is given to the distribution of BIII/BIV with respect to the composition and the temperature.

Detecting non-bridging oxygens: non-resonant inelastic X-ray scattering in crystalline lithium borates.

Probing the local environment of low-Z elements, such as oxygen, is of great interest for understanding the atomic-scale behavior in materials, but it requires experimental techniques allowing it to work with versatile sample environments. In this paper, the local environment of lithium borate crystals is investigated using non-resonant inelastic X-ray scattering (NRIXS) at energy losses corresponding to the oxygen K-edge. Large variations of the spectral features are observed close to the edge onset in the 535-540 eV energy range when varying the Li2O content. Calculations allow identification of contributions associated with bridging oxygen (BO) and non-bridging oxygen (NBO) atoms. The main result resides in the observed core-level shift of about 1.7 eV in the spectral signatures of the BO and NBO. The clear signature at 535 eV in the O K-edge NRXIS spectrum is thus an original way to probe the presence of NBOs in borates, with the great advantage of making possible the use of complex environments such as a high-pressure cell or high-temperature device for in situ measurements.

Rearrangement of the structure during nucleation of a cordierite glass doped with TiO(2).

Ordering of disordered materials occurs during the activated process of nucleation that requires the formation of critical clusters that have to surmount a thermodynamic barrier. The characterization of these clusters is experimentally challenging but mandatory to improve nucleation theory. In this paper, the nucleation of a magnesium alumino-silicate glass containing the nucleating oxide TiO(2) is investigated using neutron scattering with Ti isotopic substitution and (27)Al NMR. We identified the structural changes induced by the formation of crystals around Ti atoms and show important structural reorganization of the glassy matrix.

Environment of titanium and aluminum in a magnesium alumino-silicate glass.

The structure of the glass 2MgO-2Al(2)O(3)-5SiO(2)-TiO(2) was investigated using neutron diffraction with isotopic substitution. Reverse Monte Carlo (RMC) modeling was used to reproduce experimental structure factors derived from diffraction experiments. The local environment of titanium atoms was determined and it corresponds to an average of 5.4 ± 0.2 oxygen atoms at a mean distance of 1.86 ± 0.02 Å. This coordination number agrees with the predominance of fivefold coordination, with the coexistence of four- and sixfold coordination in similar amounts. (27)Al nuclear magnetic resonance (NMR) results revealed that the proportion of highly coordinated aluminum atoms in this titanium-bearing glass was higher than in the titanium-free sample. RMC modeling was used to interpret the structural role of these ([5])Al species and we show a trend for preferential bonding between ([5])Al and Ti atoms. This favored linkage is important to understand the role of titanium dioxide as a nucleating agent in inorganic glass-ceramics fabrication.

Boroxol Rings in Liquid and Vitreous B2O3 from First Principles.

We investigate the structural and vibrational properties of glassy B2O3 using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction f for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, 11B and 17O NMR data). We show that only the boroxol-rich model (f=75%) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about 6 kcal/(mol boroxol). Finally, the liquid state is explored in the 2,000-4,000 K range and a reduction of f to 10%-20% is obtained.

NMR heteronuclear correlation between quadrupolar nuclei in solids.

We show for the first time that it is possible to acquire high-resolution heteronuclear NMR correlation experiments in solid state between second-order-broadened half integer quadrupolar nuclei (i.e., 27Al and 17O) using the scalar J-coupling. The sensitivity of the experiment is dramatically improved at high fields (gain proportional to the fourth power of the principal field) with a combination of signal enhancement techniques. This turns a challenging experiment into a real tool. We apply this experiment to characterize a calcium aluminate glass in which we prove the presence of tricluster mu3 oxygen sites and describe the signature of their directly bonded aluminum sites. Applications involve a large range of possible pairs of quadrupolar nuclei in different materials, such as glasses, porous or mesoporous framework materials, zeolites, hybrid organic-inorganic, and bioinvolved materials.