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.

Iron coordination in liquid FeAl2O4.

The structure of aerodynamically levitated liquid [Formula: see text] was measured by neutron diffraction with isotope substitution (NDIS). Classical and ab initio molecular dynamics simulations were performed and their results were found to be in close agreement with each other and the NDIS data. The results reveal that molten [Formula: see text] may be considered as an ionic liquid without any preference for particular short-range structural motifs. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Volume nanogratings inscribed by ultrafast IR laser in alumino-borosilicate glasses.

Self-assembled nanogratings, inscribed by femtosecond laser writing in volume, are demonstrated in multicomponent alkali and alkaline earth containing alumino-borosilicate glasses. The laser beam pulse duration, pulse energy, and polarization, were varied to probe the nanogratings existence as a function of laser parameters. Moreover, laser-polarization dependent form birefringence, characteristic of nanogratings, was monitored through retardance measurements using polarized light microscopy. Glass composition was found to drastically impact the formation of nanogratings. For a sodium alumino-borosilicate glass, a maximum retardance of 168 nm (at 800 fs and 1000 nJ) could be measured. The effect of composition is discussed based on SiO2 content, B2O3/Al2O3 ratio, and the Type II processing window is found to decrease as both (Na2O + CaO)/Al2O3 and B2O3/Al2O3 ratios increase. Finally, an interpretation in the ability to form nanogratings from a glass viscosity viewpoint, and its dependency with respect to the temperature, is demonstrated. This work is brought into comparison with previously published data on commercial glasses, which further indicates the strong link between nanogratings formation, glass chemistry, and viscosity.

A spectroscopic hike in the U-O phase diagram.

The U-O phase diagram is of paramount interest for nuclear-related applications and has therefore been extensively studied. Experimental data have been gathered to feed the thermodynamic calculations and achieve an optimization of the U-O system modelling. Although considered as well established, a critical assessment of this large body of experimental data is necessary, especially in light of the recent development of new techniques applicable to actinide materials. Here we show how in situ X-ray absorption near-edge structure (XANES) is suitable and relevant for phase diagram determination. New experimental data points have been collected using this method and discussed in regard to the available data. Comparing our experimental data with thermodynamic calculations, we observe that the current version of the U-O phase diagram misses some experimental data in specific domains. This lack of experimental data generates inaccuracy in the model, which can be overcome using in situ XANES. Indeed, as shown in the paper, this method is suitable for collecting experimental data in non-ambient conditions and for multiphasic systems.

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions.

Although gas exsolution is a major driving force behind explosive volcanic eruptions, viscosity is critical in controlling the escape of bubbles and switching between explosive and effusive behavior. Temperature and composition control melt viscosity, but crystallization above a critical volume (>30 volume %) can lock up the magma, triggering an explosion. Here, we present an alternative to this well-established paradigm by showing how an unexpectedly small volume of nano-sized crystals can cause a disproportionate increase in magma viscosity. Our in situ observations on a basaltic melt, rheological measurements in an analog system, and modeling demonstrate how just a few volume % of nanolites results in a marked increase in viscosity above the critical value needed for explosive fragmentation, even for a low-viscosity melt. Images of nanolites from low-viscosity explosive eruptions and an experimentally produced basaltic pumice show syn-eruptive growth, possibly nucleating a high bubble number density.

Non network-former cations in oxide glasses spotted by Raman scattering.

The depolarized Raman spectra can be used as a probe to reveal the presence of non-network formers in oxide glasses. Two spectral responses involving the cations are observed below 400 cm-1 in more than 30 compositions of binary and ternary aluminosilicates. One of the two bands arises solely from cations close to non-bridging oxygen providing thereby a simple test for qualifying the polymerization state of the glass. The second feature involves all cations whatever their role in the glass and is found to be twofold: one contribution arises from cations charge compensating (AlO4)- tetrahedra and the other one from network modifier cations. These results confirm the net vibrational contrast of cations depending on their structural surrounding.

Cavity-melt partitioning of refractory radionuclides and implications for detecting underground nuclear explosions.

Isotopic ratios of radioxenon captured in the atmosphere can be indicators of the occurrence of an underground nuclear explosion. However, civilian sources of xenon isotopes, such as medical isotope production facilities and nuclear reactors, can interfere with detection of signals associated with nuclear testing, according to a standard model of the evolution of radioxenon isotopic abundances in a nuclear explosion cavity. We find that this standard model is idealized by not including the effects of physical processes resulting in the partitioning of the radionuclide inventory between a gas phase and rock melt created by the detonation and by ignoring seepage or continuous leakage of gases from the cavity or zone of collapse. Application of more realistic assumptions about the state of the detonation cavity results in isotopic activity ratios that differ from the civilian background more than the idealized standard model suggests, while also reducing the quantity of radioxenon available for atmospheric release and subsequent detection. Our simulations indicate that the physical evolution of the detonation cavity during the post-detonation partitioning process strongly influences isotopic evolution in the gas phase. Collapse of the cavity potentially has the greatest effect on partitioning of the refractory fission products that are precursors to radioxenon. The model allows for the possibility that post-detonation seismicity can be used to predict isotopic evolution.

Synthesis and Characterization of Double Solid Solution (Zr,Ti)2(Al,Sn)C MAX Phase Ceramics.

Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (Zr,Ti)2(Al0.5,Sn0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)2AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M6A prisms due to steric effects. The M6A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)2AlC MAX phases. The coefficients of thermal expansion along the ⟨ a⟩ and ⟨ c⟩ directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1- x,Ti x)2(Al0.5,Sn0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti2(Al0.5,Sn0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)2(Al0.5,Sn0.5)C double solid solution MAX phases as compared to the Zr2(Al0.5,Sn0.5)C and Ti2(Al0.5,Sn0.5)C end-members.

Structure and dynamics of high-temperature strontium aluminosilicate melts.

We report the study of high-temperature melts (1600-2300 °C) and related glasses in the SrO-Al2O3-SiO2 phase diagram considering three series: (i) depolymerized ([SrO]/[Al2O3] = 3); (ii) fully polymerized ([SrO]/[Al2O3] = 1); and (iii) per-aluminous ([SrO]/[Al2O3] < 1). By considering the results from high-temperature 27Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO4 units during cooling for high-silica compositions. A few five-fold coordinated VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from 27Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al-(OSi)p(OAl)(4-p) units.

Peculiar Thermal Behavior of UO2 Local Stucture.

Most materials expand with temperature because of the anharmonicity of lattice vibration, and only a few shrink with increasing temperature. UO2, whose thermal properties are of significant importance for the safe use of nuclear energy, was considered for a long time to belong to the first group. This view was challenged by recent in situ synchrotron X-ray diffraction measurements, showing an unusual thermal decrease of the U-O distances. This thermal shrinkage was interpreted as a consequence of the splitting of the U-O distances due to a change in the U local order from Fm3̅ m to Pa3̅. In contrast to these previous investigations and using an element-specific synchrotron-based spectroscopic method, we show here that the U sublattice remains locally of the fluorite type from 50 to 1265 K, and that the decrease of the first U-O bond lengths is associated with an increase of the disorder.

Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulation, Neutron Diffraction, and Nuclear Magnetic Resonance Studies.

The structure of strontium glasses with the composition (SiO2)1-2 x(Al2O3) x(SrO) x ( R = [SrO]/[Al2O3] = 1) and (SiO2)1-4 x(Al2O3) x(SrO)3 x ( R = 3) has been explored experimentally over both short- and intermediate-length scales using neutron diffraction, 27Al and 29Si nuclear magnetic resonance, and classical molecular dynamics simulations in model systems containing around 10 000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. The short- and medium-range structure such as aluminum coordination, bond angle distribution, Q( n) distribution, and oxygen speciation have been systematically studied. Two potential forms of the repulsive short-range interactions have been investigated, namely, the Buckingham and Morse forms. The comparison of these forms allows us to derive general trends independent of the particular choice of the potential form. In both cases, it is found that aluminum ions are mainly fourfold coordinated and mix with the silicon network favoring the Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO] = [Al2O3]), a small fraction of fivefold aluminum is observed both experimentally and in MD simulations, whereas the concentration of sixfold aluminum is negligible. MD shows that the fivefold aluminum units AlO5 preferentially adopt a small ring configuration and link to tricoordinated oxygen atoms whose population increases with the aluminum content and are mainly found in OAl3 and OAl2Si configurations. The modeled Sr speciation mainly involves SrO7 and SrO8 polyhedra, giving a range of average Sr2+ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of T-O-T' (T, T' = Al,Si), accounting for the population of the various oxygen speciations, reveals that both potentials predict a nearly identical Al/Si mixing.

A Li K-edge XANES study of salts and minerals.

The first comprehensive Li K-edge XANES study of a varied suite of Li-bearing minerals is presented. Drastic changes in the bonding environment for lithium are demonstrated and this can be monitored using the position and intensity of the main Li K-absorption edge. The complex silicates confirm the assignment of the absorption edge to be a convolution of triply degenerate p-like states as previously proposed for simple lithium compounds. The Li K-edge position depends on the electronegativity of the element to which it is bound. The intensity of the first peak varies depending on the existence of a 2p electron and can be used to evaluate the degree of ionicity of the bond. The presence of a 2p electron results in a weak first-peak intensity. The maximum intensity of the absorption edge shifts to lower energy with increasing SiO2 content for the lithium aluminosilicate minerals. The bond length distortion of the lithium aluminosilicates decreases with increasing SiO2 content, thus increased distortion leads to an increase in edge energy which measures lithium's electron affinity.

Percolation channels: a universal idea to describe the atomic structure and dynamics of glasses and melts.

Understanding the links between chemical composition, nano-structure and the dynamic properties of silicate melts and glasses is fundamental to both Earth and Materials Sciences. Central to this is whether the distribution of mobile metallic ions is random or not. In silicate systems, such as window glass, it is well-established that the short-range structure is not random but metal ions cluster, forming percolation channels through a partly broken network of corner-sharing SiO4 tetrahedra. In alumino-silicate glasses and melts, extensively used in industry and representing most of the Earth magmas, metal ions compensate the electrical charge deficit of AlO4- tetrahedra, but until now clustering has not been confirmed. Here we report how major changes in melt viscosity, together with glass Raman and Nuclear Magnetic Resonance measurements and Molecular Dynamics simulations, demonstrate that metal ions nano-segregate into percolation channels, making this a universal phenomenon of oxide glasses and melts. Furthermore, we can explain how, in both single and mixed alkali compositions, metal ion clustering and percolation radically affect melt mobility, central to understanding industrial and geological processes.

Highly stable silica-coated gold nanorods dimers for solution-based SERS.

The controlled assembly of anisotropic plasmonic nanoparticles (NPs) into highly SERS-active substrates remains particularly challenging for the production of long-term stable NP assemblies in suspension. In this work, we report a simple and efficient strategy to assemble gold nanorods (AuNRs) into dimers. The pH-dependent assembly was triggered using the bifunctional molecular linker BPE (1,2-bis(4-pyridyl)ethylene) and quenched with silver nitrate. The resulting AuNR dimers were encapsulated in mesoporous silica shell and proved to be stable in water for at least 5 months. Taking advantage of the large Raman scattering cross-section of the linker BPE, we conducted a detailed study of the enhancement ability of these NR dimers using solution-based surface enhanced Raman scattering (SERS). Both experimental (SERS) and theoretical (discrete dipole approximation) studies of the near-field characteristics revealed a two-orders of magnitude increase of the SERS enhancement factor for the dimers as compared to isolated AuNRs. Besides thermal and colloidal stability, mesoporous silica coating of AuNRs imparts other notable advantages due to its porosity and biocompatibility, which make these core-shell plasmonic platforms promising for future bio-applications.

Raman spectra of Martian glass analogues: A tool to approximate their chemical composition.

Raman spectrometers will form a key component of the analytical suite of future planetary rovers intended to investigate geological processes on Mars. In order to expand the applicability of these spectrometers and use them as analytical tools for the investigation of silicate glasses, a database correlating Raman spectra to glass composition is crucial. Here we investigate the effect of the chemical composition of reduced silicate glasses on their Raman spectra. A range of compositions was generated in a diffusion experiment between two distinct, iron-rich end-members (a basalt and a peralkaline rhyolite), which are representative of the anticipated compositions of Martian rocks. Our results show that for silica-poor (depolymerized) compositions the band intensity increases dramatically in the regions between 550-780 cm-1 and 820-980 cm-1. On the other hand, Raman spectra regions between 250-550 cm-1 and 1000-1250 cm-1 are well developed in silica-rich (highly polymerized) systems. Further, spectral intensity increases at ~965 cm-1 related to the high iron content of these glasses (~7-17 wt % of FeOtot). Based on the acquired Raman spectra and an ideal mixing equation between the two end-members we present an empirical parameterization that enables the estimation of the chemical compositions of silicate glasses within this range. The model is validated using external samples for which chemical composition and Raman spectra were characterized independently. Applications of this model range from microanalysis of dry and hydrous silicate glasses (e.g., melt inclusions) to in situ field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars) and submarine volcanic environments.

Steady photodarkening of thulium alumino-silicate fibers pumped at 1.07 µm: quantitative effect of lanthanum, cerium, and thulium.

By pumping thulium-doped silica-based fibers at 1.07 µm, rapid generation of absorbing centers leads to photoinduced attenuation (PIA). This detrimental effect prevents exploiting laser emissions in the visible and near infrared. We report on the characterization of the PIA versus the fiber core composition, particularly the concentration of thulium (Tm), lanthanum (La), and cerium (Ce) ions. We show that UV emission induced by Tm-Tm energy transfers is the source of photodarkening and that lanthanum and cerium are efficient hardeners against PIA.

In situ characterization of uranium and americium oxide solid solution formation for CRMP process: first combination of in situ XRD and XANES measurements.

Transmutation of americium in heterogeneous mode through the use of U1-xAmxO2±Î´ ceramic pellets, also known as Americium Bearing Blankets (AmBB), has become a major research axis. Nevertheless, in order to consider future large-scale deployment, the processes involved in AmBB fabrication have to minimize fine particle dissemination, due to the presence of americium, which considerably increases the risk of contamination. New synthesis routes avoiding the use of pulverulent precursors are thus currently under development, such as the Calcined Resin Microsphere Pelletization (CRMP) process. It is based on the use of weak-acid resin (WAR) microspheres as precursors, loaded with actinide cations. After two specific calcinations under controlled atmospheres, resin microspheres are converted into oxide microspheres composed of a monophasic U1-xAmxO2±Î´ phase. Understanding the different mechanisms during thermal conversion, that lead to the release of organic matter and the formation of a solid solution, appear essential. By combining in situ techniques such as XRD and XAS, it has become possible to identify the key temperatures for oxide formation, and the corresponding oxidation states taken by uranium and americium during mineralization. This paper thus presents the first results on the mineralization of (U,Am) loaded resin microspheres into a solid solution, through in situ XAS analysis correlated with HT-XRD.

Approximate chemical analysis of volcanic glasses using Raman spectroscopy.

The effect of chemical composition on the Raman spectra of a series of natural calcalkaline silicate glasses has been quantified by performing electron microprobe analyses and obtaining Raman spectra on glassy filaments (~450 µm) derived from a magma mingling experiment. The results provide a robust compositionally-dependent database for the Raman spectra of natural silicate glasses along the calcalkaline series. An empirical model based on both the acquired Raman spectra and an ideal mixing equation between calcalkaline basaltic and rhyolitic end-members is constructed enabling the estimation of the chemical composition and degree of polymerization of silicate glasses using Raman spectra. The model is relatively insensitive to acquisition conditions and has been validated using the MPI-DING geochemical standard glasses1 as well as further samples. The methods and model developed here offer several advantages compared with other analytical and spectroscopic methods such as infrared spectroscopy, X-ray fluorescence spectroscopy, electron and ion microprobe analyses, inasmuch as Raman spectroscopy can be performed with a high spatial resolution (1 µm2) without the need for any sample preparation as a nondestructive technique. This study represents an advance in efforts to provide the first database of Raman spectra for natural silicate glasses and yields a new approach for the treatment of Raman spectra, which allows us to extract approximate information about the chemical composition of natural silicate glasses using Raman spectroscopy. We anticipate its application in handheld in situ terrestrial field studies of silicate glasses under extreme conditions (e.g. extraterrestrial and submarine environments).

Structural transformations on vitrification in the fragile glass-forming system CaAl2O4.

The structure of the fragile glass-forming material CaAl(2)O(4) was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO(5) polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO(4) tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass.

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.