Niobium (Nb) is classified as a 'critical mineral' due to its irreplaceable applications in superalloys and superconductors but limited supply chain. This study employs high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) at the Nb K-edge to investigate Nb(V) mobility in fluids containing various ligands, namely fluoride [F-], chloride [Cl-], carbonate [CO3-], and hydroxide [NaOH], under hydro-thermal conditions (25-413 C; 800 bar). Ab initio simulations of X-ray absorption near-edge structure (XANES) spectra were combined with shell-by-shell modelling of extended X-ray absorption fine structure (EXAFS) data to elucidate the nature and geometry of the aqueous Nb(V) complexes. Key findings are that Nb(V) predominantly forms eight-coordinated hydroxyfluoride complexes (e.g., [NbF6O2] moiety) in acidic F-rich solutions, whereas high-chloride acidic conditions favour six-coordinated chlorohydroxy complexes (e.g., [NbCl4O2] moiety). Competitive ligand interactions between F-and Cl-are modulated by temperature and pH, with higher temperature or low pH promoting Cl-dominated coordination. Carbonate solutions exhibit retrograde Nb solubility and promote lower solubility than acidic F/Cl solutions above 282 C, with spectral features suggesting the presence of [NbO6]-like moieties in polynuclear clusters and/or nanoparticles. Comparative analysis with tantalum (Ta) highlights distinct geochemical behaviours: Ta dissolution depends strictly on F concentration, whereas Nb demonstrates greater coordination flexibility in Cl-bearing solution systems. These findings challenge conventional views that dismiss the role of chloride in Nb transport. Differences in Nb and Ta complexation provide a geochemical discriminator, leading to Nb/Ta ratios that are low near intrusions, increase outwards in intrusion-related hydrothermal systems, and are highest in metamorphic fluids.

Operando X‐ray absorption spectroscopy (XAS) has undergone a transformation thanks to advances in synchrotron instrumentation and reactor‐cell design, enabling studies of heterogeneous catalysts under working conditions. This topical review surveys these developments and shows how they are implemented on the BM30 (FAME‐PIX) and BM16 (FAME‐UHD) beamlines at the European Synchrotron Radiation Facility (ESRF). Both beamlines now combine high‐flux optics with catalysis‐oriented detection, including a 16‐element Mirion HPGe solid‐state detector (SSD) for sensitive XAS of active sites and a 14‐crystal analyzer spectrometer for high‐energy‐resolution fluorescence detection (HERFD) and X‐ray emission spectroscopy (XES) to track changes in oxidation state and local structure. They also include operando reactor cells capable of operating at temperatures up to 1000°C and pressures up to 100 bar, matching industrial catalytic conditions. We describe the beamline configurations, sample environments and detection modalities, and discuss how their combined use enables real‐time structural and electronic characterization of catalysts. Case studies on methane reforming, ethane dehydrogenation, CO 2 hydrogenation and methane dehydroaromatization demonstrate the scientific impact and versatility of these setups. Collectively, BM30 and BM16 are key tools for operando XAS and help bridge the gap between fundamental understanding and industrial catalytic processes.

This study investigates the dissolution kinetics of synthetic basaltic glass under circum-neutral to basic conditions (pH20 degrees C from 6 to 10) and temperatures of 30, 60 and 90 degrees C, with a particular focus on the roles of dissolved oxygen (O2(aq)) and silica (SiO2(aq)) concentrations. Surface retreats were measured using vertical scanning interferometry, and the thickness of alteration layers (amorphous silica-rich surface layer, referred to as ASSL) using X-ray reflectivity and transmission electron microscopy performed on focused ion beam-milled lamellae. As expected, the dissolution rate of basaltic glass increases with increasing pH from neutral to basic conditions. A modest influence of O2(aq) concentrations was observed, attributed to Fe(II) oxidation and the associated formation of a passivating Fe(III)-Si-rich surface layer. Most strikingly, the dissolution rate was found to decrease exponentially with increasing SiO2(aq) concentrations, which is inconsistent with the transition state theory. Instead, this behavior is consistent with a mechanism governed by classical nucleation theory in the studied conditions, resulting in the following overall dissolution rate law: r = () k0.10n.pHT.exp Ea.exp k1 k2, with r being the dissolution rate (in mol/m2/s), k0 = 552 mol/ RT |ln[SiO2(aq) ]/KTeq | m2/s, n = 0.35, Ea = 84 kJ/mol, k1 = 0.40, k2 = 3.87, pHT is the pH value at the considered temperature T, R the gas constant, and KTeq the solubility constant of amorphous silica at the considered temperature. Taken together, these findings provide new insights into the coupled effects of pH, O2(aq), and SiO2(aq) on basaltic glass reactivity, offering a refined kinetic framework for modeling glass weathering in natural and engineered environments.

Cr4+-doped Y3Al5O12 (YAG:Cr4+) is widely employed in photonic devices as a saturable absorber and has recently attracted attention as an infrared gain medium. However, its synthesis typically requires annealing at high temperature (>1573 K) to achieve the oxidation of Cr3+ into Cr4+, thus limiting cost efficiency and scalability. Here, we report the first successful preparation of YAG:Cr4+ nanopowders via the Pechini process at a temperature as low as 1173 K. Structural and morphological analyses (XRD, SEM, TEM) confirm the formation of nanocrystalline YAG with homogeneous chromium incorporation. Optical spectroscopy combined with XANES unambiguously demonstrates the presence of both Cr4+ and Cr6+ ions, the latter arising from the oxidative decomposition of organics during calcination—an unprecedented observation in YAG materials. To maximize Cr4+ concentration, we explored oxidation strategies including well-known Ca2+ co-doping and more original H2O2-assisted synthesis; both of which significantly enhance tetrahedral Cr4+ luminescence. Finally, we investigated the potential of heat treatment in a reducing atmosphere for obtaining Cr4+ ions, with the aim of better understanding the possible pathways for achieving the Cr4+ oxidation state in the YAG matrix at low temperatures, leading to very promising results. The low-temperature route studied here not only reduces synthesis costs but also provides new opportunities to tailor the oxidation state of chromium in garnet-type nanomaterials, paving the way for their integration in energy-efficient optical devices.

Knowledge of the chemical speciation of molybdenum in fluids under hydrothermal conditions is key to understanding the formation of porphyry Cu-Au-Mo deposits, which are the primary economic source of copper, molybdenum and rhenium. However, the chemical identity and thermodynamic stability of aqueous complexes of molybdenum and the role of different ligands on Mo metal transport yet remain inconsistent and incomplete, in particular for sulfur-bearing fluids typical of such environments. We have experimentally studied the role of hydrogen sulfide (H2S and HS-) and the trisulfur radical ion (S3 center dot-) in the transport of molybdenum by hydro-thermal fluids at 300 degrees C and 500 bar as a function of pH, redox conditions as well as sulfur speciation and concentration. We combined solubility measurements of molybdenite in hydrothermal reactors using fluid quenching or sampling, with in situ synchrotron X-ray absorption spectroscopy experiments and thermodynamic and molecular modeling. Our solubility and spectroscopic dataset is consistent with the formation of the tetrathiomolybdate complex, MoS42-, in reduced, H2S/HS--dominated fluids of neutral-to-alkaline pH. In contrast, a mixed-ligand complex with three sulfide ions and one trisulfur radical ion, MoS3(S3)-, prevails in more oxidized and more acidic fluids at the sulfide-sulfate transition where S3 center dot- is far more abundant. In both complexes, Mo is nominally hexavalent and in a first-shell tetrahedral coordination with sulfur atoms. The derived equilibrium constants of the formal solubility reactions (log10K): MoS2(s) + 2H2S0(aq) + 0.5 O2(g) = MoS42- + 2H+ + H2O(liq) and MoS2(s) + H2S0(aq) + S3 center dot- + 0.5 O2(g) = MoS3(S3)- + H2O(liq) at 300 degrees C and 500 bar are 0.5 +/- 0.4 and 14.6 +/- 0.4, respectively. The solubility of MoS2(s) predicted using these constants aligns well with Mo concentrations measured in natural fluid inclusions in quartz that record S-rich fluids from porphyry-epithermal systems. In contrast, other types of Mo complexes invoked so far (molybdates, alkali ion pairs, oxy-chlorides or oxysulfides) are negligible at such conditions. Thus, trisulfur radical ion complexes may be important carriers of Mo in hydrothermal fluids and would require further systematic investigation across a wide range of temperature and pressure.

Despite the growing demand for cerium-doped yttrium aluminum garnet nanocrystals (YAG:Ce), their elaboration remains challenging. Although promising results have been obtained through the glycothermal route, the nanoparticles exhibit a low internal quantum yield compared to micron-sized YAG:Ce. Top-down approaches are less studied than bottom-up ones, and little information is available regarding the resulting optical properties. In this work, a top-down approach involving high-energy colloidal ball-milling is employed to achieve cerium-doped yttrium garnet nanocrystals, and the consequences of this process have been investigated through a comparative study of samples milled for different durations, using a multitechnique characterization approach. Structural characterization reveals the rapid (<= 5 min) formation of a hydrated yttrium carbonate, as well as a progressive amorphization of the garnet lattice, starting at the surface and propagating to the core of the nanocrystals. Mechanically induced Ce3+ oxidation to Ce4+ is reported for the first time in garnet materials, to the best of our knowledge. The consequences on garnet optical properties are discussed in details. The study describes a top-down route to achieve cerium-doped garnet nanocrystals. However, it also demonstrates that the deterioration of optical properties with a decreasing crystal size is not dependent on the synthesis used to produce the nanoparticles.

The aim of this study was to investigate the contribution of Fe(III)-reducing microorganisms to the dissolution rates of Fe(III)-rich synthetic basaltic glass. Hyperthermophilic archaeon Pyrobaculum islandicum and thermophilic bacterium Thermus scotoductus were incubated for 7 or 15 days with basaltic glass, and the surface retreat of the glass was determined using vertical scanning interferometry. Pyrobaculum islandicum was shown to enhance basaltic glass dissolution rate two-fold compared to abiotic controls in 7-days incubations. However, this effect was only 1.3-fold in 15-days incubations. In contrast, Thermus scotoductus did not impact or slightly inhibited the dissolution. Accordingly, alteration microstructures of the surface were visible only with Pyrobaculum islandicum cultures which promoted the formation of (Al, P, S)-rich layers thickening with time at the surface of the glass as well as of (Fe, S)-bearing crystals. To identify the underlying mechanisms that could explain such differences, Fe(III) reduction assays were performed in cultures with Fe(III) citrate, revealing higher rates of Fe(III) reduction in incubations with Pyrobaculum islandicum compared to those with Thermus scotoductus. Collectively, these findings suggest that Fe(III)-reducing microorganisms can enhance basaltic glass dissolution, but that the process could be limited by the rate of Fe(III) reduction and by surface alteration products.

Extremophiles, microorganisms that thrive in extreme environments, have broadened our understanding of fundamental life processes. Cultivated strains of extremophiles have demonstrated the viability of life at temperatures up to 122 degrees C and pressures up to 125 MPa. These physical extremes affect intracellular mechanisms such as metabolism. At high temperatures, the key metabolite adenosine triphosphate (ATP) is subject to abiotic hydrolysis. In cells, ATP is complexed mostly with Mg2+. Although this complexation is well known, its role during abiotic ATP hydrolysis under extreme conditions has rarely been investigated. This study presents novel kinetic data which are supported by thermodynamic modeling pertaining to ATP hydrolysis at elevated temperatures and 20 MPa in presence of Mg2+ and other cations. Kinetic parameters for abiotic hydrolysis were determined using in situ Raman spectroscopy in combination with a hydrothermal diamond anvil cell and a gas-pressurized autoclave equipped with a sapphire cell. Hydrolysis rate constants were studied using Mg2+, Ca2+, and Na+ as ATP counterions at temperatures of 80 degrees C, 100 degrees C, and 120 degrees C under pressures of 20 MPa. Our findings indicate that Na+ and Ca2+ ions have negligible effects on ATP hydrolysis rates. In contrast, increasing the Mg2+ concentration to fourfold the ATP concentration resulted in a pronounced decrease of the hydrolysis rate, with reductions of approximately 30 % at 80 degrees C and 50 % at 120 degrees C. By comparison with known biotic pool turnover rates, this kinetic stabilization of ATP reinforces previous findings that its abiotic hydrolysis is not a limiting factor for life at high temperature. Furthermore, these results suggest that Mg2+-rich intracellular compositions can reduce the energy investment required to maintain ATP homeostasis in biological systems. Thermodynamic modeling revealed increasing complexation of ATP with Mg2+ ions with increasing temperature and magnesium ions concentration. Under experimental conditions of pH 2 to 3, a continuous formation of MgH2ATP complexes was calculated, leading to a deceleration of the abiotic hydrolysis rate. At pH 6 to 9, the formation of MgATP2-was calculated at equimolar concentrations of ATP and Mg2+ ions. Above 80 degrees C and pH values between 6 and 9, thermodynamic modelling indicated the formation of Mg2ATP when the Mg2+ concentration was increased above the equimolar point.

Today, the development of green phosphors is becoming a hot topic as they could be combined with blue light-emitting diodes (LEDs) to replace low-efficiency green LEDs. With its intense green emission under blue or UV excitation, Cs3MnBr5 appears to be a promising candidate. However, this phase is highly hygroscopic, and partial substitution of Mn with Zn has been proposed to overcome this challenge. In this article, the influence of zinc content on Cs3Mn1-xZnxBr5 is investigated in terms of structure, optical properties, and stability under a humid atmosphere. A pure phase is obtained for 0.4 ≤ x ≤ 0.8, and XANES and SQUID studies revealed that manganese remains in its +II state regardless of the zinc content. Zinc increases internal quantum yields but decreases blue-light absorption, and the best compromise giving the highest green emission is obtained for x = 0.5. In situ XRD results shows that the optimized solid solution Cs3Mn0.5Zn0.5Br5 is not hydrated, even at 100% relative humidity, whereas Cs3MnBr5 is fully transformed above 50% RH. More generally, by providing a better understanding of the influence of zinc on the optical properties and moisture stability of Cs3MnBr5, this work paves the way for the future development of this phosphor in (micro)-LED applications.

Chemical exchanges between magmas and volatile-rich fluids and gases are fundamental processes of magmatic and volcanic activity, but also play a critical role in the formation of various ore deposits. Yet, the composition and properties of the so-called magmatic volatile phases (MVPs) remain elusive, due to difficulties in their sampling, both in natural systems and in experimental laboratories. Here, we present a novel 'transparent' internally-heated high-pressure vessel (T-IHPV) that enables the geochemical characterization of coexisting hydrous melts and MVPs in-situ, under typical shallow magmatic conditions. The experimental design is validated through the observation of haplogranite and rhyodacite melting to 900 degrees C and 130 MPa and the in-situ X-ray absorption (XAS) study of bromine and strontium distribution and speciation in the haplogranite-H2O system to 800 degrees C and 100 MPa. These preliminary experiments confirm the efficient partitioning of Br in MVPs in differentiated systems (DMVP/melt around 41 for haplogranite) and reveal the Br stability of hydrated Br species instead of HBr in the MVPs. Coupled to other spectroscopic methods (Raman, SAXS/WAXS, XRD), we expect the T-IHPV to shed a new light on the thermodynamics and kinetics of chemical reactions at stake in shallow magmatic and hydrothermal reservoirs.

Tungsten (W) concentrations in fluids in equilibrium with crystalline tungsten oxide are used to determine thermodynamic parameters for W solubility and W species in hydrothermal fluids. The solubility data were measured in situ at high pressures and temperatures using X-ray absorption. X-ray spectroscopic data measured in situ – with X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) – were applied to characterize the symmetry and the type of atoms of the first coordination shell of W aqueous complexes present in the fluid at given temperatures and pressures. Experiments were performed at up to 400 °C and at pressures of 40, 50 and 60 MPa. With this dataset, we were able to improve constraints for the already-suggested fluid species WO42-, HWO4-, H2WO40, NaWO4- and NaHWO40. Further, we were able to introduce the H3WO4+ species that is found to be dominant in acidic fluids. No evidence was found for W species involving Cl− as a ligand. The ionic W species found in the fluid are characterized by a tetrahedral complex at alkaline conditions. In neutral to acidic conditions, W complexes with distorted octahedral symmetry are formed. These complexes may be polymerized at temperatures ≤200 °C and W concentrations >10-3 mol kg−1 H2O. X-ray spectroscopy as well as thermodynamic modeling suggests that polytungstate species are not relevant at equilibrium concentrations found in the solubility experiments of this study (≤10-3 mol W kg−1 H2O in equilibrium with tungsten oxide) or at concentrations reported for natural systems. Using the thermodynamic properties of the species mentioned above, in situ data on the solubility of scheelite can be successfully described. Thermodynamic modeling shows that scheelite solubility and wolframite solubility strongly increase with increasing salinity, especially up to 1 m NaCl (m denotes molality), and vary with pH, which is consistent with earlier reports. Overall, this study provides improved thermodynamic properties for a set of W fluid species that cover a wide range of fluid compositions, which is necessary for understanding the complex processes of W enrichment and mineralization in hydrothermal systems.

Understanding the behaviour of tantalum (Ta) in hydrothermal systems is pivotal for understanding its geochemical enrichment processes and economic extraction via hydrometallurgy. Yet, its behaviour in hydrothermal systems remains poorly characterised. This study investigates the coordination chemistry, speciation, and solubility of pentavalent Ta(V) in fluoride (F) - and chloride (Cl) -rich hydrothermal solutions up to 413 degrees C and 800 bar, utilising in-situ High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS). The results reveal the stability of high order fluoridotantalate complexes in fluoride-rich fluids solutions up to the highest investigated temperature, highlighting fluoride's paramount role in enhancing Ta solubility through the formation of stable fluoridotantalate complexes in aqueous solutions. A transition from nonafluoridotantalate tetraanion (TaF94-) to heptafluoridotantalate dianion (TaF72-) complexes was observed as a function of temperature in solutions containing >= 1 m fluoride. Conversely, our findings indicate a negligible role for chloride in Ta complexation even in high Cl (similar to 6 m) aqueous solutions, suggesting that Ta chloride complexes do not contribute significantly to Ta transport in hydrothermal systems. Existing solubility data were reinterpreted based on an updated speciation model that integrates the in-situ XAS results. This confirms that Ta(OH)(5)(aq) predominates in solutions containing <0.02 m fluoride; oxyfluoridotantalate anions such as [TaF3(OH)(3)(-)] dominate in solutions containing intermediate fluoride concentrations (0.02-1 m), and the fluoridotantalate anions [TaF94- to TaF72-] occur in more concentrated fluoride solutions (>1 m) at hydrothermal conditions (similar to 100-400 degrees C). Derived thermodynamic data for these species enable better understanding and geochemical modelling of Ta transport in hydrothermal fluids, highlighting the potential of F-rich fluids to transport significant amounts of Ta.

Extreme environments are habitats for a diverse array of microorganisms, namely extremophiles, that have evolved unique biochemical adaptations to their geological setting. Some examples of extremophiles can be found in the subseafloor or near hydrothermal vents on the ocean floor. Cultivated strains of extremophiles have demonstrated the ability to tolerate temperatures (T) up to 122 degrees C and pressures (P) up to 125 MPa. Organisms depend on the stability of key metabolites, such as adenosine triphosphate (ATP), to survive and reproduce under these conditions. In order to maintain their intracellular ATP levels, living cells must compensate for the abiotic hydrolysis of ATP, which occurs at a particularly rapid rate at high temperatures. The role of high pressures and high temperatures in abiotic hydrolysis of ATP has been rarely investigated despite the potential for this phenomenon to contribute to limit the adaptation of microorganisms to simultaneous extreme temperatures and pressures. This study presents new data on the effect of pressure on the abiotic hydrolysis of ATP to adenosine diphosphate (ADP) at elevated temperatures. In situ Raman spectra were measured at high pressure and high temperature of the hydrolysis of aqueous disodium ATP solutions in two experimental systems: a hydrothermal diamond anvil cell (HDAC) and a gas-pressurized autoclave. These two systems permitted the determination of hydrolysis rate constants of ATP into ADP up to 1670 MPa at 80 degrees C, 100 degrees C, and 120 degrees C. The data exhibited Arrhenian behavior with a slight decrease in activation energy from 0.5 MPa to 140 MPa. The effect of pressure on ATP hydrolysis rate constants was found to be vanishingly low in the so far known vital range up to 125 MPa. Abiotic hydrolysis rates of ATP showed a pronounced increase at higher pressures. For example, at 100 degrees C, a rise in pressure from 365 MPa to approximately 1670 MPa results in a nearly tenfold increase in the ATP hydrolysis rate constant. When compared with the typical ATP turnover times reported in living cells, the abiotic ATP hydrolysis rates determined in this study provide insights into the pressure and temperature conditions that could be consistent with living microorganisms.

We discuss the X-ray absorption spectroscopy (XAS) methods elaborated using the high pressure and high-temperature autoclaves installed at FAME and FAME-UHD beamlines. These methods are based on the in situ acquisition of X-ray transmission and fluorescence spectra of hydrothermal fluids and silicate melts and enable the derivation of both solubility and speciation information about metal complexes. The technological assets of our autoclaves are described across a wide range of experimental conditions spanning from different types of hydrothermal fluids, from liquid-like to vapor-like densities having metal concentrations of <1-10,000 ppm, to magmatic fluid-melt systems at 1000 degrees C. Scientific examples are presented to illustrate the use and the advantages of our spectroscopic 'micro-batch' reactors. Finally, the integration of our autoclave setups on the FAME-UHD crystal analyzer spectrometer and the benefits of this technique are demonstrated.

This paper presents the development of a novel high-pressure/high-temperature reactor cell dedicated to the characterization of catalysts using synchrotron x-ray absorption spectroscopy under operando conditions. The design of the vitreous carbon reactor allows its use as a plug-flow reactor, monitoring catalyst samples in a powder form with a continuous gas flow at high-temperature (up to 1000 °C) and under high pressure (up to 1000 bar) conditions, depending on the gas environment. The high-pressure/high-temperature reactor cell incorporates an automated gas distribution system and offers the capability to operate in both transmission and fluorescence detection modes. The operando x-ray absorption spectroscopy results obtained on a bimetallic InCo catalyst during CO2 hydrogenation reaction at 300 °C and 50 bar are presented, replicating the conditions of a conventional microreactor. The complete setup is available for users and permanently installed on the Collaborating Research Groups French Absorption spectroscopy beamline in Material and Environmental (CRG-FAME) sciences and French Absorption spectroscopy beamline in Material and Environmental sciences at ultra-high dilution (FAME-UHD) beamlines (BM30 and BM16) at the European Synchrotron Radiation Facility in Grenoble, France.