Supercritical Gallium Trichloride in Oxidative Metal Recycling: Ga2Cl6 Dimers vs GaCl3 Monomers and Rheological Behavior.

Oxidative recycling of metals is crucial for a circular economy, encompassing the preservation of natural resources, the reduction of energy consumption, and the mitigation of environmental impacts and greenhouse gas emissions associated with traditional mining and processing. Low-melting gallium trichloride appears to be a promising oxidative solvent for rare-earth metals, transuranium elements, platinum, pnictogens, and chalcogens. Typically, oxidative dissolution with GaCl3 occurs at relatively low temperatures over a few days, assuming the presence of tetrahedral Ga-Cl entities. While supercritical gallium trichloride holds the potential for advanced recycling, little is known about its structure and viscosity. Using high-energy X-ray diffraction and multiscale modeling, which includes first-principles simulations, we have revealed a dual molecular nature of supercritical gallium trichloride, consisting of tetrahedral dimers and flat trigonal monomers. The molecular geometry can be precisely tuned by adjusting the temperature and pressure, optimizing the recycling process for specific metals. The derived viscosity, consistent with the reported results in the vicinity of melting, decreases by a factor of 100 above the critical temperature, enabling fast molecular diffusion, and efficient recycling kinetics.

Gallium Trichloride Fluid: Dimer Dissociation Mechanism, Local Structure, and Atomic Dynamics.

Molten gallium trichloride emerges as a promising solvent for oxidative metal recycling. The use of supercritical fluid enhances the performance and kinetics of metal dissolution due to significantly lower viscosity in the reaction media. Additionally, the dual molecular nature of gallium trichloride, existing as edge-sharing ES-Ga2Cl6 dimers at low temperatures and high pressure, or flat trigonal GaCl3 monomers in the vicinity of the critical point and low pressures, creates the possibility to tailor the chemical geometry to a particular metallic species. Nevertheless, the mechanism of dimer dissociation, local structure, and atomic dynamics in supercritical gallium trichloride fluids are not known. Using first-principles molecular dynamics, validated by comparison with our high-energy X-ray diffraction results, we illustrate the elementary steps in dimer dissociation. These include the formation of intermediate corner-sharing CS-Ga2Cl6 dimers, the partial disproportionation of GaCl3 monomers at high temperatures and low pressures, changes in the local environment of molecular entities, and unusual atomic dynamics in supercritical fluids.

Remarkably Stable Glassy GeS2 Densified at 8.3 GPa: Hidden Polyamorphism, Contrasting Optical Properties, Raman and DFT Studies, and Advanced Applications.

Glassy GeS2, densified at 8.3 GPa, exhibits a strongly reduced bandgap, predominantly tetrahedral Ge environment, enhanced chemical disorder and partial 3-fold coordination of both germanium and sulfur, assuming two possible reaction paths under high pressure: (i) a simple dissociation 2Ge-S ⇄ Ge-Ge + S-S and (ii) a chemical disproportionation GeS2 ⇄ GeS + S. The observed electronic and structural changes remain intact for at least seven years under ambient conditions but are gradually evolving upon heating. The relaxation kinetics at elevated temperatures, up to the glass transition temperature Tg, suggests that complete recovery of the densified glassy GeS2 over a typical operational T-range of optoelectronic devices will take many thousands of years. The observed logarithmic relaxation and nearly infinite recovery time at room temperature raise questions about the nature of millennia-long phenomena in densified GeS2. Two alternative explanations will be discussed: (1) hidden polyamorphism and (2) continuous structural and chemical changes under high pressure. These investigations offer valuable insights into the behavior of glassy GeS2 under extreme conditions and its potential applications in optoelectronic devices and other advanced technologies.

Decoding the Atomic Structure of Ga2Te5 Pulsed Laser Deposition Films for Memory Applications Using Diffraction and First-Principles Simulations.

Neuromorphic computing, reconfigurable optical metamaterials that are operational over a wide spectral range, holographic and nonvolatile displays of extremely high resolution, integrated smart photonics, and many other applications need next-generation phase-change materials (PCMs) with better energy efficiency and wider temperature and spectral ranges to increase reliability compared to current flagship PCMs, such as Ge2Sb2Te5 or doped Sb2Te. Gallium tellurides are favorable compounds to achieve the necessary requirements because of their higher melting and crystallization temperatures, combined with low switching power and fast switching rate. Ga2Te3 and non-stoichiometric alloys appear to be atypical PCMs; they are characterized by regular tetrahedral structures and the absence of metavalent bonding. The sp3 gallium hybridization in cubic and amorphous Ga2Te3 is also different from conventional p-bonding in flagship PCMs, raising questions about its phase-change mechanism. Furthermore, gallium tellurides exhibit a number of unexpected and highly unusual phenomena, such as nanotectonic compression and viscosity anomalies just above their melting points. Using high-energy X-ray diffraction, supported by first-principles simulations, we will elucidate the atomic structure of amorphous Ga2Te5 PLD films, compare it with the crystal structure of tetragonal gallium pentatelluride, and investigate the electrical, optical, and thermal properties of these two materials to assess their potential for memory applications, among others.

GeTe2 Phase Change Material for Terahertz Devices with Reconfigurable Functionalities Using Optical Activation.

The phenomenon of phase change transition has been a fascinating research subject over decades due to a possibility of dynamically controlled materials properties, allowing the creation of optical devices with unique features. The present paper unravels the optical characteristics and terahertz (THz) dielectric permittivity of a novel phase change material (PCM), GeTe2, prepared by pulsed laser deposition (PLD) and their remarkable contrast in crystalline and amorphous states, in particular, a difference of 7 orders of magnitude in conductivity. The THz spectra were analyzed using the harmonic oscillator and Drude term. Using GeTe2 PLD films, we designed and prepared a THz metasurface in the form of periodic structure and revealed a possibility of tuning the THz resonance either by a thermal control or light-induced crystallization response, thus achieving the dynamic and tunable functionality of the metastructure. We propose controlling the state of metasurface by observing the intensity characteristics of the Raman peak of 155 cm-1. Density functional theory (DFT) modeling demonstrates that in the process of crystallization the mode intensity of 155 cm-1 assigned to Te-Te stretching in amorphous chain fragments decreases and disappears at full crystallization.

Potentiometric Chemical Sensors Based on Metal Halide Doped Chalcogenide Glasses for Sodium Detection.

Chalcogenide glasses are widely used as sensitive membranes in the chemical sensors for heavy metal ions detection. The lack of research work on sodium ion-selective electrodes (Na+-ISEs) based on chalcogenide glasses is due to the high hygroscopicity of alkali dopes chalcogenides. However, sodium halide doped Ga2S3-GeS2 glasses are more chemically stable in water and could be used as Na+-sensitive membranes for the ISEs. In this work we have studied the physico-chemical properties of mixed cation (AgI)x(NaI)30-x(Ga2S3)26(GeS2)44 chalcogenide glasses (where x = 0, 7.5, 15, 22.5 and 30 mol.% AgI) using density, DSC, and conductivity measurements. The mixed cation effect with shallow conductivity and glass transition temperature minimum was found for silver fraction r = Ag/(Na + Ag) ≈ 0.5. Silver addition decreases the moisture resistance of the glasses. Only (AgI)22.5(NaI)7.5(Ga2S3)26(GeS2)44 composition was suitable for chemical sensors application, contrary to the single cation sodium halide doped Ga2S3-GeS2 glasses, where 15 mol.% sodium-halide-containing vitreous alloys are stable in water solutions. The analytical parameters of (NaCl)15(Ga2S3)23(GeS2)62; (NaI)15(Ga2S3)23(GeS2)62 and (AgI)22.5(NaI)7.5(Ga2S3)26(GeS2)44 glass compositions as active membranes in Na+-ISEs were investigated, including detection limit, sensitivity, linearity, ionic selectivity (in the presence of K+, Mg2+, Ca2+, Ba2+, and Zn2+ interfering cations), reproducibility and optimal pH-range.

Transient Mesoscopic Immiscibility, Viscosity Anomaly, and High Internal Pressure at the Semiconductor-Metal Transition in Liquid Ga2Te3.

Gallium tellurides appear to be promising phase-change materials (PCMs) of the next generation for brain-inspired computing and reconfigurable optical metasurfaces. They are different from the benchmark PCMs because of sp3 gallium hybridization in both cubic Ga2Te3 and amorphous pulsed laser deposition (PLD) films. Liquid Ga2Te3 also shows a viscosity η(T) anomaly just above melting when η(T) first increases and only then starts decreasing. We used high-energy X-ray diffraction to observe a transient mesoscopic immiscibility that suggested dense metallic liquid droplets in a semiconducting melt. The η(T) shape was consistent with this finding. A vanishing first sharp diffraction peak that also shifts to a higher Q indicates a high internal pressure in the metallic melt, which produces a remarkable asymmetry of the Ga-Te nearest neighbor distances and is reminiscent of high-pressure rhombohedral Ga2Te3. The observed phenomena provide a realistic scenario for a fast, multilevel SET-RESET response, which also unravels similar trends in the purported density-driven liquid polyamorphism of water, phosphorus, sulfur, and other materials.

Deciphering Fast Ion Transport in Glasses: A Case Study of Sodium and Silver Vitreous Sulfides.

High-capacity solid-state batteries are promising future products for large-scale energy storage and conversion. Sodium fast ion conductors including glasses and glass ceramics are unparalleled materials for these applications. Rational design and tuning of advanced sodium sulfide electrolytes need a deep insight into the atomic structure and dynamics in relation with ion-transport properties. Using pulsed neutron diffraction and Raman spectroscopy supported by first-principles simulations, we show that preferential diffusion pathways in vitreous sodium and silver sulfides are related to isolated sulfur Siso, that is, the sulfur species surrounded exclusively by mobile cations with a typical stoichiometry of M/Siso ≈ 2. The Siso/Stot fraction appears to be a reliable descriptor of fast ion transport in glassy sulfide systems over a wide range of ionic conductivities and cation diffusivities. The Siso fraction increases with mobile cation content x, tetrahedral coordination of the network former and, in case of thiogermanate systems, with germanium disulfide metastability and partial disproportionation, GeS2 → GeS + S, leading to the formation of additional sulfur, transforming into Siso. A research strategy enabling to achieve extended and interconnected pathways based on isolated sulfur would lead to glassy electrolytes with superior ionic diffusion.

Laboratory study of iron isotope fractionation during dissolution of mineral dust and industrial ash in simulated cloud water.

Atmospheric deposition is a key mode of iron (Fe) input to ocean regions where low concentrations of this micronutrient limit marine primary production. Various natural particles (e.g., mineral dust, volcanic ash) and anthropogenic particles (e.g., from industrial processes, biomass burning) can deliver Fe to the ocean, and assessment of their relative importance in supplying Fe to seawater requires knowledge of both their deposition flux and their Fe solubility (a proxy for Fe bioavailability). Iron isotope (54Fe, 56Fe, 57Fe, 58Fe) analysis is a potential tool for tracing natural and anthropogenic Fe inputs to the ocean. However, it remains uncertain how the distinct Fe isotopic signatures (δ56Fe) of these particles may be modified by physicochemical processes (e.g., acidification, photochemistry, condensation-evaporation cycles) that are known to enhance Fe solubility during atmospheric transport. In this experimental study, we measure changes over time in both Fe solubility and δ56Fe of a Tunisian soil dust and an Fe-Mn alloy factory industrial ash exposed under irradiation to a pH 2 solution containing oxalic acid, the most widespread organic complexing agent in cloud- and rainwater. The Fe released per unit surface area of the ash (∼1460 µg Fe m-2) is ∼40 times higher than that released by the dust after 60 min in solution. Isotopic fractionation is also observed, to a greater extent in the dust than the ash, in parallel with dissolution of the solid particles and driven by preferential release of 54Fe into solution. After the initial release of 54Fe, the re-adsorption of A-type Fe-oxalate ternary complexes on the most stable surface sites of the solid particles seems to impair the release of the heavier Fe isotopes, maintaining a relative enrichment in the light Fe isotope in solution over time. These findings provide new insights on Fe mobilisation and isotopic fractionation in mineral dust and industrial ash during atmospheric processing, with potential implications for ultimately improving the tracing of natural versus anthropogenic contributions of soluble Fe to the ocean.

Unraveling the Atomic Structure of Bulk Binary Ga-Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications.

Binary Ge-Te and ternary Ge-Sb-Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga-Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy GaxTe1-x, 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor-metal transition above melting. We also show that a phase change in binary Ga-Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4-8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

Glassy GaS: transparent and unusually rigid thin films for visible to mid-IR memory applications.

Phase-change materials based on tellurides are widely used for optical storage (DVD and Blu-ray disks), non-volatile random access memories and for development of neuromorphic computing. Narrow-gap tellurides are intrinsically limited in the telecom spectral window, where materials having a wider gap are needed. Here we show that gallium sulfide GaS thin films prepared by pulsed laser deposition reveal good transparency from the visible to the mid-IR spectral range with optical gap Eg = 2.34 eV, high refractive index nR = 2.50 over the 0.8 ≤ λ ≤ 2.5 µm range and, unlike canonical chalcogenide glasses, the absence of photo-structural transformations with a laser-induced peak power density damage threshold above 1.4 TW cm-2 at 780 nm. The origin of the excellent damage threshold under a high-power laser and UV light irradiation resides in the rigid tetrahedral structure of vitreous GaS studied by high-energy X-ray diffraction and Raman spectroscopy and supported by first-principles simulations. The average local coordination number appears to be m = 3.44, well above the optimal connectivity, 2.4 ≤ m ≤ 2.7, and the total volume of microscopic voids and cavities is 34.4%, that is, lower than for the vast majority of binary sulfide glasses. The glass-crystal phase transition in gallium sulfide thin films may be accompanied by a drastic change in the nonlinear optical properties, opening up a new dimension for memory applications in the visible to mid-IR spectral ranges.

Chemical and Structural Variety in Sodium Thioarsenate Glasses Studied by Neutron Diffraction and Supported by First-Principles Simulations.

Sodium-conducting sulfide glasses are promising materials for the next generation of solid-state batteries. Deep insight into the glass structure is required to ensure a functional design and tailoring of vitreous alloys for energy applications. Using pulsed neutron diffraction supported by first-principles molecular dynamics, we show a structural diversity of Na2S-As2S3 sodium thioarsenate glasses, consisting of long corner-sharing (CS) pyramidal chains CS-(AsSS2/2)k, small AspSq rings (p + q ≤ 11), mixed corner- and edge-sharing oligomers, edge-sharing (ES) dimers ES-As2S4, and isolated (ISO) pyramids ISO-AsS3, entirely or partially connected by sodium species. Polysulfide S-S bridges and structural units with homopolar As-As bonds complete the glass structure, which is basically different from structural motifs predicted by the equilibrium phase diagram. In contrast to superionic silver and sodium sulfide glasses, characterized by a significant population of isolated sulfur species Siso (0.20 < Siso/Stot < 0.28), that is, sulfur connected to only mobile cations M+ with a usual M/Siso stoichiometry of 2, poorly conducting Na2S-As2S3 alloys exhibit a modest Siso fraction of 6.2%.

Mercury Thiogermanate Glasses HgS-GeS2: Vibrational, Macroscopic, and Electric Properties.

Glasses in the pseudo-binary system (HgS)x (GeS2)1-x were synthesized over the concentration range of 0.0 ≤ x ≤ 0.5. The fundamental glass properties (macroscopic, electric, and vibrational) were studied using differential scanning calorimetry (DSC), direct current (dc) electrical measurements, Raman spectroscopy supported by DFT modeling, and X-ray diffraction. Mercury species in thiogermanate glasses essentially form chain-like (HgS2/2) fragments substituting bridging sulfur between corner- and edge-sharing GeS4/2 tetrahedra. This structural evolution results in a significant monotonic decrease of the glass transition temperatures from 480 to 270 °C. The room-temperature dc conductivity changes non-monotonically with increasing HgS content x over a limited range of 4 × 10-15 to 7 × 10-13 S cm-1. The electronic transport in insulating HgS-GeS2 glasses occurs via extended electronic states. Tetrahedral HgS4/4 fragments also appear in the glass network with increasing x. Their exact population needs further advanced structural studies using diffraction techniques.

Pressure-Driven Chemical Disorder in Glassy As2S3 up to 14.7 GPa, Postdensification Effects, and Applications in Materials Design.

A small difference in energy between homopolar and heteropolar bonds and the glass-forming ability of pure chalcogens leads to unexpected trends in densification mechanisms of glassy chalcogenides compared to vitreous oxides. Using high-precision compressibility measurements and in situ high-energy X-ray diffraction up to 14.7 GPa, we show a new densification route in a canonical glass As2S3. After the first reversible elastic step with a maximum pressure of 1.3 GPa, characterized by a strong reduction of voids and cavities, a significant bonding or chemical disorder is developed under higher pressure, reaching a saturation of 30% in the population of As-As bonds above 8-9 GPa. The pressure-driven chemical disorder is accompanied by a remarkable structural relaxation and a strongly diminished optical gap and determines structural, vibrational, and optical properties under and after cold compression. The decompressed recovered glass conserves a dark color and exhibits two relaxation processes: (a) fast (a few days) and (b) slow (months/years at room temperature). The enhanced refractive index of the recovered glass is promising for optical applications with improved functionalities. A nearly permanent red shift in optical absorption after decompression can be used in high-impact-force optical sensors.

Dimeric Molecular Structure of Molten Gallium Trichloride and a Hidden Evolution toward a Possible Liquid-Liquid Transition.

Group 13 trihalides MY3 (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric M2Y6 molecular structure in the solid state and a mixture of M2Y6 dimers and MY3 monomers in the vapor phase are potential candidates for entropy-driven liquid-liquid transition M2Y6 ⇄ 2MY3 at elevated temperatures. Using pulsed neutron diffraction and high-energy X-ray scattering supported by structural modeling, we show a dimer molecular structure of liquid GaCl3 above the melting point at 351 K and midway between the boiling point (474 K) and the critical temperature (694 K) with almost hidden characteristic evolution toward a possible liquid-liquid transition. In contrast to edge-sharing (ES) dimers in solid and vapor of D2h symmetry, the ES Ga2Cl6 molecules in the melt have a puckered structure of the central four-membered ring with shorter Cl-Cl (2.90-3.09 Å) and longer Ga-Ga (3.20-3.26 Å) second-neighbor correlations. The elongation of Ga-Ga intramolecular distances with increasing temperature simultaneously with diminished Cl-Cl nearest neighbor contacts destabilizes the ES dimers, indicating the first step toward dimer dissociation.

Experimental and Theoretical Insights into the Structure of Tellurium Chloride Glasses.

The structure of the binary chalcohalide glasses Te1- xCl x (0.35 ≤ x ≤ 0.65) is considered by combining experimental and theoretical results. The structural network properties are influenced by a competition between ionic and covalent bonding in such glasses. At first, a focus is placed on the detailed information available by using the complementary high-energy X-ray and the neutron diffractions in both the reciprocal and real spaces. The main characteristic suggested by the structure factors S( Q) concerns the presence of three length scales in the intermediate range order. The total correlation function T( r) lets us also suppose that the structure of these glasses is more complicated than Te-chain fragments with terminal Cl as demonstrated in crystalline Te3Cl2. Molecular dynamics simulations were subsequently performed on Te3Cl2 and Te2Cl3, and coupled with the experimental data, a highly reticulated network of chalcogen atoms, with a fair amount of chlorine atoms bonded in a bridging mode, is proposed. The simulations clearly lead to a glass description that differs markedly from the simple structural model based on only Te atom chains and terminal Cl atoms. Solid-state NMR experiments and NMR parameters calculations allowed validation of the presence of Te highly coordinated with chlorine in these glasses.

Direct laser writing of a low-loss waveguide with independent control over the transverse dimension and the refractive index contrast between the core and the cladding.

In this Letter, we present the realization of a low-loss waveguide in a chalcogenide glass by direct laser writing technique in a particular configuration that allows the independent control over the diameter of the core and the magnitude of the refractive index contrast with the cladding. The waveguide is of multicore type and composed of 19 channels arranged on a hexagonal lattice. Each channel is obtained by stacking voxels of refractive index variation obtained by static exposure to femtosecond laser pulse burst. The distance between the channels can be used to vary the diameter of the waveguide, while the duration of laser burst controls the magnitude of the effective index and the propagation loss. We demonstrate that it can be reduced down to 0.11 dB/cm.

Direct laser writing of buried waveguide in As2S3 glass using a helical sample translation.

We report the fabrication and the characterization of buried waveguide in As(2)S(3) glass. It is well known that the interaction of femtosecond pulses with this material at high laser repetition rates results in a mainly negative refractive index variation, due to heat accumulation effect. However, we show here that a helical translation of the sample parallel to the laser beam, allows the inscription of a core of positive refractive variation, with full control over its magnitude and diameter. An example demonstrating the high symmetry of the guided mode is given.

Unraveling the atomic structure of Ge-rich sulfide glasses.

In contrast to the well-established structure of glassy GeS2, consisting of corner- and edge-sharing GeS(4/2) tetrahedra, the structural features of Ge-rich sulfide alloys remain essentially unknown. Two contrasting points of view: (1) a tetrahedral model, and (2) a distorted NaCl approach were neither confirmed nor excluded mostly because of missing advanced structural studies. Using high-energy X-ray scattering and neutron diffraction, we show the complexity of the short and intermediate range order in Ge(x)S(1-x) glasses, ⅓ ≤ x ≤ 0.47, formed by corner- and edge-sharing tetrahedra with two-fold coordinated sulfur species and a variable number of Ge-Ge bonds, and Ge-S units with three-fold coordinated sulfur at x ≥ 0.36.

Free carrier accumulation during direct laser writing in chalcogenide glass by light filamentation.

We present direct laser writing of channels in chalcogenide glass under light filamentation conditions. Because of the intrinsic properties of the filament, the positive refractive index profile of the channels exhibits a cylindrical symmetry of high quality. The role of the repetition rate is also investigated. It is shown that if the time separation between pulses is shorter than the lifetime of the plasma, the free carriers accumulate and induce a larger variation of the refractive index.