In surface soils and sediments, iron monosulfide (FeS) species, including nanocrystalline mackinawite, tend to quickly form in the presence of iron and sulfide in anoxic conditions. As such, FeS species are the main precursors for the formation of other iron sulfides such as Fe3S4 greigite and FeS2 pyrite, which are ubiquitous in surface sedimentary environments. It is known that, under prolonged aging under reducing conditions in a sulfidic aqueous medium, FeS species can evolve into crystalline mackinawite. However, the possible influence of pH on the evolution of mackinawite under such anoxic low-temperature conditions relevant to sedimentary (sub)surface environments has not been investigated yet. In this study, we used Rietveld refinement and pair distribution function analysis (PDF) of synchrotron-based X-ray powder diffraction (XRD) patterns to derive the mean coherent domain (MCD) size of mackinawite after aging under various pH conditions and X-ray absorption near-edge structure (XANES) spectroscopy at the S and Fe K-edges to study the structural and electronic properties. Moreover, in order to strengthen our interpretations, we confirmed the shape and relative energy of pre-edge features in the S K-edge XANES spectra of mackinawite (FeS) and pyrite (FeS2) model compounds via first-principle calculations. Our results show that, after FeS has precipitated from aqueous Fe(II) and H2S/HS- in a saline medium at pH 7.1, aqueous aging at the same pH over 47 d results in the formation of nanocrystalline mackinawite (MCDab=11.5 +/- 0.1 nm; MCDc=7.1 +/- 0.1 nm). When Na2S is added into the solution to reach pH 9.7 after FeS has precipitated at pH 7.1, no other Fe sulfide is observed during the aging phase, and mackinawite particles are of smaller size (MCDab=7.9 +/- 0.1 nm; MCDc=4.6 +/- 0.1 nm). In this sample, an additional weak and broad peak appears at d=10.5 & Aring; that could be interpreted as being due to either lattice expansion at the particle boundaries or a double-cell super-structure. When H+ is added as HCl to reach pH 5.1 before the aging phase, the size of mackinawite particles increases (MCDab=13.0 +/- 0.2 nm; MCDc=8.1 +/- 0.2 nm), and a fraction transforms into greigite (Fe3S4). This reaction is accompanied by a pH increase to 6.4, likely because of H+ consumption, which suggests that Fe(II) in FeS would serve as an electron donor and that H+ would serve as an electron acceptor. The calculated electronic structure of mackinawite shows partly filled Fe-3d states, which supports the fact that acidic aging conditions are favorable for Fe(II) to act as an electron donor. We propose and further discuss the fact that the formation of greigite from nanocrystalline mackinawite could result in H2 production as, for instance, observed for anoxic corrosion of zero-valent Fe at higher temperatures. Greigite has been designated in the literature either as an intermediate towards pyrite formation or as a mineralogical endmember in another reaction route. Our observations raise the question of the existence of such a reaction producing Fe3S4 and H2 in reducing sedimentary (micro)environments across geological times. In addition, the metallic character of mackinawite suggests that Fe(II) oxidation to Fe(III) by H+ in this mineral species could proceed without the need for another oxidizing agent. Although the possible formation of pyrite from greigite would require further studies on extended aging time and/or under more acid-sulfidic conditions, our findings could have implications for the understanding of the initial steps of the H2S pathway to pyrite.

In order to explain the observation of an extended superconducting region in several overdoped cuprates, which contrasts the dome scenario, by means of neutron and synchrotron x-ray powder diffraction we study the crystal structure of YBa_2Cu_3O_y, where strong oxygen overdoping up to y = 7.4 is achieved under high-pressure. A bond valence sum analysis indicates that 1/5 of the extra holes created by the excess oxygen are transferred to the CuO_2 planes, thus increasing the hole density up to p=0.27 hole/Cu, where superconductivity is expected to vanish according to the dome scenario. Instead, our data confirm a previous observation [Okai, Ono and Mitsuhashi, Physica C: Superconductivity 366, 164 (2002)] that the superconducting critical temperature, T_c, remains constant with y. Our data analysis accounts for this discrepancy in terms of the much shorter bond between the apical oxygen and the planar Cu ion, which suggests that the extra holes occupy the a_1-symmetry states formed by d_3z^2-r^2 orbitals, instead of the usual b_1-symmetry Zhang-Rice singlet states formed by d_x^2-y^2 orbitals. Suitable spectroscopic measurements on single crystals may support such a two-band scenario, which would require a totally different theoretical approach to explain superconductivity in cuprates.

Nanometer-scale modulations can spontaneously emerge in complex materials when multiple degrees of freedom interact. Here we demonstrate that ferroelectric Sr_{1-x}Ca_{x}TiO_{3} lies in close proximity to a finite-q lattice instability associated with a tendency toward structural modulation. Using inelastic neutron and x-ray scattering, we show that upon cooling, dipolar fluctuations strongly couple to and soften the c_{44} transverse acoustic mode. We identify the wave vector at which this softening is maximal, thereby defining the characteristic length scale of the instability. Calcium substitution enhances both the amplitude and the wavevector of the softening by strengthening the ferroelectric and antiferrodistortive instabilities. Our results demonstrate that the flexoelectric phonon coupling promotes a tendency toward a dynamically modulated state that cooperates with, rather than competes against, the other lattice instabilities in SrTiO_{3}.

Using cobalt(II) complexes and a simple coordination reaction at the surface of maghemite nanoparticles, molecule-driven control of the effective magnetic anisotropy can be achieved. This functionalization strategy is explored for nanoparticles ranging from 4 to 8 nm and performed under soft synthetic conditions (mild temperature and in air) and in aqueous media. It preserves colloidal stability and permits an acute control of the magnetic properties with an increase in the blocking temperature and of the coercive field values. This effect is correlated to the quantity of complexes coordinated at the surface and the increase in the surface anisotropy of the nanoparticles. Magnetometry studies show that the effective magnetic anisotropy constant, K eff, can be modulated from 32 to 168 kJm-3 and with a few kJm-3 accuracy.

Nanometer-scale modulations can spontaneously emerge in complex materials when multiple degrees of freedom interact. Here we demonstrate that ferroelectric Sr_1-xCa_xTiO_3 lies in close proximity to an incipient structurally modulated phase. Using inelastic neutron and X-ray scattering, we show that upon cooling, dipolar fluctuations strongly couple to and soften the c_44 transverse acoustic mode. We identify the wavevector at which this softening is maximal, thereby defining the characteristic length scale of the modulation. Calcium substitution enhances both the amplitude and the wavevector of the softening by strengthening the ferroelectric and antiferrodistortive instabilities. Our results demonstrate that nonlinear flexoelectric phonon coupling tends to stabilize a modulated state that cooperates with, rather than competes against, the other lattice instabilities in SrTiO_3.

Chemical sediments such as Banded Iron Formations (BIFs) are potential recorders of biogeochemical cycles and nutrient availability in ancient oceans. In particular, their metal and nutrient contents have been used to reconstruct concentrations of dissolved elements in seawater. However, chemical sediments represent non-continuous and non-homogeneous archives throughout Earth history, and their formation still remains largely controversial, challenging our ability to reconstruct early geochemical cycles. For instance, the mineralogy of their precursors is highly uncertain, and how such primary mineralogy transformed through diagenesis and metamorphism is equally unclear. Here, we explored the mineralogical transformations occurring upon rising pressure (P) and temperature (T) on key Fe-bearing minerals, commonly reported as ubiquitous in Archean, anoxic and Fe-rich oceans. Specifically, we performed P-T experiments in diamond anvil cells, spanning 25–300 °C and 0 to 3 GPa to simulate sediment burial and metamorphism, on a range of Fe bearing minerals including ferrihydrite, goethite, hematite, carbonate green rust and greenalite, and used in-situ X-Ray Diffraction to characterize and discuss the evolution of mineralogical assemblages. Our results indicate that, in the absence of organic matter, the entire range of dominant, BIF-forming minerals can be achieved with an initial mixture of carbonate green rust and Fe silicate.

Mercury telluride (HgTe) nanocrystals are cornerstone materials for infrared optoelectronics, yet all previously reported forms of HgTe have crystallized in the zinc blende phase. Here, we develop a comprehensive cation exchange route to access metastable wurtzite (WZ) HgTe in both spherical and nanorod morphologies. Structural and spectroscopic characterizations show that WZ HgTe NCs retain the strong confinement tunability of their optical properties while introducing non-cubic lattice and distinct electronic topology. Ab initio modeling reveals that bulk WZ HgTe is a Dirac semimetal, whereas quantum confinement opens a direct gap that enables bright short-wave infrared emission. High-pressure studies demonstrate an irreversible WZ-to-zinc blende phase transition, consistent with its metastable nature, while the WZ phase remains stable at cryogenic temperatures. Electrically driven light-emitting diodes based on WZ HgTe nanorods exhibit superior electroluminescence beyond 2 μm, establishing a platform bridging topological semimetals and confined infrared emitters.

By means of a specific heat, susceptibility and high-pressure electrical resistivity study, we show that the local magnetic moments of the intercalated V ions in V_5S_8 realize a prototype of Kondo lattice system, where an antiferromagnetic order of the moments coexists with a Fermi liquid in the VS_2 layers with intermediate heavy Fermion properties. The antiferromagnetic order and the Fermi-liquid behavior are simultaneously suppressed at a critical pressure, P_c =10 GPa, signature of a quantum critical point, which supports a Kondo lattice scenario and raises the question whether, in the paramagnetic phase at higher pressures, the heavy quasiparticles survive or form a non-Fermi liquid phase governed by the Kondo interaction.

By means of Raman scattering and infrared reflectivity spectroscopies under high pressure, we investigate the metallic phase induced by pressure at Pcr = 17.5 GPa in the Mott insulator (NaMn3)Mn4O12 (NMO). Upon approaching Pcr, two signatures of the presence of mobile carriers become increasingly pronounced: a Fano-like lineshape of the Raman modes and a Drude-like peak in the optical conductivity spectra. Remarkably, all Raman and infrared modes display a continuous evolution with pressure up to 22 GPa, indicating that no structural distortion occurs at the transition owing to the very compact quadruple perovskite structure of NMO. We therefore propose a unique scenario of a second-order Mott phase transition where only the electronic degrees of freedom are active, which opens the possibility of probing the effective Hamiltonian of the transition disentangled from the lattice degrees of freedom.

Supergene minerals preserve a record of protracted exposure and weathering, rendering them valuable for understanding and reconstructing continental surface evolution and palaeoclimatic history. Determining when and how these minerals precipitated is fundamental for reconstructing the timing, nature, and controlling factor of weathering processes recorded in weathering profiles. In this study, we investigate a well-preserved, 5-m-thick lateritic ferruginous duricrust developed on low-relief uplands (similar to 1100-m elevation) of the Brazilian Central Plateau (BCP). The BCP represents a high-standing postorogenic surface in southeastern Brazil, where numerous geochronological data provide a framework for regional comparison. Using (U-Th)/He geochronology on 100 haematite and goethite grains from nodular, pisolitic, and protopisolitic facies at three depths (similar to 0.5 to similar to 5 m), we provide new constraints on the timing and possible controls of discrete weathering episodes in the BCP. Two successive weathering phases during the Cenozoic Era were identified. The older phase, recorded predominantly by haematite, occurred between ca. 35 and 24 Ma (Late Eocene-Oligocene) under seasonally contrasted tropical conditions. The younger phase, dated between ca. 17 and 8 Ma (Middle to Late Miocene), is characterized by widespread goethite precipitation under more humid and cooler climatic conditions that influenced the entire profile. These findings are consistent with (U-Th)/He datasets from nearby sites and confirm the spatial extent and synchronicity of these weathering events across the BCP. Comparison with geochronological data from different lithologies reveals a strong control of basement composition on weathering style and age distribution: duricrusts developed over igneous and sedimentary rocks yield well-clustered ages, whereas profiles over complex lithologies, such as cangas formed on Banded Iron Formations, show scattered and broadly distributed ages. This study contributes to refining the understanding of Cenozoic weathering dynamics and long-term landscape evolution across the BCP.

We present a comprehensive study of the high-pressure behavior of ReO3 using x-ray and neutron diffraction, Raman scattering, and first-principles calculations. We show that the ambient pressure Pm3m structure converts at 0.7 GPa in a continuous phase transition directly to a cubic phase with space group Im3, which is then stable up to at least 15 GPa. We show that previous reports of monoclinic C2/c and rhombohedral R3c structures in this pressure range are an artifact due to an alteration of the sample by high-flux synchrotron x-ray radiation. The structural pressure dependence of the Im scattering data of both natural and isotopically enriched 18O samples are presented. The data shed light on the unusual transition and densification mechanism due to progressive tilting of essentially rigid ReO6 octahedra.

Superconducting domes, ubiquitous across a variety of quantum materials, are often understood as a window in which pairing is favored, opened by the fluctuations of competing orders. Yet, the understanding of how such a window closes is missing. Here, we show that inelastic neutron scattering, by quantifying a length scale associated with the dipoles correlation, ℓ0, addresses this issue. We find that, within the experimental precision, the end of the superconducting dome coincides with the end of a highly polarizable state (in which ℓ0 is longer than the interatomic distance). Thus, the superconducting dome is driven by the competition between the increase in the density of states and the inevitable collapse of the quantum paraelectric phase. This is compatible with a crucial role played by the soft ferroelectric mode in driving superconductivity. Such a scenario may also be at work in other quantum paraelectric materials, either bulk or at interfaces.

Here we describe the synthesis of a series of 1D anisotropic Al metal-organic framework (i.e., DUT-5(Al)) nanostructures differing in their crystal dimensions and aspect ratios. By using graphene oxide (GO) nanoscrolls as structure-directing agents, we synthesized monodisperse DUT-5(Al) nanowires (NWs) that are >6 μm in length. Shorter DUT-5(Al) nanorods (∼50 nm) and polydisperse samples with broader size distributions were prepared without GO. Remarkably, these nanostructures also differed by their dynamic properties upon the adsorption of guest molecules. While DUT-5(Al) nanorods exhibited a rigid large pore (lp) phase, it was revealed that DUT-5(Al) NWs could present a gating behavior characterized by a structural transition from a disordered narrow pore (np) phase to the lp phase upon adsorption. The structure of DUT-5(Al) NWs and their guest-responsive structural flexibility were fully characterized by coupling high-resolution TEM, 3D electron diffraction, high-resolution synchrotron PXRD and PDF, adsorption isotherms, and in situ PXRD. To the best of our knowledge, this study delivers an unprecedented design of flexible DUT-5(Al) NWs, revising the conventional view of this MOF as a rigid porous material. Moreover, this demonstrates that tuning the aspect ratio of MOF particles can influence their structural flexibility.

Insertion of argon, nitrogen, and oxygen guests in the large 12 & Aring; pores of the aluminophosphate AlPO4-54 under high pressure was studied by synchrotron, X-ray powder diffraction. Structure refinements using the Rietveld method indicate that the number of guests per unit cell saturates at 24-28, which is greater than the values observed in adsorption experiments at low temperature due to the higher fluid densities at high pressure. Guest insertion suppresses transformations to crystalline forms at high pressure, such as AlPO4-8, and the beginning of amorphization is shifted to a much higher pressure as compared to the empty pore material.

Manufactured infrastructures of urban areas, including buildings and roads, are contributors of solid particles to the environment due to wear processes and further weathering. Mineral dusts produced by such mechanisms are transported by air or water across urban compartments until they accumulate in surrounding natural and artificial sediment reservoirs, mixing with other minerals of geogenic sedimentary origin. With the expansion of artificialized urban surfaces over time, the contribution of urban-sourced minerals is expected to increase in sediment fluxes, thus taking an increasing importance in biogeochemical cycles. In this study, we postulate that mineral particles emitted from specific man-made materials could be traced in different compartments of urban environments on the basis of their mineralogical signature. Such identified urban mineralogical components could then serve as useful markers to monitor urbanization wear processes and subsequent emprise of urbanization at the regional scale. Here, we have analyzed a collection of urban samples, which comprises urban dusts, road sediment deposits, suspended particulate matter from the Seine and Orge rivers near Paris, and sediments accumulating in stormwater basins along high traffic roads in the Paris region (N118, N104). In almost all of the solid samples studied (n = 34), whose sampling span over a ten-year period, we show by powder X-ray diffraction (XRD) the presence of minerals belonging to the amphibole group, which are necessarily derived from human activities since these minerals do not belong to the Parisian sedimentary basin. Detailed analysis of a mineral pellet embedded in bitumen of road treads sampled in a Paris street by analytical electron microscopies and Rietveld refinement analysis of powder XRD pattern show that a ferro-magnesio-actinolite is a major constituent (17 wt%) of this road material. Further analysis of an amphibole grain in a road dust sample by single crystal X-ray diffraction also points to such FeMg-actinolite of Ca2.15Mg2.44Fe2.56Si8O22(OH)2 composition. Other samples collected in the vicinity of areas subjected to road water runoff also contain amphibole minerals of close crystal-chemical composition to this FeMg-actinolite, likely designating road aggregates as sources of amphiboles in our broad set of samples. A large distribution of sizes was observed for amphibole particles using electron microscopy, from massive (100-10 mu m) to micrometric packages of elongated mineral particles, likely produced by cleavage of massive particles. The presence of micrometric minerals with elongated fiber habit raises questions about public exposure to such urban dusts. This amphibole signature is also detected in samples of river suspended particulate matter from strongly artificialized urban areas, including in a punctual sample collected in the Seine River, which emphasizes the pervasive occurrence of such minerals in this urban environment. Additionally, the presence of amphibole is suggested by X-ray diffraction on a sample taken on a building roof, which calls for a quantitative investigation of amphibole transport pathways, including air transport, in urban areas. Finally, we propose that this amphibole mineralogical pattern could be used as a mineralogical tracer of city wear and urbanization influence on sedimentary fluxes produced by urban materials.

The thermal conductivities of crystals and glasses vary strongly and with opposite trends upon heating, decreasing in crystals and increasing in glasses. Here, we show that the dominant conduction mechanisms of crystals (particle-like propagation) and glasses (wave-like tunneling) can compensate in materials with crystalline bond order and nearly glassy bond geometry, yielding a hybrid crystal-glass conductivity that is constant from the quantum to the classical regime (i.e., from below to above the Debye temperature). We showcase these arguments with a combined theoretical and experimental study on meteoritic silica (a tridymite carved from a sample found in Steinbach, Germany, in 1724) and on a geometrically amorphous tridymite phase found in refractory bricks used in furnaces for steel smelting. Our results prove that temperature-invariant conductivities are not limited to the classical regime, and pave the way to understand or control heat-transport phenomena in solids exposed to extreme temperature variations, ranging from planetary cooling to heating protocols to reduce the carbon footprint of industrial furnaces.

Trace element contents in authigenic pyrite (FeS2) are often considered as a reliable geochemical archive of past marine conditions. For instance, cobalt (Co) abundance in marine sedimentary pyrite may track back the extent of past ocean anoxia and is considered as a reverse proxy for the rise of atmospheric oxygen. However, the molecular-scale route of Co incorporation in pyrite at low temperature is not well documented. Thus, any insight on this aspect are expected to help better assess the actual role of pyrite in Co cycling in modern and past subsurface environments. In this study, a series of pyrites were synthesized in solution via the polysulfide pathway under anoxic conditions and at ambient temperature, with various initial aqueous Co concentrations (Co:Fe = 0.13-5). Rietveld refinement analysis of the powder X-ray diffraction (XRD) patterns shows that pyrite is the principal component (69(5) wt%) of the final solid products, with small fractions of marcasite (17(4) wt%) and FeS (10(2) wt%). High Energy Resolution Fluorescence Detected (HERFD) Co K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray absorption fine structure (EXAFS) analysis indicate that a minor fraction (20-36 %) of Co substitutes for Fe in the pyrite structure as compared with a theoretical spectrum of a Co-substituted pyrite supercell calculated using Density Functional Theory (DFT). Besides, in the final products, the major part (64-80 %) of Co persists in the form of an amorphous CoSn-polysulfide precursor phase that represents the whole Co speciation before pyrite nucleation. In this precursor observed at the monosulfide FeS pre-pyrite stage, Co early adopts an octahedral coordination as attested by Co pre-edge data, whereas Co is in tetrahedral coordination in our Co-doped mackinawite FeS reference compound. The local structure of the CoSnpolysulfide precursor is further elucidated by EXAFS shell-by-shell analysis that points to monomeric units (< 1 nm), where Co is octahedrally coordinated to first neighboring S atoms with at least three of these S neighbors belonging to a polysulfide chain of undetermined length. Aggregation of such monomeric units into an amorphous CoSn-polysulfide phase is supported by X-ray scattering-pair distribution function analysis (XRD-PDF) of an analogous amorphous CoSn compound. These results could have important implications on our understanding of pyrite nucleation mechanisms via the polysulfide pathway since the observed octahedral CoSn-polysulfide precursor differs from the generally proposed models of tetrahedral FeS precursors. In addition, the persistence of these peculiar species and the observed delay of Co incorporation in pyrite highlights the importance of trace elements in pyrite formation kinetics at low temperature. Lastly, our results illustrate the high affinity of Co for polysulfides and raise questions on the possible presence and evolution of this non-pyrite CoSn phase in sedimentary archives. In this regard, this study may provide new mechanistic insights that could help explaining the moderate affinity for authigenic pyrite generally reported for Co, in particular in a sedimentary context where other possible bearing phases such as clay minerals and Mn oxides are involved.

Superconducting domes, ubiquitous across a variety of quantum materials, are often understood as a window favorite for pairing opened by the fluctuations of competing orders. Yet, a quantitative understanding of how such a window closes is missing. Here, we show that inelastic neutron scattering, by quantifying a length scale associated with polar fluctuations, $\ell_0$, addresses this issue. We find that the superconducting dome of strontium titanate definitely ends when $\ell_0$ vanishes. Moreover, the product of $\ell_0$ and the Fermi wavevector peaks close to the maximum critical temperature. Thus, this superconducting dome stems from the competition between the increase of the density of states and the unavoidable collapse of the quantum paraelectric phase, both induced by doping. The successful quantitative account of both the peak and the end of the superconducting dome implies a central role in the pairing mechanism played by the soft ferro-electric mode and its hybridisation with the acoustic branch. Such a scenario may also be at work in other quantum paraelectric materials, either bulk or interfaces.

Boron carbide (B4+delta C) possesses a large potential as a structural material owing to its lightness, refractory character, and outstanding mechanical properties. However, its large-scale industrialization is set back by its tendency to amorphize when subjected to an external stress. In the present work, we design a path toward nanostructured boron carbide with greatly enhanced hardness and resistance to amorphization. The reaction pathway consists of triggering an isomorphic transformation of covalent nanocrystals of Na1-x B5-x C1+x (x = 0.18) produced in molten salts. The resulting 10 nm B4.1C nanocrystals exhibit a 4-fold decrease of size compared to previous works. Solid-state 11B and 13C NMR coupled to density functional theory (DFT) reveal that the boron carbide nanocrystals are made of a complex mixture of atomic configurations, which are located at the covalent structural chains between B11C icosahedral building units. These nanocrystals are combined with a spark plasma-sintering-derived method operated at high pressure. This yields full densification while maintaining the particle size. The nanoscaled grains and high density of grain boundaries provide the resulting nanostructured bodies with significantly enhanced hardness and resistance to amorphization, thus delivering a superhard material.