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International audience; Data of quasi-specular reflection of cold neutrons, prompt-γ neutron analysis, X-ray Raman scattering (XRS), and neutron pair distribution function (PDF) analysis with powder of detonation nanodiamonds are analyzed to collect their structural properties and chemical composition. Both as-synthesized and purified samples were studied using fluorination samples. Removal of both the sp2 amorphous carbon shell and the hydrogen atoms is evidenced respectively by the change of neutron-nuclei optical potentials of nanoparticles and the increase of their neutron reflectivity. Moreover, sp3 diamond cores of nanoparticles stay intact during the fluorination as revealed by similar scattering patterns, PDF, and XRS data. Quasi-specular reflection, PDF, and XRS data are complementary for the study of nanomaterials and in good agreement with conventional characterization techniques (infrared spectroscopy and solid-state NMR).

We have explored the effect of high pressure post-treatment in optimizing the properties of carbon nanotube yarns and found that the application of dry hydrostatic pressure reduces porosity and enhances electrical properties. The CNT yarns were prepared by the dry-spinning method directly from CNT arrays made by the hot filament chemical vapour deposition (HF-CVD) process. Mechanical hydrostatic pressure up to 360 MPa induces a decrease in yarn resistivity between 3% and 35%, associated with the sample’s permanent densification, with CNT yarn diameter reduction of 10%–25%. However, when increasing the pressure in the 1–3 GPa domain in non-hydrostatic conditions, the recovered samples show lower electrical conductivity. This might be due to concomitant macroscopic effects such as increased twists and damage to the yarn shown by SEM imaging (caused by strong shear stresses and friction) or by the collapse of the CNTs indicated by in situ high pressure Raman spectroscopy data.

International audience; Competition between microscopic point defect (vacancy and interstitial) configurations is inherent to crystalline phases of increased structural complexity. Phase transitions that preserve symmetry between them belong to a specific class of isostructural transitions. Type-I silicon clathrates are representatives of such structurally complex crystalline phases showing an intriguing structural transition at high pressure associated with an abrupt reduction of volume with no indication for any breakage of symmetry. Using isothermal high-pressure X-ray diffraction performed on a single crystal of the simplest representative type-I silicon clathrates, binary Ba 8 Si 46 , we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure. An analysis based on the Landau theory gives a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.

International audience; Nanoscale coal deformation affects thegeomechanics response of coal structure under external stress conditions. In this study, insitu small-angle neutron scattering (SANS) is used to characterize the evolution of nanopore structures under uni-axial compression on an Illinois coal specimen. Porod invariant mapping is used to estimate apparent porosities at different azimuthal angles. Structure-free orientation factors, including the alignment factor and Hermans’ orientation factor, are used to characterize orientation degree in nanopores quantitatively. The nanoscale pore deformation and evolution with stress have not been directly measured before, and the insitu SANS technique offers the unique capability to probe the nanoscale rock deformation. From the results, higher values of apparent porosities are shown near the coal bedding direction.At dif-ferent azimuthal angles, the apparent porosities increase with increasing uniaxial stress in the elastic deformation range (54–575bar) for the measured coal. The pore-scale degree of orientation is a size-dependent parameter, and pore orientation along the coal bedding generally increases as pore size increases. The degree of orientation in nanopores on average (in a size range between ~ 2 × 103Å and ~ 15Å) tends to increase first and then decreases with increasing stress loading, although different from that of macropores, indicating higher stresses on the nanopore edges along the bedding direction and then on the nanopore body along the normal direction. There is no distinct permanent change ofporosity within the measured stress range (0–575 bar)after the stress relief

International audience; C 2 F graphite fluoride has a unique mutable structure when subjected to external applied pressure. In the present study, we examine C 2 F using X-ray Raman scattering (XRS) up to 6.5 GPa coupled with theoretical simulations. Using the XRS technique, we follow the in situ high pressure evolution of the energy loss corresponding to the C and F K-edge. Significant variations occur at 2.9 GPa, remain up to 6.5 GPa and persist at ambient conditions after decompression. The permanent changes are related to an increased planarity of the graphitic layers and a modulation of the fluorine configuration. The all-electron density of a C 2 F-sp 3 slab obtained from the DFT simulation and the Quantum Theory of Atoms in Molecules (QTAIM), reveals the appearance of bond-critical points between in-plane and out of plane F-F, suggesting an increasingly ionic character of the structure under biaxial isotropic strain. Pressure can be used as an alternative to chemical synthesis for tuning C-F bonding and metastabilizing new intercalated fluorine compounds with potentially improved electrochemical properties.

International audience; Mesoporous materials may be considered as the negative image of a collection of nanoparticles. In both cases, the large surface area induces an increase of the system energy. Pressure-induced phase transitions are used to study the differences between mesoporous TiO 2 and a collection of TiO 2 nanoparticles. The synthesis and characterization of mesoporous TiO 2 with a nanosponge-like structure are presented and the inputs of different spectroscopic techniques (Terahertz, Brillouin and Raman) are highlighted. The results show that the phase stability of mesoporous TiO 2 under pressure is very similar to that observed in nanoparticles. 2

International audience; In the present paper, graphite fluoride compounds CxF with various degrees of fluorination (x = 4, 2 and 1), i.e., with different C-F bonding characteristics, were systematically investigated using X-ray Raman Spectroscopy (XRS) and X-ray and neutron diffraction by means of Pair Distribution Function (PDF) analysis. For both techniques, ab-initio calculations allow the experimental results and structural/bonding assignments to be explained and/or confirmed. Presented here for the three stoichiometries are results for the carbon and fluorine first-neighbour distances, the evolution of the electronic band structure due to the change of the carbon hybridization upon fluorination, and also the nature of the final states for 1s core-electron excitations. Since fluorination is limited by the diffusion of fluorine in the carbon lattice, the process is less effective in the bulk, where a graphitic core remains. The cross-checked XRS, PDF data and Density Functional Theory (DFT) simulations appear also to be a powerful method to highlight the presence of a residual graphitic core, even at the highest fluorine content, in such semi-disordered and structurally complex compounds.

International audience; The evolution of the pore structure of compressed expanded vermiculite was investigated in situ under uniaxial stress up to 3200 bar using small-angle neutron and X-ray scattering. The studied samples have a lamellar texture in which the presence of oriented oblate pores was revealed by the anisotropic small-angle scattering patterns. The initial pore size distribution and the mechanical behaviour of the pellets at working pressure conditions are strongly related to the applied initial stresses used to pre-compact the vermiculite powder. Our study shows that the porous structure of the pellets was not modified under compression up to the pre-compaction pressure. Higher densification is associated with increasing anisotropy of the pore–matrix interface structure. The pore–matrix interface could be described with fractal geometry. The stress dependence of the system fractal dimension, apparent specific surface area and total porosity was determined and related to the meso- and macropore evolution under uniaxial stress.

International audience; Pressure and temperature phase transitions of nanomaterials often differ significantly from those of their bulk parents, offering novel approaches for the engineering of original materials. The importance or even the dominance of surface atoms in the nanoworld enhances the effects of environment, geometry, and intercalation. In the present article, we explore the current knowledge of these effects, as evidenced in the high pressure phase diagrams of nanomaterials such as nanocrystals, carbon nanotubes, fullerites, graphene, and other 2D systems, as well as nanoporous structures like clathrates or zeolites. Recent advances and future challenges in the use of extreme thermodynamic conditions to develop new functional nanomaterials, composites, or devices will be reviewed, along with the specificities of the experimental environment required for these investigations.

International audience; In this paper the structural and vibrational properties of intercalated fluorine graphite (C4F)n under compression up to 40 GPa are investigated by means of Raman spectroscopy, X-ray diffraction and ab initio calculations. The theoretical study was performed within the framework of the density functional theory, taking into account van der Waals corrections to the total energy. We studied the effect of anisotropic and isotropic deformation on the crystal properties, in relation to the layered structure of the sample and in particular to the CF bonding. The evolution with pressure of the phonon modes at the center of the Brillouin zone was calculated and compared with experimental data. Both our experimental data and theoretical models indicate that a structural transition occurs around 10 GPa. Above this pressure the (C4F)n structure becomes dynamically unstable, characterized by a “soft mode” and a change in symmetry due to anisotropic strain components.

We have studied a small, very unusual stone, here named “Hypatia”, found in the area of southwest Egypt where an extreme surface heating event produced the Libyan Desert Glass 28.5 million years ago. It is angular, black, shiny, extremely hard and intensely fractured. We report on exploratory work including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy with EDS analysis, deuteron nuclear reaction analysis, C-isotope and noble gas analyses. Carbon is the dominant element in Hypatia, with heterogeneous O/C and N/C ratios ranging from 0.3 to 0.5 and from 0.007 to 0.02, respectively. The major cations of silicates add up to less than 5%. The stone consists chiefly of apparently amorphous, but very hard carbonaceous matter, in which patches of sub- μ m diamonds occur. δ 13 C values (ca. 0‰) exclude an origin from shocked terrestrial coal or any variety of terrestrial diamond. They are also higher than the values for carbonaceous chondrites but fall within the wide range for interplanetary dust particles and comet 81P/Wild2 dust. In step heating, 40Ar/36Ar ratios vary from 40 to the air value (298), interpreted as a variable mixture of extraterrestrial and atmospheric Ar. Isotope data of Ne, Kr and Xe reveal the exotic noble gas components G and P3 that are normally hosted in presolar SiC and nanodiamonds, while the most common trapped noble gas component of chondritic meteorites, Q, appears to be absent. An origin remote from the asteroid belt can account for these features. We propose that the Hypatia stone is a remnant of a cometary nucleus fragment that impacted after incorporating gases from the atmosphere. Its co-occurrence with Libyan Desert Glass suggests that this fragment could have been part of a bolide that broke up and exploded in the airburst that formed the Glass. Its extraordinary preservation would be due to its shock-transformation into a weathering-resistant assemblage.

XPS+EQUIPE-VIDE+LCS; International audience; We studied the high pressure behavior of poly(dicarbon monofluoride) (C2F)n up to 37 GPa both theoretically, within the first-principle framework of the density functional theory, taking into account van der Waals corrections to the total energy, and experimentally, combining in situ Raman spectroscopy, XRD and X-ray photoelectron spectroscopy (XPS) measurements. The (C2F)n structure has a complex mixed sp2 sp3 character where graphene layers are intercalated among sp3 carbon hybridized structural domains. We observed three phase transitions in the high pressure range explored. The first transition is observed around 12 GPa and is dominated by strain effects. Above 16 GPa the intercalated graphene layers corrugate and change from the sp2 to sp3 hybridization state. Above 25–27 GPa the new pressure-induced phase becomes unstable. We find that, by varying applied pressure or by changing the hybridization due to the fluorination of graphite, the band structure of (C2F)n can be modified, resulting in valence and conduction bands touching at the K point, as in graphene, thus turning (C2F)n into a semimetal. This could provide a new pathway to reversibly engineer the band structure and conductivity for applications in high energy density batteries.

The high-pressure superconducting properties and the local structure of the Ba8Si46 clathrate have been studied using electrical resistance and x-ray absorption spectroscopy measurements up to more than 20 GPa. At pressures above 10–13 GPa, corresponding to a well-known volume collapse phase transformation, a sudden increase in the critical temperature leads to a maximum value of the superconducting critical temperature Tc of ∼8.5–9 K at 20 GPa. In the low-pressure clathrate phase, the superconducting critical temperature decreases as pressure is applied with an electron-phonon coupling constant λ=1.1 derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the Ba@Si24 cages of the structure is observed for pressures beyond ∼5 GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of Ba8Si46 is then associated with local structural changes and shows enhanced superconducting properties.

Phengite 3T (Dora-Maira massif, Italian western Alps), with chemical composition K0.96Na0.01Al1.44Mg0.56(Si3.59 Al0.41)O10(OH)2 has been investigated using powder neutron diffraction, at the ILL (Institut Laue-Langevin) Facility, on the D2B diffractometer. Data sets were collected at 293 and 873 K, and the present results are compared with those obtained previously using the time-of-flight (TOF) technique, on the same compound. In the octahedral sheet, Al tends to order into the M2 site, in accordance with the measurements mentioned above, whereas the Al/Si tetrahedral ordering we observe (i.e. Si fully into T1 site) is at variance with that determined previously. These discrepancies are ascribed to the presence of talc impurities in the sample used previously (∼6 wt.%), which affected the results obtained.

$\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{39}\mathrm{G}{\mathrm{e}}_{7}$ and $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{29}\mathrm{G}{\mathrm{e}}_{17}$ have been studied at high pressure using x-ray diffraction and x-ray absorption spectroscopy (XAS) at the Ge $K$ edge. In $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{39}\mathrm{G}{\mathrm{e}}_{7}$, a transition is observed similar to the one in $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$, apparently isostructural. However, the XAS data analysis shows that the transformation is related to the off-centering of the Ba atoms. A theoretical model based on a Landau potential suggests that this transition is second order, with a symmetry-breaking mechanism related to the Ba displacement probably initiated by the vacancy creation or local distortion predicted theoretically. This analysis gives a coherent picture of the phase transition mechanism. In the case of $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{29}\mathrm{G}{\mathrm{e}}_{17}$, such phase transition is not observed as the Ba atoms appear already off-center at ambient pressure.

Combined XRD and Mossbauer effect spectroscopy studies to high pressures of ∼30 GPa of vacancy ordered maghemite are presented. The vacancy ordered superstructure is robust and remains intact up to the pressure-induced onset transition to hematite at 13–16 GPa. The pressure transformed hematite is shown to be crystallographically textured, unlike the randomised low pressure maghemite phase. This arises out of a pressure or stress instigated topotactic transformation of the cubic-spinel to hexagonal-corundum structure. The textured sample permits us to obtain information on the spin reorientation behavior of the pressure transformed hematite in compression and decompression sequences. Spin reorientation is restricted to ∼15° over wide pressure ranges, attributable to the effect of entrapped vacancies in the high pressure structure. Thus there are structural and magnetic peculiarities specific to pressure transformed hematite not evident in pressurized hematite starting material. These are triggered by the maghemite→hematite transformation.

We demonstrate that laser irradiation on the narrow-gap insulator Li4C60 and Na4C60 polymers results in metallic monomeric structures, having the same Raman features as metallic fcc K3C60 and Rb3C6 ...

K-vermiculite pellets were obtained by compression (80 MPa, 25 °C or 200 °C) of submicrometer size vermiculite particles. The evolution of the structural and textural properties of the compressed materials was studied as a function of the annealing temperature in the 400–1000 °C range. Small Angle Neutron Scattering patterns of the pressed samples suggest a preferential orientation of the vermiculite particles for which the basal planes were arranged perpendicular to the compaction axis. Thermogravimetric analyses, Scanning Electron Micrographs, density and porosity measurements show that the most significant changes occurred for annealing temperatures higher than 800 °C. When heated above 800 °C, the pressed vermiculite samples undergo a simultaneous increase in density and decrease in porosity concomitant to a structural evolution, leading to a more three dimensional-based structure.

International audience; Vermiculite materials were obtained by uniaxial pressing of potassic vermiculite powders obtained by sonication without any binder addition. The vermiculite powders, made of aggregates of particles with nanometric thick-ness and micrometric in plane dimension, were pressed in the range 17.7–80 MPa at room temperature and 200 °C, and further annealed in the range 400–800 °C. Pressing powders at 200 °C instead of 25 °C allowed the slight increase of the density of the formed materials (from 1.9 to 2 g·cm −3) due to the desorption of the water molecules from the interaggregate and interparticle spaces, allowing a higher densification. The density was also increased by tailoring the particle size distribution. The pressed materials were formed of oriented arrangement of vermiculite aggregates. The porous structure, characterized by mercury porosimetry, scanning electron microscopy, and small angle X-ray scattering, was modelled by oriented oblate spheroidal pores formed in the voids between the stacked aggregates organized in a structure possessing a cylindrical symmetry. The porous structure was found to vary with the pressure and the annealing temperature.

International audience; Auguste Bravais by his fundamental work on lattice has pioneered modern crystallography. He was born in 1811 in Annonay (France) at a short distance from Lyon and Saint-Etienne. He was then educated at the College Stanislas in Paris and entered the École Polytechnique in Paris in 1829. In 1832, he joined the French Navy as an officer and took part at scientific explorations to the Algerian Coast and Northern Europe. In 1837 he defended a PhD in Astronomy at the Faculty of Sciences in Lyon where he became Professor in 1841 to teach mathematics in astronomy. Then, in 1845, he moved at Ecole Polytechnique in Paris to take the chair of Physics, which he held till 1856. He published his first studies dealing with crystal lattice in 1849 in a short paper [1] and later wrote a book where he developed his theory fully based on geometrical theorems [2]. He died prematurely in 1863 exhausted by the loss of his only son. Like many scientists of that time Auguste Bravais was universalist and has been successively astronomer, geologist, mathematician, physicist, mineralogist, and crystallographer as well as an explorer from Lapland to the top of Mont Blanc [3]. In this communication, the steering committee Lyon-Saint-Etienne will recall the contribution of Auguste Bravais to crystallography and will show some aspect of his life that may be less known in our community.

International audience; In a previous work [1] we elucidated the in situ evolution of the porosity of out-of-plane com- pressed flexible graphite under uniaxial pressure up to 1000 bar using small-angle neutron scattering (SANS) technique. In order to understand the influence of shear effect on the properties of flexible graphite we study, in the present paper, the in situ behaviour of in-plane compressed flexible graphite under a uniaxial pressure. The sample had a pleated layered structure in which anisotropic SANS patterns revealed a distribution of differently oriented ellipsoid pores. Uniaxial compression generates important shear effects in this kind of sample.We have determined the evolution of the system fractal dimension, pore size distribu- tion and apparent specific surface area with applied pressure which together allow us to describe the meso and macro pore structure evolution. Under pressure, the irreversible col- lapse and splitting of larger pores into smaller size ones which is characteristic of out-of- plane uniaxial compression [1], is accompanied, in the presence of shear stress compo- nents, by an in-plane slipping mechanism giving rise to cracks and consequently to inter- face formation.

Two low temperature (15 K) neutron powder diffraction data sets from 2 M 1 and 3 T phengite samples were collected on the D2B beamline at ILL (Grenoble, France), to investigate the cation ordering and the occurrence of fractional occupants at the M 1-site. Low temperature has been used in order to reduce the thermal motion. The complexity of this study poses problems to property quantify our results, but on the basis of the issues from several refinement schemes we obtained indications in favour of occupancy factors other than zero for the formally empty in dioctahedral micas M 1-sites (roughly ≈ 0.12 and ≈ 0.18 atom per M 1-site, in case of 2M 1 and 3 T polytype, respectively). The atomic displacement parameters, and the large uncertainties upon the M 1-O bond lengths, suggest the large M 1 site to be positionally disordered in both polytypes. These results are to be taken up with due care as stacking faults resulting in staggering of the TOT layers might mimic a fictitious M 1-O site occupancy.

X-ray powder diffraction measurements have been carried out at ESRF (Grenoble, France) on the ID30 beamline to study the equation of state of 3T phengite (Dora-Maira massif, Italian western Alps) by a large volume cell up to P = 50 kbar and T = 1000 K. Several equations of state (EOS) models (the Vinet EOS, the Birch-Murnaghan EOS and its variants, a VT-polynomial expansion) have been used to interpolate the experimental data and discussed in the light of the results achieved. The thermoelastic properties of 3T phengite (bulk modulus, its derivatives versus pressure or temperature, bulk thermal expansion) have been obtained and an isochoric curve with slope P/T = 0.02 kbar/K has been calculated by means of the Vinet EOS. This slope value supports either the occurrence at the peak conditions (about 30 kbar and 1000 K) of an originally Mg/Si-richer and stiffer phengite or a non-isochoric P-T retrograde path.

Magnetic-electronic studies of mixed-valence Fe${}_{2}$OBO${}_{3}$ have shown that ionic charge order (CO) is disrupted at \ensuremath{\sim}16 GPa. The pertinent minority-spin carrier exhibits persistent intersite electron exchange Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$ to well beyond this pressure. Temperature-dependent electrical transport measurements over an extended pressure range presented here demonstrate that the electronic structure remains gapped to well beyond 16 GPa. Extrapolation of data to higher pressure suggests that metallization will only prevail at $Pg50$ GPa. Both the persistent gapped electronic state across the CO instability and signature of carrier confinement to Fe-Fe dimers in the Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$ electron exchange are rationalized as crossover from a Wigner crystal (site centered) insulator to a dimer Mott (bond centered type) insulator---``Wigner-Mott transition'' at \ensuremath{\sim}16 GPa. The dimer insulating state is a consequence of modulation of the relevant hopping parameter $t$ in quasi-low-dimensional features in the structure (ribbons and chains). Complementary structural studies suggest that the $a$ axis is appreciably more compressible than other crystallographic directions of the original monoclinic unit cell. Therefore, such a modulation in $t$ may arise from Peierls type distortions along the $a$ axis or else stems from intrinsic modulation in the $c$ axis direction of the unit cell. This is aided by a monoclinic ($P$2${}_{1}/c$) \ensuremath{\rightarrow} orthorhombic (Pmcn) structural adjustment that is concurrent across the electronic transition. Pressure tuning of relative values of on-site $U$/$t$ and intersite $V$/$t$ Coulomb interaction parameters of the quasi-low-dimensional features evolve the system from site-centered to dimer-centered electron localization.

In this article we study the alkali metal-intercalated 2D polymeric Li4C60 and the monomeric Li12C60 under pressure up to 40 GPa at room temperature, using x-ray diffraction and Raman spectroscopy. Li4C60 undergoes several transitions in the studied pressure range. At pressures lower than 8 GPa, we observed changes in both diffraction patterns and Raman scattering spectra, probably due to the displacement of Li atoms. At 8 GPa another structural and electronic transition occurs. We observe an enhancement of background and a broadening of diffraction peaks. Raman modes weaken and broaden considerably. An important structural transition occurs at around 16 GPa, in which new Raman bands exhibit features similar to those of a reported 3D C60 polymeric structure. The XRD data shows a collapse in volume with the simultaneous formation of amorphous material. The cell parameters deviate from their early pressure evolution and become less compressible. The high pressure study of highly doped monomeric Li12C60 shows that its structural integrity is retained up to 13 GPa, with increasing pressure-induced structural distortion and disorder. Above 13 GPa, Li12C60 transforms to a highly disordered state.

International audience; The present article studies the in situ evolution of the pore structure of compressed expanded graphite under a uniaxial stress up to 1000 bar using small-angle neutron scattering (SANS). The uniaxial stress was applied in the direction of the average c-axis of the graphite crystallites composing the sample. Ex situ characterization by electrical resistivity and mercury intrusion porosimetry was performed on the compressed samples. The anisotropic SANS pattern indicates the presence of spheroidal pores in the 4–100 nm detectable range. The stress dependence of the different extractable parameters (fractal dimension, apparent specific surface area and apparent porosity) was related to the meso and macro pore structure evolution. In particular, the fractal dimension increases irreversibly with the applied stress. We propose a model of evolution under uniaxial load in which the irreversible collapse and splitting of larger pores into smaller size ones provides a coherent description of the experimental observations.

Valence order and fluctuations in the mixed-valence warwickite Fe${}_{2}$OBO${}_{3}$ have been explored by ${}^{57}$Fe M\"ossbauer effect spectroscopy at pressures up to 30 GPa in diamond anvil cell experiments. At room temperature a drastic disruption of charge order is evident at \ensuremath{\sim}11 GPa. There is coexistence of charge order and a progressively increasing abundance of fluctuating valence states in the range extending to \ensuremath{\sim}16 GPa. At $P$g16 GPa only signatures of electron exchange relaxation, Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$, where ``$\ensuremath{\Leftrightarrow}$'' represents the resonating mobile carrier, are discerned. Spectral signatures indicate that electron hopping is on a timescale of \ensuremath{\sim}50 ns, that is, in a time window to which the nuclear resonance technique is particularly sensitive. Low-temperature quenching (\ensuremath{\sim}110 K) at these high pressures (i) is not sufficient to inhibit electron exchange for charge order to reemerge and (ii) reveals that magnetic ordering typical of the charge-ordered phase at low pressure is completely altered to entail new spin dynamics. This evidences the strong interplay between charge order and magnetism and establishes $P$\ensuremath{\sim}16 GPa as a new electronic phase transition boundary for this system. Nanosecond valence fluctuation signatures persist upon further pressurization to \ensuremath{\sim}30 GPa at 300 K, suggestive of continued confinement of the mobile carrier to the Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$ pair at these extremes.

In this article the alkali metal-intercalated two-dimensional (2D) polymer Na4C60 is studied under pressure up to 41 GPa at room temperature by Raman spectroscopy and x-ray diffraction (XRD) measur ...

By laser-irradiating polymeric Li(4)C(60) and Na(4)C(60), we have obtained pure monomeric metallic phases stable at ambient conditions. Based on a systemic Raman analysis, we have determined the electron-phonon coupling constant for both metallic phases. The e-p coupling constants of Li- and Na-intercalated metallic fullerides are smaller than those of superconductive K(3)C(60) and Rb(3)C(60) and comparable to or slightly higher than that of ambient-pressure non-superconductive Cs(3)C(60). We predict that Na-doped fulleride could exhibit superconductivity with T(c) ∼ 10 K. Much lower T(c) or even no superconductivity can be expected for the Li-doped fulleride which exhibits a strong Li(+)-C interaction. These results contribute to the understanding of superconductivity in light alkali metal intercalated fullerides.

MATERIAUX+SDA; We investigate the interface energy impact on phase stability using the shining example of TiO2 nanoparticles under pressure. We revisit the previously reported phase diagram of this system and propose a new mechanism allowing the control of pressure-induced amorphization of TiO2 ultrafine particles. We demonstrate that the size effect is necessary for stabilizing the amorphous state but is not sufficient in the sense that surface chemical functionalization of nanoparticles is determinant. This discovery opens the possibility to select the high-pressure phase in nanomaterials and, consequently, the recovered structure under ambient conditions.

Machon, Denis Daniel, Marlene Pischedda, Vittoria Daniele, Stephane Bouvier, Pierre LeFloch, Sylvie; Two different nanometric (6 nm) TiO2 compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13-16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation HDA2 -> HDA1 at similar to 30 GPa. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.

We developed a high pressure cell for the in situ study of the porosity of solids under high uniaxial strain using neutron small angle scattering. The cell comprises a hydraulically actioned piston and a main body equipped with two single-crystal sapphire windows allowing for the neutron scattering of the sample. The sample cavity is designed to allow for a large volume variation as expected when compressing highly porous materials. We also implemented a loading protocol to adapt an existing diamond anvil cell for the study of porous materials by X-ray small angle scattering under high pressure. The two techniques are complementary as the radiation beam and the applied pressure are in one case perpendicular to each other (neutron cell) and in the other case parallel (X-ray cell). We will illustrate the use of these two techniques in the study of lamellar porous systems up to a maximum pressure of 0.1 GPa and 0.3 GPa for the neutron and X-ray cells, respectively. These devices allow obtaining information on the evolution of porosity with pressure in the pore dimension subdomain defined by the wave-numbers explored in the scattering process. The evolution with the applied load of such parameters as the fractal dimension of the pore-matrix interface or the apparent specific surface in expanded graphite and in expanded vermiculite is used to illustrate the use of the high pressure cells.

The bulk elastic properties of natural andradite and grossular have been investigated by synchrotron radiation powder diffraction at high pressure conditions, up to 37 GPa, on the ID9 beamline (ESRF). The P-V data have been interpolated by the Birch-Murnaghan EOS and the Vinet EOS, and the bulk modulus and its first derivative versus P determined. The results obtained by the different EOS models are compared with one another, and with earlier issues. For andradite and grossular the Birch-Murnaghan EOS yields K 0 = 158.5(±1.0) GPa and K 0 ’ = 5.77(±0.10) [ K 0 “=−0.056 GPa −1 implied value], and K 0 =169.3(±1.2) and K 0 ’ = 5.92(±0.14) [ K 0 ”=-0.056 GPa −1 implied value], respectively. The elastic properties here reported are compared with previous issues. The cell edge compressibilities, which have been determined by fitting the experimental lattice parameters with polynomials in P , are β a = 0.00208(2) GPa −1 , for andradite, and β a = 0.00194(3) GPa −1 , for grossular.

The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola, and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K0=201.7(±0.9) GPa, K ′0=7.62(±0.09) and K ″0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and 0.0041(2) GPa−1 at P=0, respectively] are discussed.

The porous structure of expanded graphite and expanded vermiculite has been studied in situ under uniaxial stress. The properties of a porous material being related to the matrix and to the porosity, the in situ evolution under of the crystalline structure (of the matrix) under high pressure have been first investigated using diamond anvil cell. The equation of state of expanded vermiculite has been established. This first part of this work allowed giving a particular insight to the study of the unsolved high pressure phase of graphite. Combining Raman scattering data and calculations, a new structure, called Z-Carbon, has been proposed. Thanks to the specific technical developments of this work, the porosities of expanded graphite and expanded vermiculite based systems have been studied in situ under uniaxial stress. The used of fractal model in data analysis allowed following the evolution of the fractal dimension and of the apparent specific surface The studied samples were made of compressed forms of expanded graphite and expanded vermiculite in which the basal plane of the crystallites have a preferential orientation. The uniaxial stress was taken perpendicular and parallel to this preferential direction. The porous structure of the expanded graphite sample was found to undergo an irreversible collapse of the pores or a cracks and creation and propagation. Additional electrical and porosity measurements supported the proposed models. In the expanded vermiculite based systems, the crack apparition was observed under uniaxial stress., Résumé : L'évolution des structures poreuses du graphite et de la vermiculite expansés a été étudiée in situ sous pression uniaxiale. Les propriétés d'un matériau résultant des propriétés intrinsèques à la matrice et de celles dues à la porosité, les études faites dans ce travail ont porté sur deux échelles différentes. Les évolutions structurales de la structure cristalline du graphite et de la vermiculite ont d'abord été étudiées à haute pression en cellule à enclumes de diamant. Cette partie du travail a permis d'établir les équations d'état de la vermiculite et de contribuer, de manière significative, à la caractérisation de la phase haute pression du graphite: une nouvelle phase, le Carbone Z, a été proposée après l'analyse des données de la spectroscopie Raman couplée aux simulations. Des développements techniques ont été particulièrement réalisés pour permettre d'étudier in situ l'évolution de la porosité sous pression par diffusion aux petits angles sous pression. L'application du modèle fractale à l'analyse des données a permis de suivre l'évolution de la dimension fractale et de la surface spécifique apparente. Les échantillons étudiés sont des formes comprimées de graphite et de vermiculite expansés dans lesquelles les plans basaux des cristallites ont une orientation préférentielle. Sous pression uniaxiale, la structure poreuse du graphite expansé comprimé évolue à travers un effondrement irréversible des pores ou un cisaillement de la matrice suivant l'orientation de la pression appliquée par rapport à l'orientation préférentielle des plans basaux des cristallites. Des expériences complémentaires de mesures électriques et de mesures de la porosité par intrusion de mercure ont permis de confirmer ces modèles proposés. Dans la vermiculite expansée comprimée, les fissures apparaissent, de manière générale, sous l'effet de la pression uniaxiale.

The porous structure of expanded graphite and expanded vermiculite has been studied insitu under uniaxial stress. The properties of a porous material being related to the matrix and to theporosity, the in situ evolution under of the crystalline structure (of the matrix) under high pressurehave been first investigated using diamond anvil cell. The equation of state of expanded vermiculitehas been established. This first part of this work allowed giving a particular insight to the study of theunsolved high pressure phase of graphite. Combining Raman scattering data and calculations, a newstructure, called Z-Carbon, has been proposed. Thanks to the specific technical developments of thiswork, the porosities of expanded graphite and expanded vermiculite based systems have been studiedin situ under uniaxial stress. The used of fractal model in data analysis allowed following the evolutionof the fractal dimension and of the apparent specific surface The studied samples were made ofcompressed forms of expanded graphite and expanded vermiculite in which the basal plane of thecrystallites have a preferential orientation. The uniaxial stress was taken perpendicular and parallel tothis preferential direction. The porous structure of the expanded graphite sample was found to undergoan irreversible collapse of the pores or a cracks and creation and propagation. Additional electrical andporosity measurements supported the proposed models. In the expanded vermiculite based systems,the crack apparition was observed under uniaxial stress.; L’évolution des structures poreuses du graphite et de la vermiculite expansés a été étudiée insitu sous pression uniaxiale. Les propriétés d'un matériau résultant des propriétés intrinsèques à lamatrice et de celles dues à la porosité, les études faites dans ce travail ont porté sur deux échellesdifférentes. Les évolutions structurales de la structure cristalline du graphite et de la vermiculite ontd'abord été étudiées à haute pression en cellule à enclumes de diamant. Cette partie du travail a permisd'établir les équations d'état de la vermiculite et de contribuer, de manière significative, à lacaractérisation de la phase haute pression du graphite: une nouvelle phase, le Carbone Z, a étéproposée après l’analyse des données de la spectroscopie Raman couplée aux simulations. Desdéveloppements techniques ont été particulièrement réalisés pour permettre d’étudier in situl'évolution de la porosité sous pression par diffusion aux petits angles sous pression. L’application dumodèle fractale à l’analyse des données a permis de suivre l’évolution de la dimension fractale et de lasurface spécifique apparente. Les échantillons étudiés sont des formes comprimées de graphite et devermiculite expansés dans lesquelles les plans basaux des cristallites ont une orientation préférentielle.Sous pression uniaxiale, la structure poreuse du graphite expansé comprimé évolue à travers uneffondrement irréversible des pores ou un cisaillement de la matrice suivant l'orientation de la pressionappliquée par rapport à l'orientation préférentielle des plans basaux des cristallites. Des expériencescomplémentaires de mesures électriques et de mesures de la porosité par intrusion de mercure ontpermis de confirmer ces modèles proposés. Dans la vermiculite expansée comprimée, les fissuresapparaissent, de manière générale, sous l’effet de la pression uniaxiale.