Phase boundary between Na-Si clathrates of structures I and II at high pressures and high temperatures

Understanding the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. Silicon clathrates have the archetype structures that can serve an existant model compounds for superhard clathrate frameworks "Si-B", "Si-C", "B-C" and "C" with intercalated atoms (e.g. alkali metals or even halogenes) that can assure the metalic properties. Here we report the in situ and ex situ studies of high-pressure formation and stability of clathrates Na8Si46 (structure I) and Na24+xSi136 (structure II). Experiments have been performed using standard Paris-Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in Na-Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration have been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~6 GPa, Na24+xSi136 (structure II) is stable at lower temperatures as compared to Na8Si46 (structure I)

Stockage des éléments alcalins et devenir de la croûte océanique dans le manteau inférieur

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

The Anomalous Seismic Behavior of Aqueous Fluids Released during Dehydration of Chlorite in Subduction Zones

International audienceDehydration and fluid circulation are integral parts of subduction tectonics that govern the dynamics of the wedge mantle. The knowledge of the elastic behavior of aqueous fluid is crucial to understand the fluid–rock interactions in the mantle through velocity profiles. In this study, we investigated the elastic wave velocities of chlorite at high pressure beyond its dehydrating temperature, simulating the progressive dehydration of hydrous minerals in subduction zones. The dehydration resulted in an 8% increase in compressional (Vp) and a 5% decrease in shear wave (Vs) velocities at 950 K. The increase in Vp can be attributed to the stiffening of the sample due to the formation of secondary mineral phases followed by the dehydration of chlorite. The fluid-bearing samples exhibited Vp/Vs of 2.45 at 950 K. These seismic parameters are notably different from the major mantle minerals or hydrous silicate melts and provide unique seismic criteria for detecting mantle fluids through seismic tomography

Synthesis of boron carbide from its elements up to 13 GPa

International audienceThe synthesis of boron carbide from its elements (boron and carbon) has been studied under pressures up to 13 GPa and optimum parameters have been determined by varying the (P, T, reactants) conditions. Stoichiometric mixtures of amorphous boron and amorphous carbon have been subjected to a range of temperatures from 1673 K to 2273 K at the pressure values of 2 GPa and 5 GPa. The formation temperatures have been compared to those obtained from mixtures of β rhombohedral boron and graphite, and β rhombohedral boron and amorphous carbon at 2 GPa and 5 GPa. The formation temperature is thus shown to be affected by the pressure and the choice of the reactants. The carbon concentration of boron carbide is also shown to be affected by the pressure at which it is synthesised from elements, and we propose pressure as a means to control the carbon content. Temperature cycling has also been shown to reduce the formation temperature of boron carbide at 13 GPa. The formation of α rhombohedral boron as an intermediate phase is seen at 5 GPa before the formation of boron carbide

Thermoelastic properties of post-perovskite phase MgSiO3 determined experimentally at core-mantle boundary P-T conditions

International audienceWe have performed in-situ X-ray diffraction measurements on MgSiO3 up to 144.5 GPa and 2535 K to obtain a precise experimental determination of the elastic properties of the post-perovskite polymorph of MgSiO3 at high pressure and temperature. The complete dataset is comprised of 24 data points at ambient temperature and an equal number at high temperature. Fitted physical properties are the room temperature bulk modulus, the Grüneisen parameter with its volume dependency, the thermal expansion with its temperature dependency, and the linear incompressibilities as well as thermal expansions and their temperature dependency on each axis. Elastic anisotropy is observed, the structure being much more compressible along the b axis, and less compressible along the c axis, in agreement with theoretical calculations. We also demonstrate with the present study that experiments can provide precise information on the elasticity of complex materials at extreme pressure and temperature condition

Thermoelastic equation of state and melting of Mg metal at high pressure and high temperature

International audienceThe p-V-T equation of state of magnesium metal has been measured up to 20 GPa and 1500 K using both multianvil and opposite anvil techniques combined with synchrotron X-ray diffraction. To fit the experimental data, the model of Anderson-Grüneisen has been used with fixed parameter δ T. The 300-K bulk modulus of B 0 = 32.5(1) GPa and its first pressure derivative, B 0 ' = 3.73(2), have been obtained by fitting available data up to 20 GPa to Murnaghan equation of state. Thermal expansion at ambient pressure has been described using second order polynomial with coefficients a = 25(2)×10-6 K-1 and b = 9.4(4)×10-9 K-2. The parameter describing simultaneous pressure and temperature impact on thermal expansion coefficient (and, therefore, volume) is δ T = 1.5(5). The good agreement between fitted and experimental isobars has been achieved to relative volumes of 0.75. The Mg melting observed by X-ray diffraction and in situ electrical resistivity measurements confirms previous results and additionally confirms the p-T estimations in the vicinity of melting

Crystallography under high pressure using synchrotron radiation

International audienceThis review provides an overview of high pressure crystallography using synchrotron radiation. Such experiments concern most of the scientific domains (Physics, Chemistry, Biology, Earth Science, Material Science...). After a description of the most used high pressure apparatus and of the different techniques of diffraction, adapted to high pressure environment, we present few examples of scientific results, selected from these different scientific domains. The perspective opened by recent experimental developments is also discussed

Melting of Fe-Ni-Si and Fe-Ni-S alloys at megabar pressures: implications for the core-mantle boundary temperature

International audienceHigh pressure melting behavior of three Fe-alloys containing 5 wt% Ni and (1) 10 wt% Si, (2) 15 wt% Si or (3) 12 wt% S was investigated up to megabar pressures by in situ X-ray diffraction and laser-heated diamond anvil cell techniques. We observe a decrease in melting temperature with increasing Si content over the entire investigated pressure range. This trend is used to discuss the melting curve of pure Fe. Moreover, our measurements of eutectic melting in the Fe-Fe3S system show a change in slope around 50 GPa concomitant with the fcc- hcp phase transition in pure solid iron. Extrapolations of our melting curve up to the core-mantle boundary pressure yield values of 3,600-3,750 K for the freezing temperature of plausible outer core compositions

A New Reference for the Thermal Equation of State of Iron

International audienceThe high-pressure, high-temperature behavior of iron was investigated to 140 GPa and 3500 K with in situ synchrotron X-ray diffraction. Iron samples were compressed in diamond-anvil cells and heated up with the double-sided laser-heating system installed at the high-pressure ID27 of the European Synchrotron Radiation Facility (ESRF). Three different structures, namely α-bcc, γ-fcc or ε-hcp Fe were identified as a function of pressure and temperature in the domain we explored. At pressures above 90 GPa, it is clearly shown that ε-iron is the single stable solid phase up to 160 GPa at high temperatures. The analysis of the P-V-T relationship allows us to propose a reliable experimental thermal equation of state (EoS) for iron. We also show that the addition of low pressure points to our EoS refinement yields more robust constrain on the determination of the reference volume V 0 of the ε-hcp structure, which has important implications on the final parametrization of the equation of state. The extrapolation of the proposed EoS to core pressure conditions indicates that a pure iron core would have an excess of density of 3% compared to the PREM density profile

Following the phase transitions of iron in 3D with X-ray tomography and diffraction under extreme conditions

International audienc