Compressibility and Electronic Structure of MgB2 up to 8 GPa

The lattice parameters of MgB2 up to pressures of 8 GPa were determined using high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk modulus, B0, was determined to be 151 +-5 GPa. Both experimental and first-principles calculations indicate nearly isotropic mechanical behavior under pressure. This small anisotropy is in contrast to the 2 dimensional nature of the boron pi states. The pressure dependence of the density of states at the Fermi level and a reasonable value for the average phonon frequency account within the context of BCS theory for the reduction of Tc under pressure.Comment: REVTeX file. 4 pages, 4 figure

Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study

A recent experiment by Shimizu et al. has provided evidence of a superconducting phase in hcp Fe under pressure. To study the pressure-dependence of this superconducting phase we have calculated the phonon frequencies and the electron-phonon coupling in hcp Fe as a function of the lattice parameter, using the linear response (LR) scheme and the full potential linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the Eliashberg functions $\alpha^2 F$ indicate that conventional s-wave electron-phonon coupling can definitely account for the appearance of the superconducting phase in hcp Fe. However, the observed change in the transition temperature with increasing pressure is far too rapid compared with the calculated results. For comparison with the linear response results, we have computed the electron-phonon coupling also by using the rigid muffin-tin (RMT) approximation. From both the LR and the RMT results it appears that electron-phonon interaction alone cannot explain the small range of volume over which superconductivity is observed. It is shown that ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from magnetic impurities (spin-ordered clusters) can account for the observed values of the transition temperatures but cannot substantially improve the agreeemnt between the calculated and observed presure/volume range of the superconducting phase. A simplified treatment of p-wave pairing leads to extremely small ($\leq 10^{-2}$ K) transition temperatures. Thus our calculations seem to rule out both $s$- and $p$- wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR

Structure and Dynamics of Liquid Iron under Earth's Core Conditions

First-principles molecular dynamics simulations based on density-functional theory and the projector augmented wave (PAW) technique have been used to study the structural and dynamical properties of liquid iron under Earth's core conditions. As evidence for the accuracy of the techniques, we present PAW results for a range of solid-state properties of low- and high-pressure iron, and compare them with experimental values and the results of other first-principles calculations. In the liquid-state simulations, we address particular effort to the study of finite-size effects, Brillouin-zone sampling and other sources of technical error. Results for the radial distribution function, the diffusion coefficient and the shear viscosity are presented for a wide range of thermodynamic states relevant to the Earth's core. Throughout this range, liquid iron is a close-packed simple liquid with a diffusion coefficient and viscosity similar to those of typical simple liquids under ambient conditions.Comment: 13 pages, 8 figure

Lattice modes of solid nitrogen to 104 GPa

Lattice modes of solid nitrogen were studied by Raman spectroscopy at room temperature to 104 GPa using the diamond anvil technique. Changes in the lattice mode spectral features correlate with those observed in the vibronic spectra suggesting symmetry changes of the crystal lattice. The changes in the spectral features mainly appear as branchings of existing modes supporting the view of a close structural relationship among these high pressure phases

Potential of Earth’s core as a reservoir for noble gases: Case for helium and neon

Co-auteur étrangerInternational audiencedoi: 10.7185/geochemlet.2028 lower mantle upper mantle OIB MORB outer core Low 3 He/ 22 Ne High 3 He/ 4 He Co re C on tr ib ut io n This study investigates metal-silicate partitioning of neon (D Ne) under the likely conditions of early Earth's core formation: up to 16 GPa, ∼ 3000 K and an oxygen fugacity near IW-2 (2 log units below the Iron-Wüstite buffer). We find that the D Ne coefficients range between 10 −2 and 10 −1. These partition coefficients are only one of the controlling factors of noble gas distributions within the early Earth: because, even if D He and D Ne are low (∼10 −4), there may have been sufficient noble gases present in the mantle to supply a significant quantity of He and Ne to the core. Assuming gas-melt equilibrium of the molten proto-Earth with a nebular gas composition and con-comitant metal-silicate differentiation, the core would have inherited and maintained throughout Earth's history high 3 He/ 4 He ratios and low 3 He/ 22 Ne ratios (<0.6), making the core a potential source of primordial light noble gases in mantle plumes

Metal-silicate partitioning of iodine at high pressures and temperatures: Implications for the Earth's core and 129*Xe budgets

International audienceThe partition coefficients of iodine Dmet/sil between molten metal and molten silicate were investigated using a Laser Heated Diamond Anvil Cell (LHDAC) at pressures between 2 and 20 GPa and at ~2800 K. No pressure dependence of Dmet/sil was observed within this range, but the composition of the Fe-Ni alloy liquid phase was shown to have an effect. When the metallic liquid was alloyed with S, O and Si, there was an increase in iodine solubility in the metal. Iodine exhibited mildly siderophile behaviour across all the investigated conditions, with Dmet/sil=1.25±0.65 (2 s.d.) (Fe metal system) and Dmet/sil=4.33±1.41 (2 s.d.) (Fe-alloy). In conjunction with a revised bulk silicate Earth (BSE) concentration, it is calculated that the core could be a significant reservoir for iodine, with up to 82% of the bulk Earth's iodine budget in the core, depending on the light element content of the metal phase and the process of core formation. The composition of the metal phase appears to have a greater effect on the partitioning and sequestration of iodine than the style of core segregation. As the core likely formed while 129I was still extant, the core can also be a reservoir for radiogenic 129Xe from the decay system 129I-129Xe (T1/2=15.7 Myr). Preliminary modelling indicates that the decay of 129I in the core has the potential to generate radiogenic 129Xe concentrations that are at least two orders of magnitude greater than what has been estimated for the depleted mantle. While this may have a significant impact on the isotopic signatures of the overlying mantle, it is not yet clear how flux from the core fits within the overall picture of mantle noble gas evolutio

Equation of state, elasticity, and shear strength of pyrite under high pressure

Physical properties including the equation of state, elasticity, and shear strength of pyrite have been measured by a series of X-ray diffraction in diamond-anvil cells at pressures up to 50 GPa. A Birch-Murnaghan equation of state fit to the quasi hydrostatic pressure-volume data obtained from laboratory X-ray source/film techniques yields a quasihydrostatic bulk modulus K-0T = 133.5 (+/-5.2) GPa and bulk modulus first pressure derivative K-0T' = 5.73 (+/-0.58). The apparent equation of state is found to be strongly dependent on the stress conditions in the sample. The stress dependency of the high-pressure properties is examined with anisotropic elasticity theory from subsequent measurements of energy-dispersive radial diffraction experiments in the diamond-anvil cell. The calculated values of K-0T depend largely upon the angle between the diffracting plane normal and the maximum stress axis. The uniaxial stress component in the sample, t = sigma(3) - sigma(1), varies with pressure as t = -3.11 + 0.43P between 10 and 30 GPa. The pressure derivatives of the elastic moduli dC(11)/dP = 5.76 (+/-0.15), dC(12)/dP = 1.41 (+/-0.11) and dC(44)/dP = 1.92 (+/-0.06) are obtained from the diffraction data assuming previously reported zero-pressure ultrasonic data (C-11 = 382 GPa, C-12 = 31 GPa, and C-44 = 109 GPa)

Nitrogen solubility in molten metal and silicate at high pressure and temperature

International audienceNitrogen is the dominant gas in Earth atmosphere, but its behavior during the Earth' differentiation is poorly known. To aid in identifying the main reservoirs of nitrogen in the Earth, nitrogen solubility was determined experimentally in a mixture of molten CI-chondrite model composition and (Fe, Ni) metal alloy liquid. Experiments were performed in a laser-heated diamond-anvil cell at pressures to 18 GPa and temperatures to 2850 ± 200 K. Multi-anvil experiments were also performed at 5 and 10 GPa and 2390 ± 50 K. The nitrogen content increases with pressure in both metal and silicate reservoirs. It also increases with the iron content of the (Fe, Ni) alloy. Sieverts' formalism successfully describes the nitrogen solubility in metals up to 18 GPa. Henry's law applies to nitrogen-saturated silicate melts up to 4-5 GPa. Independently of these solubility models, it is shown that the partition coefficient of nitrogen between metal and silicate melts changes from almost 104 at ambient pressure to about 10-20 for pressures higher than 1 GPa. The pressure-dependence of the nitrogen partitioning can explain the recently suggested depletion of nitrogen relative to other volatiles in the bulk silicate Earth

Equation of state of solid neon from x-ray diffraction measurements to 110 GPa

This paper briefly discusses the pressure-volume properties of condensed neon. X-ray diffraction techniques are used to determine solid neon equation of state and crystal structure. 16 refs., 2 figs. (LSP