On the High-Pressure Phase Transition in GaPO4

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a=7.306 A, b=5.887 A and c=5.124 A.Comment: 7 pages, 2 figures, 1 tabl

XAS Study of the High Pressure Behaviour of Quartzlike Compounds

EXAFS spectroscopy experiments have been carried out on quartz-like GaAsO4 and AlAsO4 at high pressure and room temperature. It has been shown that these materials exhibit two structural phase transitions; the first transition to a high pressure crystalline form occurs at 9 GPa and is reversible upon decompression, whereas the second transition occurs at higher pressures and is irreversible. In GaAsO4, EXAFS measurements agree with the predicted transition from four- to six-fold coordination of oxygen atoms around the cations, but the two local coordination transformations are not dissociated; in fact, both As and Ga atoms exhibit a coordination change at the onset of the first phase transition, the rate of transformation being significantly higher for Ga atoms. In both cases, the average bond length increases very rapidly with pressure thus yielding the first compression stage after the transition. In the second stage, the average bond lengths increase slowly, ultimately reaching six-fold coordination above 28 GPa and 24 GPa for As and Ga respectively. The behaviour of the As K-edge EXAFS is the same for both compounds, and enables us to link the behaviour of Ga and Al atoms. The local transformations are well described and a direct link with phosphate berlinites seems timely.Comment: 5 pages, 3 figures, LaTeX2e, J. de Physique (in press

A seismologically consistent compositional model of Earth's core

International audienceEarth's core is less dense than iron, and therefore it must contain " light elements, " such as S, Si, O, or C. We use ab initio molecular dynamics to calculate the density and bulk sound velocity in liquid metal alloys at the pressure and temperature conditions of Earth's outer core. We compare the velocity and density for any composition in the (Fe–Ni, C, O, Si, S) system to radial seismological models and find a range of compositional models that fit the seismo-logical data. We find no oxygen-free composition that fits the seismological data, and therefore our results indicate that oxygen is always required in the outer core. An oxygen-rich core is a strong indication of high-pressure and high-temperature conditions of core differentiation in a deep magma ocean with an FeO concentration (oxygen fugacity) higher than that of the present-day mantle. mineral physics | first principles | geophysic

Metal-ligand interplay in strongly-correlated oxides: a parametrized phase diagram for pressure induced spin transitions

We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO and NiO under very high pressure by x-ray emission spectroscopy at the K\beta line. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The pressure dependence of the emission line is well accounted for by changes of the ligand field acting on the d electrons and allows us to extract parameters like local d-hybridization strength, O-2p bandwidth and ionic crystal field across the magnetic transition. This approach allows a first-hand insight into the mechanism of the pressure induced spin transition.Comment: 5 pages, 3 figure

Experimental Evidence for a High-Pressure Isostructural Phase Transition in Osmium

International audienceWe have measured the isothermal equation of state (EOS) of osmium to 75 GPa under hydrostatic conditions at room temperature using angle-dispersive x-ray diffraction. A least-squares fit of this data using a third-order Birch-Murnaghan EOS yields an isothermal bulk modulus K 0 411 6 GPa, showing osmium is more compressible than diamond. Most importantly, we have documented an anomaly in the compressibility around 25 GPa associated with a discontinuity in the first pressure derivative of the c=a ratio. This discontinuity plausibly arises from the collapse of the small hole-ellipsoid in the Fermi surface near the L point

Seconds after impact: Insights into the thermal history of impact ejecta from diffusion between lechatelierite and host glass in tektites and experiments

Tektites contain inclusions of lechatelierite, nearly pure SiO_2 glass formed by quenching of quartz grains melted during hypervelocity impacts. We report the discovery in a tektite of chemically zoned boundary layers (ca 20 μm) between lechatelierite and host felsic glass. These boundary layers in tektites formed by chemical diffusion between molten silicainclusions (quenched to lechatelierite on cooling) and surrounding felsic melt. We reproduced the details of these boundary layers via experiments on mixtures of powdered natural tektite plus quartz grains heated to 1800–2400 °C for 1–120 s using an aerodynamic levitation laser heating furnace. The results of these experiments were used to provide quantitative constraints on possible thermal histories of the natural sample. The experiments successfully reproduced all major aspects of the concentration profiles from the natural sample including diffusion length scale, strong asymmetry of the concentration profiles with respect to the Matano plane (due to the strong concentration dependence of the diffusivities of all oxides on SiO_2 content), similarities in lengths of the diffusive profiles (due to control by the diffusion of SiO_2 on the diffusivity of the other oxides), and differences in the shapes of the profiles among the oxides (including a maximum in the diffusion profile of K_2O due to uphill diffusion). The characteristic lengths of all non-alkali oxide profiles are proportional to t from which diffusivities and activation energies can be derived; these results are consistent with measurements in melts with lower SiO_2 contents and at lower temperatures reported in the literature. We also fit the experimental profiles of SiO_2 and Al_2O_3 using simple formulations of the dependence of their diffusivities on SiO_2 content and temperature, yielding results similar to those obtained from the t dependence of the characteristic profile lengths. The quantitative characterization of diffusion in boundary layers based on our experiments allow us to set limits on the thermal history of the natural tektite in which the boundary layers were discovered. If the interdiffusion between the silica and felsic melts occurred at constant temperature, the duration of heating experienced by the natural tektite we studied depends on temperature; possible solutions include heating at ∼2000 °C for ∼70 s, −2400 °C for ∼3 s. We also explored non-isothermal, asymptotic cooling histories; for a maximum temperature of 2400 °C, a characteristic cooling time scale of ∼50 s is implied, whereas, for 2000 °C, the time scale is ∼1400 s. Further, a maximum temperature of ∼2360 °C yields an effective diffusive time scale of ∼5 s, a cooling time scale of ∼90 s, and a cooling rate at the glass transition temperature of ∼5 °C/s; results that are consistent with independent estimates of cooling time scales for ∼1 cm clasts (Xu and Zhang, 2002), as well as cooling rates at the glass transition temperature (Wilding et al., 1996) – thus satisfying all currently available relevant data. More complex T-t paths are possible and can also be modeled using our experimental results and compared with and used as tests of the accuracy of physical models of tektite-forming impact events

Composition of the Earth's inner core from high-pressure sound velocity measurements in Fe-Ni-Si alloys

Editor: R.D. van der Hilst Keywords: Fe-Ni-Si alloy aggregate compressional and shear sound velocities high pressure inner core light elements We performed room-temperature sound velocity and density measurements on a polycrystalline alloy, Fe 0.89 Ni 0.04 Si 0.07 , in the hexagonal close-packed (hcp) phase up to 108 GPa. Over the investigated pressure range the aggregate compressional sound velocity is ∼ 9% higher than in pure iron at the same density. The measured aggregate compressional (V P ) and shear (V S ) sound velocities, extrapolated to core densities and corrected for anharmonic temperature effects, are compared with seismic profiles. Our results provide constraints on the silicon abundance in the core, suggesting a model that simultaneously matches the primary seismic observables, density, P-wave and S-wave velocities, for an inner core containing 4 to 5 wt.% of Ni and 1 to 2 wt.% of Si

Composition of the low seismic velocity E ′ layer at the top of Earth's core

Using ab initio simulations on Fe-Ni-S-C-O-Si liquids, we constrain the origin and composition of the low-velocity layer E′ at the top of Earth's outer core. We find that increasing the concentration of any light element always increases velocity and so a low-velocity and low-density layer (for stability) cannot be made by simply increasing light element concentration. This rules out barodiffusion or simple sedimentation of a light phase for its origin. However, exchanging elements can—depending on the elements exchanged—produce such a layer. We evaluate three possibilities. First, crystalization of a phase from a core may make such a layer, but only if the core contains more than one light element and only if crystalizing phase is very Fe rich. Second, the E′ layer may result from incomplete mixing of an early Earth core with a late impactor, depending on the light element compositions of the impactor and Earth's core. Third, using thermodynamic models for metal-silicate partitioning, we show that a reaction between the core and an FeO-rich basal magma ocean can result in a light and slow layer

Strength, Anisotropy, and preferred orientation of solid13; argon at high pressures

The elasticity and plasticity of materials at high pressure are of great importance for the fundamental insight they provide on bonding properties in dense matter and for applications ranging from geophysics to materials technology. We studied pressure-solidified argon with a boronx2013;epoxyx2013;beryllium composite gasket in a diamond anvil cell (DAC). Employing monochromatic synchrotron13; x-radiation and imaging plates in a radial diffraction geometry (Singh et al 1998 Phys. Rev. Lett. 80 2157; Mao et al 1998 Nature 396 741), we observed low strength in solid argon below 20 GPa, but the strength increases drastically with applied pressure, such that at 55 GPa, the shear strength exceeded 2.7 GPa. The elastic anisotropy at 55 GPa was four times higher than the extrapolated value from 30 GPa. Extensive (111) slip develops under uniaxial compression, as manifested by the preferred crystallographic orientation of (220) in the compression direction. These macroscopic properties reflect basic changes in van der Waals bondings under ultra high pressures