Guoyin Shen

Toward an internally consistent pressure scale.

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earths interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earths interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure–temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earths interior.

Melting and crystal structure of iron at high pressures and temperatures

High-pressure melting, phase transitions and structures of iron have been studied to 84 GPa and 3500 K with an improved laser heated diamond anvil cell technique and in situ high P-T x-ray diffraction. At pressures below 60 GPa, the lower bound on the melting curve is close to those measured by Boehler [1993] and Saxena et al. [1993]; however, at pressures above 60 GPa our data indicate melting at higher temperatures than these studies, but still lower than the melting curve of Williams et al [1990]. The e-γ-1 triple point is 60(±5) GPa and 2800(±200) K, based on our data of the e-γ phase transition and the observation of melting by in situ x-ray diffraction. No solid phases other than e-Fe and γ-Fe were observed in situ at high temperatures (>1000 K) and pressures to 84 GPa. However, the diffraction patterns of temperature quenched products at high pressure can be fit to other structures such as dhcp.

Elasticity and rheology of iron above 220 GPa and the nature of the Earth's inner core

Recent numerical-modelling and seismological results have raised new questions about the dynamics, and magnetism, of the Earths core. Knowledge of the elasticity and texture of iron, at core pressures is crucial for understanding the seismological observations, such as the low attenuation of seismic waves, thelow shear-wave velocity, and the anisotropy of compressional-wave velocity. The density and bulk modulus of hexagonal-close-packed iron have been previously measured to core pressures by static and dynamic, methods. Here we study,using radial X-ray diffraction and ultrasonic techniques, the shear modulus, single-crystal elasticity tensor, aggregate compressional- and shear-wave velocities, and orientation dependence of these velocities in iron. The inner core shear-wave velocity is lower than the aggregate shear-wave velocity of iron, suggesting the presence of low-velocity components or anelastic effects in the core. Observation of a strong lattice strain anisotropy in iron samples indicates a large (∼24%) compressional-wave anisotropy under the isostress assumption, and therefore a perfect alignment of crystals would not be needed to explain the seismic observations. Alternatively the strain anisotropy may indicate stress variation due to preferred slip systems.

Laser heated diamond cell system at the Advanced Photon Source for in situ x-ray measurements at high pressure and temperature

We describe a laser heated diamond anvil cell system at the GeoSoilEnviroCARS sector at the Advanced Photon Source. The system can be used for in situ x-ray measurements at simultaneously ultrahigh pressures (to >150 GPa) and ultrahigh temperatures (to >4000 K). Design goals of the laser heating system include generation of a large heating volume compared to the x-ray beam size, minimization of the sample temperature gradients both radially and axially in the diamond anvil cell, and maximization of heating stability. The system is based on double-sided laser heating technique and consists of two Nd:YLF lasers with one operating in TEM00 mode and the other in TEM01* mode, optics to heat the sample from both sides, and two spectroradiometric systems for temperature measurements on both sides. When combined with an x-ray microbeam (3–10 μm) technique, a temperature variation of less than 50 K can be achieved within an x-ray sampled region for longer than 10 min. The system has been used to obtain in situ str...

The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity

The 660-km seismic discontinuity in the Earths mantle has long been identified with the transformation of (Mg,Fe)2SiO4 from γ-spinel (ringwoodite) to (Mg,Fe)SiO3-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures. But the first in situ examination of this phase transformation in Mg2SiO4 using a multi-anvil press indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to ∼600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg2SiO4 at pressures of 20–36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from γ-Mg2SiO4 to MgSiO3-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study, we find that the pressure and temperature of the post-spinel transformation in Mg2SiO4 is consistent with seismic observations for the 660-km discontinuity.

Superhard B–C–N materials synthesized in nanostructured bulks

We report here the high-pressure synthesis of well-sintered millimeter-sized bulks of superhard BC 2 N and BC 4 N materials in the form of a nanocrystalline composite with diamond-like amorphous carbon grain boundaries. The nanostructured superhard B–C–N material bulks were synthesized under high P–T conditions from amorphous phases of the ball-milled molar mixtures. The synthetic B–C–N samples were characterized by synchrotron x-ray diffraction, high-resolution transmission electron microscope, electron energy-loss spectra, and indentation hardness measurements. These new high-pressure phases of B–C–N compound have extreme hardnesses, second only to diamond. Comparative studies of the high P – T synthetic products of BC 2 N, BC 4 N, and segregated phases of diamond + c BN composite confirm the existence of the single B–C–N ternary phases.

Synchrotron X-ray Study of Iron at High Pressure and Temperature

X-ray synchrotron experiments with in situ laser heating of iron in a diamond-anvil cell show that the high-pressure ε phase, a hexagonal close-packed (hcp) structure, transforms to another phase (possibly a polytype double-layer hcp) at a pressure of about 38 gigapascals and at temperatures between 1200 and 1500 kelvin. This information has implications for the phase relations of iron in Earths core.

The stability and P–V–T equation of state of CaSiO3 perovskite in the Earth's lower mantle

Energy dispersive X-ray diffraction measurements for polycrystalline CaSiO3 perovskite were carried out at in situ transition zone and lower mantle P-T conditions (P = 18–96 GPa, T= 1238–2419 K) using the diamond anvil cell and double-sided laser heating at the GeoSoilEnviro Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source. An analysis of the temperature error sources in laser heating reveals that the axial and radial thermal gradients are the greatest error source. We have used measurements where the combined temperature error (1σ) from all sources is <150 K. By obtaining X-ray diffraction patterns at 8–22 GPa and 300–2200 K range, the high-temperature phase boundary between CaSi2O5+Ca2SiO4 and CaSiO3 perovskite was determined to be 14–16 GPa, in contrast to the results of previous large-volume press (LVP) measurements (9–11 GPa). The stability of cubic CaSiO3 perovskite was confirmed to 2300 km depth in the Earths interior. No evidence of phase transformation or break down to oxides was observed. The proposed tetragonal distortion, and hence the phase transformation from distorted phase to cubic, was not observed. The combined data set of this study and earlier LVP measurements was fit to a Birch-Murnaghan-Debye equation. By fixing V0 = 27.45 cm3/mol, KT0 = 236 GPa, and K′T0 = 3.9 from recent static compression data and θ0 = 1000 K, we obtain γ0 = 1.92±0.5 and q = 0.6±0.3. Although data to 69 GPa and 2380 K were used in the fitting, this result is also consistent with measurements to 96 GPa. This result yields not only density and bulk modulus but also higher-order thermoelastic parameters, such as thermal expansivity and temperature dependence of bulk modulus, at lower mantle P-T condition in an internally consistent way. This direct measurement of the equation of state at lower mantle condition verifies that the density and bulk modulus of CaSiO3 perovskite at lower mantle P-T conditions are very close to seismic values (within 1.5 and 3.0%, respectively). These differences are sufficiently small that the abundance of CaSiO3 perovskite will have negligible effects on density and bulk modulus profiles for the mantle.

Elasticity, shear strength, and equation of state of molybdenum and gold from x-ray diffraction under nonhydrostatic compression to 24 GPa

Lattice strains were measured as a function of the angle ψ between the diffracting plane normal and the stress axis of a diamond anvil cell in a layered sample of molybdenum and gold. The sample was compressed over the range 5–24 GPa and the lattice strains were measured using energy-dispersive x-ray diffraction. As ψ is varied from 0° to 90°, the mean lattice parameter of molybdenum increases by up to 1.2% and that of gold increases by up to 0.7%. A linear relationship between Q(hkl), which is related to the slope of the measured d spacing versus 1−3 cos2 ψ relation, and 3Γ(hkl), a function of the Miller indices of the diffracting plane, is observed for both materials as predicted by theory. The pressure dependence of the uniaxial stress t for gold from this and other recent studies is given by t=0.06+0.015P, where P is the pressure in GPa. The uniaxial stress in molybdenum can be described by t=0.46+0.13P. Using gold as an internal pressure standard, the equation of state of molybdenum depends strongly ...

Pressure-induced transformations of cristobalite

X-ray in situ studies in electrically and laser-heated diamond anvil cells (DACs) at pressures over 80 cpa and temperatures above 2500 K were used to determine stable silica phase at extreme conditions. We demonstrate that so far unidentified phases obtai