Jinfu Shu

Effect of pressure, temperature, and composition on lattice parameters and density of (Fe,Mg)SiO3‐perovskites to 30 GPa

High-pressure, high-temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation-of-state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298-K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero-pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P - T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −6.3(±0.5)×10−2 GPa/K. Analyses of the high-temperature data give a value for the Anderson-Gruneisen parameter δT of 6.5–7.5, which is significantly higher than that used in recent lower mantle models.

Analysis of lattice strains measured under nonhydrostatic pressure

The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp e-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures.

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.

The plastic deformation of iron at pressures of the Earth's inner core

Soon after the discovery of seismic anisotropy in the Earths inner core, it was suggested that crystal alignment attained during deformation might be responsible. Since then, several other mechanisms have been proposed to account for the observed anisotropy, but the lack of deformation experiments performed at the extreme pressure conditions corresponding to the solid inner core has limited our ability to determine which deformation mechanism applies to this region of the Earth. Here we determine directly the elastic and plastic deformation mechanism of iron at pressures of the Earths core, from synchrotron X-ray diffraction measurements of iron, under imposed axial stress, in diamond-anvil cells. The ε-iron (hexagonally close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity theory predicts an alignment of c-axes parallel to the compression direction as a result of basal slip, if basal slip is either the primary or a secondary slip system. The experiments provide direct observations of deformation mechanisms that occur in the Earths inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.

Simultaneous high‐P, high‐T X ray diffraction study of β‐(Mg,Fe)2SiO4 to 26 GPa and 900 K

The lattice parameters of β phase [(Mg0.84Fe0.16)2SiO4] have been determined by X ray diffraction using synchrotron radiation under simultaneous high-pressure and high-temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao-Bell type diamond anvil cell using neon gas as pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high-pressure calibrant at high temperatures. Using 4.0 for (∂KT/∂P)T, we found that the 300-K isothermal bulk modulus, KT0, and its temperature derivative at constant pressure, (∂KT/∂T)P, are 174(±3) GPa and −2.7(±0.3)×10−2 GPa/K, respectively. The value of the Anderson-Gruneisen parameter δT of the β phase is 5.1(±0.8) above the Debye temperature. The experimental results provide direct measurements of the pressure effect on the thermal expansivity and the temperature dependence of bulk modulus for the β phase.

Pressure-Induced High-Spin to Low-Spin Transition in FeS Evidenced by X-Ray Emission Spectroscopy

We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe Kb emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphidelike phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure. [S0031-9007(99)08946-2]

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 ...

X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earths interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earths interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earths interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earths lower mantle.

The wüstite enigma

High-pressure energy dispersive X-ray diffraction of wustite has been obtained with three types of diamond cells; each one is designed to optimize a type of in situ study, namely single-crystal X-ray diffraction, deviatoric strain measurement, or simultaneous high-P-T experimentation. The results demonstrate that above 17 GPa at 300 K wustite undergoes a displacive transition from B1 to a rhombohedral structure, and above 90 GPa at 600 K, a second transition to the NiAs structure. Many of the earlier inconsistencies concerning the structure and properties of wustite at high pressures can be attributed to the extreme softening of the C44 elastic modulus at pressures above 10 GPa.

NATURAL OCCURRENCE AND SYNTHESIS OF TWO NEW POSTSPINEL POLYMORPHS OF CHROMITE

A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe2O4-type (CF) structure. The unit-cell parameters are a = 8.954(7) Å, b = 2.986(2) Å, c = 9.891(7) Å, V = 264.5(4) Å3 (Z = 4) with space group Pnma. The new phase has a density of 5.62 g/cm3, which is 9.4% denser than chromite-spinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites.