Jiuhua Chen

Pressure-induced slip-system transition in forsterite: Single-crystal rheological properties at mantle pressure and temperature

Abstract Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373.1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (ε) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the “puzzling” seismic-anisotropy anomalies commonly observed in subduction zones

X-ray strain analysis at high pressure: Effect of plastic deformation in MgO

3^011& at different critical resolved shear stress ratios for the different slip systems. The prediction of the models is correlated with the results of x-ray diffraction measurements. Uniaxial deformation experiments on polycrystalline and single-crystal MgO samples were conducted in situ using white x-ray diffraction with a multielement detector and multianvil high-pressure apparatus at a pressure up to 6 GPa and a temperature of 500 °C. A deformation DIA was used to generate pressure and control at a constant deformation rate. Elastic strains and plastic strains were monitored using x-ray diffraction spectra and x-ray imaging techniques, respectively. The correlation of the data and models suggests that the plastic models need to be used to describe the stress‐strain observations with the presence of plasticity, while the Reuss and Voigt models are appropriate for the elastic region of deformation, before the onset of plastic deformation. The similarity of elastic strains among different lattice planes suggests that the

Effect of plasticity on elastic modulus measurements

111% slip system is the most significant slip system in MgO at high pressure and high temperature.

Raman spectroscopy study of ammonia borane at high pressure

[1] Recent developments in high-pressure X-ray studies enable the measurement of the elastic strain anisotropy induced by a macroscopic differential stress in an aggregate sample. Such data are commonly used to constrain the single-crystal elastic moduli under the assumptions of the Reuss-Voigt state. We find this procedure is valid only for samples that have not been plastically deformed. Measured elastic strain anisotropy for MgO at stress levels below the yield point agree well with the Reuss elastic model. Plastic deformation effects a change in the stress field for subpopulations of grains that represent different crystallographic orientations with respect to the applied stress field. Our data for the plastically deformed sample are consistent with the predictions of a self-consistent aggregate model for deforming polycrystals. Such models may be useful as a guide to define the elastic properties in light of the active slip systems. INDEX TERMS: 3902 Mineral Physics: Creep and deformation; 3909 Mineral Physics: Elasticity and anelasticity; 3924 Mineral Physics: High-pressure behavior; 3954 Mineral Physics: X ray, neutron, and electron spectroscopy and diffraction; 3994 Mineral Physics: Instruments and techniques. Citation: Weidner, D. J., L. Li, M. Davis, and J. Chen (2004), Effect of plasticity on elastic modulus measurements, Geophys. Res. Lett., 31, L06621, doi:10.1029/ 2003GL019090.

The strength of Mg0.9Fe0.1SiO3 perovskite at high pressure and temperature

Ammonia borane, NH(3)BH(3), has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH(3)BH(3) was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH(3) proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH(3) proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH(3) and NH(3) bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in d nu/d P which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials.

Rietveld structure refinement of perovskite and post-perovskite phases of NaMgF3 (Neighborite) at high pressures

The Earths lower mantle consists mainly of (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowüstite, with the perovskite taking up at least 70 per cent of the total volume. Although the rheology of olivine, the dominant upper-mantle mineral, has been extensively studied, knowledge about the rheological behaviour of perovskite is limited. Seismological studies indicate that slabs of subducting oceanic lithosphere are often deflected horizontally at the perovskite-forming depth, and changes in the Earths shape and gravity field during glacial rebound indicate that viscosity increases in the lower part of the mantle. The rheological properties of the perovskite may be important in governing these phenomena. But (Mg,Fe)SiO3 perovskite is not stable at high temperatures under ambient pressure, and therefore mechanical tests on (Mg,Fe)SiO3 perovskite are difficult. Most rheological studies of perovskite have been performed on analogous materials, and the experimental data on (Mg,Fe)SiO3 perovskite are limited to strength measurements at room temperature in a diamond-anvil cell and microhardness tests at ambient conditions. Here we report results of strength and stress relaxation measurements of (Mg0.9Fe0.1)SiO3 perovskite at high pressure and temperature. Compared with the transition-zone mineral ringwoodite at the same pressure and temperature, we found that perovskite is weaker at room temperature, which is consistent with a previous diamond-anvil-cell experiment, but that perovskite is stronger than ringwoodite at high temperature.

A large-volume press facility at the Advanced Photon Source: diffraction and imaging studies on materials relevant to the cores of planetary bodies

Abstract Neighborite (NaMgF3) with the perovskite structure, transforms to a post-perovskite (ppv) phase between 27 and 30 GPa. The ppv phase is observed to the highest pressures achieved (56 GPa) at room temperature and transforms to an as yet unknown phase upon heating. Rietveld structure refinement using monochromatic synchrotron X-ray diffraction data provide models for the perovskite and postperovskite structures at high pressure. The refined models at 27(1) GPa indicate some inter-octahedral F-F distances rival the average intra-octahedral distance, which may cause instability in the perovskite structure and drive the transformation to the post-perovskite phase. The ratio of A-site to B-site volume (VA/VB) in perovskite structured NaMgF3 (ABX3), spans from 5 in the zero-pressure high-temperature cubic perovskite phase to 4 in this high-pressure perovskite phase at 27(1) GPa, matching the VA/VB value in post-perovskite NaMgF3. Using Rietveld refinement on post-perovskite structure models, we observe discrepancies in pattern fitting, which may be described in terms of development of sample texture in the diamond-anvil cell, recrystallization, or a change of space group to Cmc21, a non-isomorphic subgroup of Cmcm-the space group describing the structure of CaIrO3.

Erratum: Ionic high-pressure form of elemental boron

A new large-volume, high-pressure facility is being utilized and developed as part of GeoSoilEnviroCARS at a third-generation synchrotron, the Advanced Photon Source. This user facility consists of two large-volume presses (LVP), a 2.5 MN (250 ton) LVP installed at the bending magnet beamline, and a 10 MN (1000 ton) LVP at the insertion device beamline. Here we report some techniques currently being developed with the 10 MN LVP and the latest scientific results obtained using the 2.5 MN LVP.

Studies of Local and Intermediate Range Structure in Crystalline and Amorphous Materials at High Pressure Using High-Energy X-rays

This corrects the article DOI: 10.1038/nature07736

Flow-law for ringwoodite at subduction zone conditions

The method of high-energy total elastic X-ray scattering to determine the atomic structure of nanocrystalline, highly disordered, and amorphous materials is presented. The current state of the technique, its potential, and limitations are discussed with two successful studies on the pressure induced phase transition in mackinawite (FeS) and the high-pressure behavior of liquid gallium.