Showing papers by "Wendy L. Mao published in 2013"
TL;DR: In this article, structural and electronic spin transitions in high-pressure e -FeOOH were studied using a combination of high pressure X-ray emission spectroscopy (XES), XRD, and density functional theory (DFT) calculations.
60 citations
TL;DR: In this article, a 3D coherent x-ray diffraction imaging of a molten Fe-rich alloy and crystalline olivine sample, synthesized at 6 GPa and 1800 degrees C, with nanoscale resolution is presented.
Abstract: We report quantitative 3D coherent x-ray diffraction imaging of a molten Fe-rich alloy and crystalline olivine sample, synthesized at 6 GPa and 1800 degrees C, with nanoscale resolution. The 3D mass density map is determined and the 3D distribution of the Fe-rich and Fe-S phases in the olivine-Fe-S sample is observed. Our results indicate that the Fe-rich melt exhibits varied 3D shapes and sizes in the olivine matrix. This work has potential for not only improving our understanding of the complex interactions between Fe-rich core-forming melts and mantle silicate phases but also paves the way for quantitative 3D imaging of materials at nanoscale resolution under extreme pressures and temperatures.
54 citations
TL;DR: In this article, three-dimensional imaging of samples experimentally subjected to high pressures reveals that liquid iron forms interconnected melt networks at lower mantle conditions, suggesting pathways through which iron can percolate towards the core.
Abstract: The differentiation of the Earth into mantle and core implies that there is a mechanism to separate iron from silicates. Three-dimensional imaging of samples experimentally subjected to high pressures reveals that liquid iron forms interconnected melt networks at lower mantle conditions, suggesting pathways through which iron can percolate towards the core.
53 citations
TL;DR: The observed seismic anisotropy in the Earth’s inner core has been explained by the preferential alignment of grains by plastic deformation as discussed by the authors, which has been shown to be a mechanism for dislocation creep.
Abstract: The observed seismic anisotropy in the Earth’s inner core has been explained by the preferential alignment of grains by plastic deformation. Measurements of the strength of iron at core pressures suggest that the inner core is weaker than previously thought and deforms by dislocation creep.
47 citations
TL;DR: In this article, Li 15 Si 4 was synthesized in a Li-ion battery by electrochemically lithiating a thin Si wafer and its stability and compressional behavior were investigated using in situ high-pressure synchrotron X-ray diffraction (XRD) at room temperature.
37 citations
TL;DR: The in situ single-crystal structure determination of (Mg,Fe)SiO3 postperovskite (ppv) at high P and after temperature quenching in a diamond anvil cell shows the feasibility of the in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device.
Abstract: Knowledge of the structural properties of mantle phases is critical for understanding the enigmatic seismic features observed in the Earth’s lower mantle down to the core–mantle boundary. However, our knowledge of lower mantle phase equilibria at high pressure (P) and temperature (T) conditions has been based on limited information provided by powder X-ray diffraction technique and theoretical calculations. Here, we report the in situ single-crystal structure determination of (Mg,Fe)SiO3 postperovskite (ppv) at high P and after temperature quenching in a diamond anvil cell. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, crystallographic orientations of over 100 crystallites were simultaneously determined at high P in a coarse-grained polycrystalline sample containing submicron ppv grains. Conventional single-crystal structural analysis and refinement methods were applied for a few selected ppv crystallites, which demonstrate the feasibility of the in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device. The similarity of structural models for single-crystal Fe-bearing ppv (∼10 mol% Fe) and Fe-free ppv from previous theoretical calculations suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase.
30 citations
TL;DR: An “Absorption Correlation Tomography” (ACT) method based on the correlation of the material absorption across multiple edges is presented, which overcomes the significant limitation caused by overlapping absorption edges, significantly expands the capabilities of TXM, and makes it possible for fully quantitative nano‐scale 3D structural investigation with chemical/elemental sensitivity.
Abstract: Transmission X-ray microscopy (TXM) is a rapidly developing technique with the capability of nanoscale three dimensional (3D) real-space imaging. Combined with the wide range in energy tunability from synchrotron sources, TXM enables the retrieval of 3D microstructural information with elemental/chemical sensitivity that would otherwise be inaccessible. The differential absorption contrast above and below absorption edges has been used to reconstruct the distributions of different elements, assuming the absorption edges of the interested elements are fairly well separated. Here we present an "Absorption Correlation Tomography" (ACT) method based on the correlation of the material absorption across multiple edges. ACT overcomes the significant limitation caused by overlapping absorption edges, significantly expands the capabilities of TXM, and makes it possible for fully quantitative nano-scale 3D structural investigation with chemical/elemental sensitivity. The capability and robustness of this new methodology is demonstrated in a case study of an important type of rare earth magnet (Nd₂Fe₁₄B).
26 citations
TL;DR: In this paper, the crystal chemistry and physical properties of Rhodochrosite at high pressure were studied and shown to exhibit a series of high-pressure transitions between 15 and 30 GPa and at 50 GPa at ambient temperature.
Abstract: Atmospheric carbon is critical for maintaining the climate and life equilibrium on Earth. The concentration of this carbon is controlled by the deep carbon cycle, which is responsible for the billion year-scale evolution of the terrestrial carbon reservoirs of the planet. Understanding the crystal chemistry and physical properties of carbonates at mantle conditions is vital as they represent the main oxidized carbon-bearing phases in the Earth’s mantle. Here we present data on the crystal chemistry and physical properties of rhodochrosite at high pressure. Rhodochrosite (MnCO 3 ) exhibits a series of high-pressure transitions between 15 and 30 GPa and at 50 GPa at ambient temperature as observed by in situ Raman spectroscopy, X-ray diffraction (XRD), and X-ray emission spectroscopy (XES). A transition is observed to begin at 15 GPa and complete at 30 GPa, which may be due to several possibilities: modifications in the magnetic order, changes in the compression mechanism, and/or a structural transition resulting from disorder. We also observed a first-order phase transition of MnCO 3 at 50 GPa, which is not accompanied by any changes in the electronic spin state. These results highlight the unique behavior of MnCO 3 , which we found to be quite different from other common carbonates such as siderite, magnesite, and calcite.
21 citations
TL;DR: In this article, the effect of pressure on the volume change in GeO2 glass up to 38.5 GPa was studied and a transition was observed at approximately 10−13 GPa, where the material displays distinct compression behaviors.
Abstract: Nanoscale transmission x-ray microscopy measurements have been performed to determine the effect of pressure (P) on the volume (V) change in GeO2 glass up to 38.5 GPa. The P-V data show a continuous increase upon compression, indicating that the density-driven structural transformation is a gradual process. Over the pressure range studied, a transition is observed at approximately 10–13 GPa, where the material displays distinct compression behaviors. The pressure-induced densification that involves the coordination number change has been discussed. Using this newly developed high-pressure imaging technique with tens of nanometer resolution, we have provided a direct and unequivocal way for measuring density of amorphous materials to much higher pressures with accuracy rivaling x-ray diffraction of crystalline solids.
21 citations
TL;DR: In this paper, high-pressure in situ synchrotron x-ray diffraction experiments were performed on Ag${}_{2}$Te up to 42.6 GPa at room temperature, and four phases were identified.
Abstract: High-pressure in situ synchrotron x-ray diffraction experiments were performed on Ag${}_{2}$Te up to 42.6 GPa at room temperature, and four phases were identified. Phase I (\ensuremath{\beta}--Ag${}_{2}$Te) transformed into isostructural phase II at 2.4 GPa, and phase III and phase IV emerged at 2.8 and 12.8 GPa, respectively. Combined with first-principles calculations, we solved the phase II and phase III crystal structures and determined the compressional behavior of phase III. Electronic band structure calculations show that the insulating phase I with a narrow band gap first transforms into the semimetallic phase II with the perseverance of topologically nontrivial nature and then to the bulk metallic phase III. Density of states calculations indicate the contrasting transport behavior for Ag${}_{2\ensuremath{-}\ensuremath{\delta}}$Te and Ag${}_{2+\ensuremath{\delta}}$Te under compression. Our results highlight pressure's dramatic role in tuning Ag${}_{2}$Te's electronic band structure and its novel electrical and magnetotransport behaviors.
20 citations
TL;DR: In this article, the phonon density of states of pure iron (57Fe) was measured under hydrostatic conditions using nuclear resonant inelastic X-ray scattering (NRIXS) at pressures up to 136 GPa.
Abstract: [1] The phonon density of states of pure iron (57Fe) was measured under hydrostatic conditions using nuclear resonant inelastic X-ray scattering (NRIXS) at pressures up to 136 GPa. Extracting shear (Vs) and compressional (Vp) wave speeds from the Debye velocity and equation of state, we find the hydrostatic shear wave speed trend above previously collected NRIXS data under nonhydrostatic conditions by roughly 5%–6% in the measured pressure range. Using the Birch Murnaghan finite strain approach to fit pressure-dependent adiabatic bulk and shear moduli, we extrapolated our velocities to inner Earth core densities and found that our shear wave speeds are 3% higher than those in previous studies. Our results on pure iron provide a more accurate and precise baseline to which added complications (e.g., Ni concentration, inclusion of various light elements, and temperature effects) can be considered when comparing experimental elasticity measurements to inner core seismic data.
TL;DR: In this article, the authors used high pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge2Sb2Te5, GeSb 2Te4, and SnSb 1Te4 and found that the critical pressure for triggering amorphization decreases with increasing vacancy concentration.
Abstract: We used high-pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge2Sb2Te5, GeSb2Te4, and SnSb2Te4. We found that the critical pressure for triggering amorphization in the PCMs decreases with increasing vacancy concentration, demonstrating that the presence of vacancies, rather than differences in the atomic covalent radii, is crucial for pressure-induced amorphization in PCMs. Compared to the as-deposited amorphous phase, the pressure-induced amorphous phase has a similar vibrational spectrum but requires much lower laser power to transform into the crystalline phase, suggesting different kinetics of crystallization, which may have implications for applications of PCMs in non-volatile data storage.
TL;DR: In this article, x-ray, neutron scattering, and heat capacity data reveal a coupled first-order magnetic and structural phase transition of the metastable mixed-valence postspinel compound Mn3O4 at 210 K.
Abstract: We present x-ray, neutron scattering, and heat capacity data that reveal a coupled first-order magnetic and structural phase transition of the metastable mixed-valence postspinel compound Mn3O4 at 210 K. Powder neutron diffraction measurements reveal a magnetic structure in which Mn3+ spins align antiferromagnetically along the edge-sharing a axis, with a magnetic propagation vector k = [1/2,0,0]. In contrast, the Mn2+ spins, which are geometrically frustrated, do not order until a much lower temperature. Although the Mn2+ spins do not directly participate in the magnetic phase transition at 210 K, structural refinements reveal a large atomic shift at this phase transition, corresponding to a physical motion of approximately 0.25 angstrom, even though the crystal symmetry remains unchanged. This "giant" response is due to the coupled effect of built-in strain in the metastable postspinel structure with the orbital realignment of the Mn3+ ion.
TL;DR: In this paper, in situ high-pressure x-ray powder diffraction on the mixed-valence compound Cs${}_{2}$Au${}^{\mathrm{I}}$CsAuI${}_3}$ was reported for pressures up to 21 GPa in a diamond anvil cell under hydrostatic conditions.
Abstract: Results of in situ high-pressure x-ray powder diffraction on the mixed-valence compound Cs${}_{2}$Au${}^{\mathrm{I}}$Au${}^{\mathrm{III}}$I${}_{6}$ (CsAuI${}_{3}$) are reported for pressures up to 21 GPa in a diamond-anvil cell under hydrostatic conditions. We find a reversible pressure-induced tetragonal-to-orthorhombic structural transition at 5.5--6 GPa and reversible amorphization at 12--14 GPa. Two alternative structures are proposed for the high-pressure orthorhombic phase and are discussed in the context of a possible Au valence transition.
TL;DR: In this article, high pressure x-ray diffraction measurements on CexAl1−x (x = 65, 70, and 80 %) MGs were performed and it was found that PID is very sensitive to compositions and can only exist over narrow compositional ranges.
Abstract: Long-range topological order (LRTO) was recently revealed in a Ce75Al25 metallic glass (MG) by a pressure-induced devitrification (PID) at 300 K. However, what compositions may have PID and an understanding of the physical and chemical controls behind PID are still not clear. We performed in situ high pressure x-ray diffraction measurements on CexAl1−x (x = 65, 70, and 80 at. %) MGs. Combining our experimental results and simulations, we found PID is very sensitive to compositions and can only exist over narrow compositional ranges. These results provide valuable guidance for searching for PID in MGs.
TL;DR: In this article, high pressure synchrotron x-ray diffraction experiments on Co3O4 were conducted, and two new monoclinic phases with the P121/c1 and C12/m1 structures were found.
Abstract: Co3O4 represents an intriguing strongly correlated system since it is a spin liquid originating from A-site magnetic frustration. We conducted in-situ high pressure synchrotron x-ray diffraction experiments on Co3O4, and found two new monoclinic phases with the P121/c1 and C12/m1 structures. These high pressure phases are distinct from the high pressure structures that were previously reported for other 3d transition metal spinel oxides. The charge transfer does not drive the normal spinel to transform into a symmetry equivalent inverse spinel, and instead we observe transformations into the new monoclinic phases which have two distinct Co2+ and Co3+ octahedral sites.
TL;DR: In this article, a review of the techniques for reaching ultrahigh pressure-temperature conditions for carbon-bearing materials is presented. But the authors do not address the problem of how carbon moves within the planet's deep interior.
Abstract: > “Who looks outside, dreams; who looks inside, awakes.”
>
> — Carl Jung
Surficial observations reveal carbon in a great variety of organic, inorganic, and biological forms that subduct with descending slabs and rise and erupt in volcanoes. Due to the lack of experimental means for studying carbon under extreme deep Earth conditions, we have limited information on the density and bonding nature of carbon-bearing fluids, and virtually no information on the texture and porosity of fluid-rock assemblages. Our knowledge on some of the most fundamental questions surrounding the deep carbon cycle becomes increasingly tenuous as we move into the planet. For example, in what form do carbon-bearing materials exist deep within Earth (Oganov et al. 2013)? How does carbon move within the planet’s deep interior (Dasgupta 2013)? To address these types of questions, we need to improve our understanding of carbon-bearing phases at the extreme pressure-temperature conditions existing in Earth. As the fourth most abundant element in the universe, the backbone of organic matter and major energy carriers, pure carbon forms a variety of allotropes including both crystalline and disordered structures such as diamond, graphite, graphene, buckyballs, nanotubes and glassy carbon with numerous and exciting potential in technological applications. Adding the more than 370 other carbon-bearing mineral species (Hazen et al. 2013), this represents a huge range of structures and bonding and fascinating (as well as complex) physics and chemistry. Currently much is unknown about the behavior of carbon-bearing phases at high pressures and temperatures. Experimental study of materials behavior at extreme conditions requires the ability to reach simultaneous high pressure-temperature conditions, and the development and implementation of a battery of micro/nanoscale probes to characterize samples. In addition, studying carbon brings its own set of complications and considerations.
In this chapter we first review some of the techniques for reaching ultrahigh …
TL;DR: In this article, high pressure measurements on the graphitic C 3 N 4 ( g -C 3 n 4 ) phase using Brillouin light scattering (BLS) up to 41.5 GPa and X-ray Raman scattering (XRS), up to 26 GPa, were conducted to probe the behavior of sp 2 bonds.
TL;DR: CeRuPO, a ferromagnetic Kondo lattice, undergoes two pressure-induced structural phase transitions as discussed by the authors, and the collapsed tetragonal phase transforms into an orthorhombic Cmma structure.
Abstract: CeRuPO, a ferromagnetic Kondo lattice, undergoes two novel pressure-induced structural phase transitions. CeRuPO undergoes an isostructural transition to a collapsed tetragonal structure near 7.5 GPa. Since pressure suppresses the initial ferromagnetic ordering of Ce3+, our observation suggests a quantum critical point for CeRuPO near this pressure. The collapsed tetragonal phase transforms into an orthorhombic Cmma structure. This phase is isostructural with the low temperature phase of LaFeAsO, which is superconducting under high pressure. Since CeRuPO exhibits a similar relationship between the two phases of LaFeAsO, the collapsed tetragonal and orthorhombic phases of CeRuPO may adopt a novel magnetic ground state.