Wendy L. Mao

Stanford University, Palo Alto, California, United States

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Publications (169)930.94 Total impact

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    ABSTRACT: Metallic glass (MG) is an important new category of materials, but very few rigorous laws are currently known for defining its "disordered" structure. Recently we found that under compression, the volume (V) of an MG changes precisely to the 2.5 power of its principal diffraction peak position (1/q1). In the present study, we find that this 2.5 power law holds even through the first-order polyamorphic transition of a Ce68Al10Cu20Co2 MG. This transition is, in effect, the equivalent of a continuous "composition" change of 4f-localized "big Ce" to 4f-itinerant "small Ce," indicating the 2.5 power law is general for tuning with composition. The exactness and universality imply that the 2.5 power law may be a general rule defining the structure of MGs.
    Full-text · Article · Feb 2016 · Proceedings of the National Academy of Sciences
  • Zhidan Zeng · Nian Liu · Qiaoshi Zeng · Seok Woo Lee · Wendy L. Mao · Yi Cui
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    ABSTRACT: Stress is a long standing challenge for the applications of silicon (Si) anodes in lithium (Li) ion batteries. Nanostructured Si are important materials to address mechanical stress issues in batteries although their stress were only calculated and no experimental data are available. Using in situ Raman microscopy to monitor the shift of the first-order Raman peak of Si, we were able to measure for the first time the lithiation-induced stress in Si nanoparticles. The shift of Raman peak of Si under hydrostatic stress was calibrated via an in situ high pressure Raman experiment. We observed a transition in the stress in Si core of nanoparticles during lithiation, from tensile to compressive. At the beginning of lithiation, the reduction of the native oxide surface layer of the Si particle results in a tensile stress of approximately 0.2 GPa in Si. During the formation of amorphous LixSi in the outer layer of the nanoparticles, an increasing compressive stress up to 0.3 GPa is introduced to the Si core. This evolving stress explains the cracks that developed in the amorphous LixSi layer during lithiation of the Si nanoparticles, and is also consistent with modeling results. These results improve our understanding of lithiation-induced stress in nanostructured Si anodes, and provide valuable information for their theoretical study and future design.
    No preview · Article · Feb 2016 · Nano Energy
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    Full-text · Dataset · Oct 2015
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    ABSTRACT: Metallic glasses are metallic alloys that exhibit exotic material properties. They may have fractal structures at the atomic level, but a physical mechanism for their organization without ordering has not been identified. We demonstrated a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures using in situ x-ray diffraction, tomography, and molecular dynamics simulations. A specific class of fractal, the percolation cluster, explains the structural details for several metallic-glass compositions. We postulate that atoms percolate in the liquid phase and that the percolating cluster becomes rigid at the glass transition temperature.
    No preview · Article · Sep 2015 · Science
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    ABSTRACT: Pressure- and temperature-induced phase transitions have been studied for more than a century but very little is known about the non-equilibrium processes by which the atoms rearrange. Shock compression generates a nearly instantaneous propagating high-pressure/temperature condition while in situ X-ray diffraction (XRD) probes the time-dependent atomic arrangement. Here we present in situ pump-probe XRD measurements on shock-compressed fused silica, revealing an amorphous to crystalline high-pressure stishovite phase transition. Using the size broadening of the diffraction peaks, the growth of nanocrystalline stishovite grains is resolved on the nanosecond timescale just after shock compression. At applied pressures above 18 GPa the nuclueation of stishovite appears to be kinetically limited to 1.4±0.4 ns. The functional form of this grain growth suggests homogeneous nucleation and attachment as the growth mechanism. These are the first observations of crystalline grain growth in the shock front between low- and high-pressure states via XRD.
    Full-text · Article · Sep 2015 · Nature Communications
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    Full-text · Dataset · Jul 2015
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    Full-text · Dataset · Jul 2015
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    ABSTRACT: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~ 60 GPa using multiple experimental techniques and ab -initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
    Full-text · Article · Jun 2015 · Nature Communications
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    Full-text · Technical Report · Jun 2015
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    ABSTRACT: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
    Full-text · Dataset · May 2015
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    ABSTRACT: Sb2O3-based materials are of broad interest in materials science and industry. High-pressure study using diamond anvil cells shows promise in obtaining new crystal and electronic structures different from their pristine states. Here, we conducted in situ angle dispersive synchrotron x-ray-diffraction and Raman spectroscopy experiments on α-Sb2O3 up to 50 GPa with neon as the pressure transmitting medium. A first-order structural transition was observed in between 15 and 20 GPa, where the cubic phase I gradually transformed into a layered tetragonal phase II through structural distortion and symmetry breaking. To explain the dramatic changes in sample color and transparency, we performed first-principles calculations to track the evolution of its density of states and electronic structure under pressure. At higher pressure, a sluggish amorphization was observed. Our results highlight the structural connections among the sesquioxides, where the lone electron pair plays an important role in determining the local structures.
    Full-text · Article · May 2015 · Physical Review B
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    ABSTRACT: The high‐pressure behavior of manganese‐rich carbonate, rhodochrosite, has been characterized up to 62 GPa by synchrotron‐based midinfrared spectroscopy and X‐ray diffraction. Modifications in both the infrared spectra and the X‐ray diffraction patterns were observed above ~35 GPa, indicating the presence of a high‐pressure phase transition at these pressures. We found that rhodochrosite adopts a structure close to CaCO3‐VI with a triclinic unit cell (a = 2.87 Å, b = 4.83 Å, c = 5.49 Å, α = 99.86°, β = 94.95°, and γ = 90.95° at 62 GPa). Using first‐principles calculations based on density functional theory, we confirmed these observations and assigned modes in the new infrared signature of the high‐pressure phase. These results suggest that high‐pressure metastable phase of calcite may play an important role in carbon storage and transport in the deep Earth. We present high‐pressure infrared spectroscopy and DFT calculations on MnCO3MnCO3 adopts a metastable high‐pressure (HP) structure of calcite above 35 GPaMetastable HP structures of calcite may represent carbon host in deep Earth
    Full-text · Article · May 2015 · Journal of Geophysical Research: Solid Earth
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    ABSTRACT: Li15Si4, the only crystalline phase that forms during lithiation of the Si anode in lithium-ion batteries, is found to undergo a structural transition to a new phase at 7 GPa. Despite the large unit cell of Li15Si4 (152 atoms in the unit cell), ab initio evolutionary metadynamics (using the USPEX code) successfully predicts the atomic structure of this new phase (β-Li15Si4), which has an orthorhombic structure with an Fdd2 space group. In the new β-Li15Si4 phase Si atoms are isolated by Li atoms analogous to the original cubic phase (α-Li15Si4), whereas the atomic packing is more efficient owing to the higher SiLi coordination number and shorter SiLi, LiLi bonds. β-Li15Si4 has substantially larger elastic moduli compared with α-Li15Si4, and has a good electrical conductivity. As a result, β-Li15Si4 has superior resistance to deformation and fracture under stress. The theoretical volume expansion of Si would decrease 25% if it transforms to β-Li15Si4, instead of α-Li15Si4, during lithiation. Moreover, β-Li15Si4 can be recovered back to ambient pressure, providing opportunities to further investigate its properties and potential applications.
    Full-text · Article · Apr 2015 · Advanced Energy Materials
  • Wendy L Mao

    No preview · Article · Feb 2015 · Nature Materials
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    ABSTRACT: Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in the Earth’s crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO3. Using ab-initio calculations, we assign the new infrared signature to C–O bands associated with tetrahedrally coordinated carbon with asymmetric C–O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low-pressure threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
    Full-text · Article · Feb 2015 · Nature Communications
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    ABSTRACT: Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in the Earth’s crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO3. Using ab-initio calculations, we assign the new infrared signature to C–O bands associated with tetrahedrally coordinated carbon with asymmetric C–O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low-pressure threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
    Full-text · Article · Feb 2015 · Nature Communications
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    ABSTRACT: NaYF4:Yb(3+),Er(3+) nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the non-optimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. While hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly twofold increase in efficiency. In-situ x-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.
    No preview · Article · Feb 2015 · Nano Letters
  • Adam Jaffe · Yu Lin · Wendy L Mao · Hemamala I Karunadasa
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    ABSTRACT: Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d9 Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper (II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder x-ray diffraction, electronic absorption and vibrational spectroscopy, dc conductivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other CuII-Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in CuII-Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10-4 S•cm-1 at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0-eV bandgap semiconductor at 39.7 GPa, and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression and ambient pressure powder x-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.
    No preview · Article · Jan 2015 · Journal of the American Chemical Society
  • Yu Lin · Wendy L. Mao
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    ABSTRACT: As a promising candidate material for hydrogen storage, ammonia borane (NH3BH3) has attracted significant interest in recent years due to its remarkably high hydrogen content. Subjecting this material to high pressure not only enables the formation of novel phases and compounds with exotic properties, but also improves our basic understanding of material’s behavior at different levels of atomic and molecular interactions. This review focuses on the perspective of high-pressure chemical hydrogen storage related to NH3BH3-based materials. Four main aspects are discussed: the structures and bonding of NH3BH3 over a wide pressure–temperature space, thermolysis of NH3BH3 at high pressure, the formation of a novel high-pressure H-rich compound as a result of storage of additional molecular H2 in NH3BH3, and the potential rehydrogenation of the thermally decomposed NH3BH3 under the extreme of pressure.
    No preview · Article · Dec 2014 · Chinese Science Bulletin
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    ABSTRACT: The high-pressure behavior of diamantane was investigated using angle-dispersive synchrotron x-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells. Our experiments revealed that the structural transitions in diamantane were extremely sensitive to deviatoric stress. Under non-hydrostatic conditions, diamantane underwent a cubic (space group Pa3) to a monoclinic phase transition at below 0.15 GPa, the lowest pressure we were able to measure. Upon further compression to 3.5 GPa, this monoclinic phase transformed into another high-pressure monoclinic phase which persisted to 32 GPa, the highest pressure studied in our experiments. However, under more hydrostatic conditions using silicone oil as a pressure medium, the transition pressure to the first high-pressure monoclinic phase was elevated to 7-10 GPa, which coincided with the hydrostatic limit of silicone oil. In another experiment using helium as a pressure medium, no phase transitions were observed to the highest pressure we reached (13 GPa). In addition, large hysteresis and sluggish transition kinetics were observed upon decompression. Over the pressure range where phase transitions were confirmed by XRD, only continuous changes in the Raman spectra were observed. This suggests that these phase transitions are associated with unit cell distortions and modifications in molecular packing rather than the formation of new carbon-carbon bonds under pressure.
    No preview · Article · Oct 2014 · The Journal of Chemical Physics

Publication Stats

3k Citations
930.94 Total Impact Points

Institutions

  • 2007-2015
    • Stanford University
      • Department of Geological and Environmental Sciences
      Palo Alto, California, United States
    • Los Alamos Medical Center
      Los Alamos, New Mexico, United States
    • Carnegie Institute
      Washington, Washington, D.C., United States
  • 2010
    • Zhejiang University
      • Department of Material Science and Engineering
      Hangzhou, Zhejiang Sheng, China
  • 2006-2007
    • Los Alamos National Laboratory
      • Los Alamos Neutron Science Center
      Лос-Аламос, California, United States
    • James Cook University
      Townsville, Queensland, Australia
  • 2002-2005
    • University of Chicago
      • Department of Geophysical Sciences
      Chicago, Illinois, United States
  • 2004
    • University of Illinois at Chicago
      Chicago, Illinois, United States
  • 2002-2004
    • Carnegie Institution for Science
      • Geophysical Laboratory
      Washington, West Virginia, United States