Wendy L. Mao

University of Kentucky, Lexington, Kentucky, United States

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Publications (140)696.47 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Journal of the American Chemical Society 01/2015; · 11.44 Impact Factor
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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.
    The Journal of Chemical Physics 10/2014; 141(15):154305. · 3.12 Impact Factor
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    ABSTRACT: Mn$_3$O$_4$ is a spin frustrated magnet that adopts a tetragonally distorted spinel structure at ambient conditions and a CaMn$_2$O$_4$-type postspinel structure at high pressure. We conducted both optical measurements and \emph{ab} \emph{initio} calculations, and systematically studied the electronic band structures of both the spinel and postspinel Mn$_3$O$_4$ phases. For both phases, theoretical electronic structures are consistent with the optical absorption spectra, and display characteristic band-splitting of the conduction band. The band gap obtained from the absorption spectra is 1.91(6) eV for the spinel phase, and 0.94(2) eV for the postspinel phase. Both phases are charge-transfer type insulators. The Mn 3\emph{d} $t_2$$_g$ and O 2\emph{p} form antibonding orbitals situated at the conduction band with higher energy.
  • Phys. Rev. B. 06/2014; 89(24):245109.
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    ABSTRACT: The mineralogical constitution of the Earth's mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases.
    Science 05/2014; 344(6186):877-82. · 31.48 Impact Factor
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    ABSTRACT: As a fundamental property of a material, density is controlled by the interatomic distances and the packing of microscopic constituents. The most prominent atomistic feature in a metallic glass (MG) that can be measured is its principal diffraction peak position (q_{1}) observable by x-ray, electron, or neutron diffraction, which is closely associated with the average interatomic distance in the first shell. Density (and volume) would naturally be expected to vary under compression in proportion to the cube of the one-dimensional interatomic distance. However, by using high pressure as a clean tuning parameter and high-resolution in situ techniques developed specifically for probing the density of amorphous materials, we surprisingly found that the density of a MG varies with the 5/2 power of q_{1}, instead of the expected cubic relationship. Further studies of MGs of different compositions repeatedly produced the same fractional power law of 5/2 in all three MGs we investigated, suggesting a universal feature in MG.
    Physical Review Letters 05/2014; 112(18):185502. · 7.73 Impact Factor
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    ABSTRACT: We conducted in situ angle dispersive high pressure x-ray diffraction experiments on Sr3Ir2O7 up to 23.1 GPa at 25 K with neon as the pressure transmitting medium. Pressure induces a highly anisotropic compressional behavior seen where the tetragonal plane is compressed much faster than the perpendicular direction. By analyzing different aspects of the diffraction data, a second-order structural transition is observed at approximately 14 GPa, which is accompanied by the insulating state to nearly metallic state at 13.2 GPa observed previously (Li et al 2013 Phys. Rev. B 87 235127). Our results highlight the coupling between electronic state and lattice structure in Sr3Ir2O7 under pressure.
    Journal of Physics Condensed Matter 05/2014; 26(21):215402. · 2.22 Impact Factor
  • Journal of Power Sources 05/2014; 253:431. · 5.21 Impact Factor
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    ABSTRACT: We investigated the effect of pressure on the structure and bonding of [121] tetramantane up to 20 GPa via in situ angle-dispersive synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells (DACs). A phase transition from the starting monoclinic P21/n structure to a triclinic P1 structure was observed beginning at 13 GPa. Upon decompression, the transition pressure showed large hysteresis. After fully releasing pressure, [121] tetramantane was found to recover into a different polymorph from the starting phase. Continuous changes of the vibration modes associated with the CCC bending and CC stretching regions suggest that this phase transition was induced by the intramolecular level distortions and modifications in the molecular packing. These results provide guidance for understanding the inter- and intramolecular interactions in diamondoids under pressure and shed light on the possibility of using pressure as a tuning parameter for synthesizing higher diamondoids.
    The Journal of Physical Chemistry C 03/2014; 118(14):7683–7689. · 4.84 Impact Factor
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    ABSTRACT: Phase transitions in indentation induced Si-III/XII phases were investigated using a diamond anvil cell and nanoindentation combined with micro-Raman spectroscopy. The in situ high pressure Raman results demonstrate that the Si-III and Si-XII phases have very similar Raman spectra, indicating their relative amount cannot be determined if they are both present in a sample. The Si-III and Si-XII phases coexist in the indentations produced by a nanoindenter on a single crystalline silicon wafer as a result of the local residual compressive stresses near 1 GPa. High power laser annealing on the indentations can initiate a rapid Si-III/XII → Si-I phase transition. The newly formed polycrystalline Si-I phase initially has very small grain size, and the grains grow when the annealing time is extended. Si-IV phase was not observed in our experiment.
    Journal of Applied Physics 02/2014; 115(10). · 2.19 Impact Factor
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    ABSTRACT: The strain derivatives of $T_c$ along the $a$ and $c$ axes have been determined for HgBa$_2$CuO$_{4+\delta}$ (Hg1201), the simplest monolayer cuprate with the highest $T_c$ of all monolayer cuprates ($T_c$ = 97 K at optimal doping). The underdoped compound with the initial $T_c$ of 65 K has been studied as a function of pressure up to 20 GPa by magnetic susceptibility and X-ray diffraction (XRD). The observed linear increase in $T_c$ with pressure is the same as previously been found for the optimally-doped compound. The above results have enabled the investigation of the origins of the significantly different $T_c$ values of optimally doped Hg1201 and the well-studied compound La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), the latter value of $T_c$ = 40 K being only about 40% of the former. Hg1201 can have almost identical CuO$_6$ octahedra as LSCO if specifically strained. When the apical and in-plane CuO$_2$ distances are the same for the two compounds, a large discrepancy in their $T_c$ remains. Differences in crystal structures and interactions involving the Hg-O charge reservoir layers of Hg1201 may be responsible for the different $T_c$ values exhibited by the two compounds.
    Physical Review B 01/2014; 89:024515. · 3.66 Impact Factor
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    ABSTRACT: Colossal negative thermal expansion was recently discovered in BiNiO3 associated with a low density to high density phase transition under high pressure. The varying proportion of co-existing phases plays a key role in the macroscopic behavior of this material. Here, we utilize a recently developed X-ray Absorption Near Edge Spectroscopy Tomography method and resolve the mixture of high/low pressure phases as a function of pressure at tens of nanometer resolution taking advantage of the charge transfer during the transition. This five-dimensional (X, Y, Z, energy, and pressure) visualization of the phase boundary provides a high resolution method to study the interface dynamics of high/low pressure phase.
    Applied Physics Letters 01/2014; 104(4):043108. · 3.52 Impact Factor
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    ABSTRACT: We combine ultrafast pump-probe spectroscopy with a diamond-anvil cell to decouple the insulator-metal electronic transition from the lattice symmetry changing structural transition in the archetypal strongly correlated material vanadium dioxide. Coherent phonon spectroscopy enables tracking of the photo-excited phonon vibrational frequencies of the low temperature, monoclinic (M 1)-insulating phase that transforms into the metallic, tetragonal rutile structured phase at high temperature or via non-thermal photo-excitations. We find that in contrast with ambient pressure experiments where strong photo-excitation promptly induces the electronic transition along with changes in the lattice symmetry, at high pressure, the coherent phonons of the monoclinic (M 1) phase are still clearly observed upon the photo-driven phase transition to a metallic state. These results demonstrate the possibility of synthesizing and studying transient phases under extreme conditions. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862197] Strongly correlated electron materials exhibit intriguing and remarkable properties 1,2 due to the strong, complex inter-play between electron charge, spin, orbital, and lattice degrees of freedom. Vanadium dioxide (VO 2) is a model sys-tem in which strong electron-electron repulsion coupled with the lattice distortion cause the insulator-metal transition (IMT) and monoclinic (M 1)-rutile structural transition to occur concomitantly at ambient pressure and T $ 340 K. 2 In the low-temperature monoclinic (M 1) insulating phase, V atoms are dimerized and tilted with respect to the c-axis (space group P2 1/c) with an electronic band gap of $0.6 eV. 3
    Applied Physics Letters 01/2014; 104:021917. · 3.52 Impact Factor
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    ABSTRACT: We conducted in situ angle dispersive high pressure x-ray diffraction experiments on Sr 3 Ir 2 O 7 up to 23.1 GPa at 25 K with neon as the pressure transmitting medium. Pressure induces a highly anisotropic compressional behavior seen where the tetragonal plane is compressed much faster than the perpendicular direction. By analyzing different aspects of the diffraction data, a second-order structural transition is observed at approximately 14 GPa, which is accompanied by the insulating state to nearly metallic state at 13.2 GPa observed previously (Li et al 2013 Phys. Rev. B 87 235127). Our results highlight the coupling between electronic state and lattice structure in Sr 3 Ir 2 O 7 under pressure.
    Journal of Physics: Condensed Matter. 01/2014; 26(21):215402.
  • Yu Lin, Qiaoshi Zeng, Wenge Yang, Wendy L. Mao
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    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.
    Applied Physics Letters 12/2013; 103(26):261909-261909-5. · 3.52 Impact Factor
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    ABSTRACT: Pure body-center-cubic structured metastable polycrystalline Li15Si4 was synthesized in a Li-ion battery by electrochemically lithiating a thin Si wafer. Its stability and compressional behavior were investigated using in-situ high-pressure synchrotron X-ray diffraction (XRD) at room temperature. No phase transition was observed in polycrystalline Li15Si4 up to 5.8 GPa. A bulk modulus of 28.4(6) GPa was obtained for Li15Si4 by fitting the XRD data to a third order Birch–Murnaghan equation of state. We found that the bulk modulus of Li15Si4 follows a linear interpolation relationship between the bulk moduli for pure Li and Si. These results provide valuable experimental data to validate the theoretical calculation and are also important inputs for modeling the Li–Si system.
    Journal of Power Sources 11/2013; 242:732. · 5.21 Impact Factor
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    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.
    Applied Physics Letters 11/2013; 103:191908. · 3.52 Impact Factor
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    ABSTRACT: Core formation represents the most significant differentiation event in Earth's history. Our planet's present layered structure with a metallic core and an overlying mantle implies that there must be a mechanism to separate iron alloy from silicates in the initially accreted material. At upper mantle conditions, percolation has been ruled out as an efficient mechanism because of the tendency of molten iron to form isolated pockets at these pressures and temperatures. Here we present experimental evidence of a liquid iron alloy forming an interconnected melt network within a silicate perovskite matrix under pressure and temperature conditions of the Earth's lower mantle. Using nanoscale synchrotron X-ray computed tomography, we image a marked transition in the shape of the iron-rich melt in three-dimensional reconstructions of samples prepared at varying pressures and temperatures using a laser-heated diamond-anvil cell. We find that, as the pressure increases from 25 to 64GPa, the iron distribution changes from isolated pockets to an interconnected network. Our results indicate that percolation could be a viable mechanism of core formation at Earth's lower mantle conditions.
    Nature Geoscience 10/2013; 6(11):971-975. · 11.67 Impact Factor
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    ABSTRACT: 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 bil-lion 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.
    American Mineralogist 10/2013; 98:1817-1823. · 2.06 Impact Factor
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    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 (Nd2 Fe14 B). Microsc. Res. Tech., 2013. © 2013 Wiley Periodicals, Inc.
    Microscopy Research and Technique 08/2013; · 1.59 Impact Factor

Publication Stats

2k Citations
696.47 Total Impact Points


  • 2014
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, Kentucky, United States
  • 2008–2014
    • Stanford University
      • Department of Geological and Environmental Sciences
      Palo Alto, California, United States
  • 2013
    • Center for High Pressure Science and Technology Advanced Research
      Pootung, Shanghai Shi, China
  • 2002–2012
    • Carnegie Institution for Science
      • Geophysical Laboratory
      Washington, WV, United States
  • 2009–2011
    • Zhejiang University
      • Department of Material Science and Engineering
      Hangzhou, Zhejiang Sheng, China
  • 2007
    • Los Alamos Medical Center
      Los Alamos, New Mexico, United States
    • Carnegie Institute
      Washington, Washington, D.C., United States
  • 2006
    • Los Alamos National Laboratory
      • Los Alamos Neutron Science Center
      Los Alamos, NM, United States
  • 2002–2005
    • University of Chicago
      • Department of Geophysical Sciences
      Chicago, IL, United States
  • 2004
    • University of Illinois at Chicago
      Chicago, Illinois, United States