Showing papers by "Wendy L. Mao published in 2011"

Long-Range Topological Order in Metallic Glass

Abstract: Glass lacks the long-range periodic order that characterizes a crystal. In the Ce75Al25 metallic glass (MG), however, we discovered a long-range topological order corresponding to a single crystal of indefinite length. Structural examinations confirm that the MG is truly amorphous, isotropic, and unstrained, yet under 25 gigapascals hydrostatic pressures, every segment of a centimeter-length MG ribbon devitrifies independently into a face-centered cubic (fcc) crystal with the identical orientation. By using molecular dynamics simulations and synchrotron x-ray techniques, we elucidate that the mismatch between the large Ce and small Al atoms frustrates the crystallization and causes amorphization, but a long-range fcc topological order still exists. Pressure induces electronic transition in Ce, which eliminates the mismatch and manifests the topological order by the formation of a single crystal.

Amorphous diamond: a high-pressure superhard carbon allotrope.

Abstract: Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully $s{p}^{3}$-bonded amorphous structure and diamondlike strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced $s{p}^{2}$-to-$s{p}^{3}$ bonding change, while x-ray diffraction confirmed the perseverance of noncrystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of $g70\text{ }\text{ }\mathrm{GPa}$ has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope.

Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in LaMnO 3

Abstract: High pressure, low temperature Raman measurements performed on LaMnO3 up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that LaMnO3 is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that LaMnO3 becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.

Compressional Behavior of Bulk and Nanorod LiMn2O4 under Nonhydrostatic Stress

Abstract: We studied the effect of pressure on LiMn{sub 2}O{sub 4} commercial powders and well-characterized nanorods using angle-dispersive synchrotron X-ray diffraction (XRD) in diamond anvil cells and found that spinel LiMn{sub 2}O{sub 4} is extremely sensitive to deviatoric stress induced by external applied pressure. Under nonhydrostatic conditions, bulk LiMn{sub 2}O{sub 4} underwent an irreversible phase transformation at pressures as low as 0.4 GPa from a cubic Fd-3m to tetragonal I4{sub 1}/amd structure driven by the Jahn-Teller effect. In contrast, bulk LiMn{sub 2}O{sub 4} under hydrostatic conditions experienced a reversible structural transformation beginning at approximately 11 GPa. Well-characterized LiMn{sub 2}O{sub 4} nanorods with an average diameter of 100-150 nm and an average length of 1-2 {micro}m were investigated under the same experimental conditions and showed a similar structural behavior as the bulk material confirming that LiMn{sub 2}O{sub 4} displays an extremely sensitive structural response to deviatoric stress. Scanning electron microscope (SEM) images of the samples especially the nanorods that were recovered from high pressure demonstrated a link between the changing morphology of the materials and the origin of the phase transition. We also found that nanostructured materials can accommodate more stress compared to their bulk counterparts. Our comparative study of bulk andmore » nanorod LiMn{sub 2}O{sub 4} improves our understanding of their fundamental structural and mechanical properties, which can provide guidance for applied battery technology. In addition, LiMn{sub 2}O{sub 4} represents a strongly correlated system, whose structural, electronic, and magnetic properties at high pressure are of broad interest for fundamental chemistry and condensed matter physics.« less

High-pressure EXAFS measurements of crystalline Ge using nanocrystalline diamond anvils

Abstract: High-pressure extended x-ray absorption fine structure (EXAFS) measurements on crystalline Ge demonstrate that the use of nanocrystalline diamond anvils can solve the glitch problem from single crystal diamond anvils and improve the quality of the data. Our results indicate that using nanocrystalline diamond anvils for high-pressure EXAFS research can provide a large enough energy range for structural study up to four coordination shell distances. In particular, we obtained the pressure evolution of mean square relative displacement for the first neighbor shells of crystalline Ge and observed different correlation effects for different coordination shells. The use of nanocrystalline diamond anvils will provide a breakthrough for high-pressure EXAFS study, especially for amorphous compounds in which only limited structural information can be obtained by diffraction techniques.

The effect of compressive strain on the Raman modes of the dry and hydrated BaCe0.8Y0.2O3 proton conductor

Abstract: The BaCe0.8Y0.2O3-{\delta} proton conductor under hydration and under compressive strain has been analyzed with high pressure Raman spectroscopy and high pressure x-ray diffraction. The pressure dependent variation of the Ag and B2g bending modes from the O-Ce-O unit is suppressed when the proton conductor is hydrated, affecting directly the proton transfer by locally changing the electron density of the oxygen ions. Compressive strain causes a hardening of the Ce-O stretching bond. The activation barrier for proton conductivity is raised, in line with recent findings using high pressure and high temperature impedance spectroscopy. The increasing Raman frequency of the B1g and B3g modes thus implies that the phonons become hardened and increase the vibration energy in the a-c crystal plane upon compressive strain, whereas phonons are relaxed in the b-axis, and thus reveal softening of the Ag and B2g modes. Lattice toughening in the a-c crystal plane raises therefore a higher activation barrier for proton transfer and thus anisotropic conductivity. The experimental findings of the interaction of protons with the ceramic host lattice under external strain may provide a general guideline for yet to develop epitaxial strained proton conducting thin film systems with high proton mobility and low activation energy.

Effect of Compressive Strain on the Raman Modes of the Dry and Hydrated BaCe0.8Y0.2O3 Proton Conductor

Abstract: The BaCe0.8Y0.2O3−δ proton conductor under hydration and under compressive strain has been analyzed with high-pressure Raman spectroscopy and high-pressure X-ray diffraction. The pressure-dependent variation of the Ag and B2g bending modes from the O–Ce–O unit is suppressed when the proton conductor is hydrated, affecting directly the proton transfer by locally changing the electron density of the oxygen ions. Compressive strain causes a hardening of the Ce–O stretching bond, with the pressure coefficient Δν/Δp = 4.32 ± 0.05 cm–1/GPa being the same for the dry and hydrated sample. As a result of this hardening of the lattice vibrations, the activation barrier for proton conductivity is raised, in line with recent findings using high-pressure and high-temperature impedance spectroscopy. Hydration also offsets slightly the Ce–O B1g and B3g stretching modes by ∼2 cm–1 toward higher wave numbers, revealing an increase in the bond strength of Ce–O. The (20–2) Bragg reflections do not change during pressurizing...

Low temperature transport properties of Ce-Al metallic glasses

Abstract: The low temperature transport properties of Ce75−xAl25+x (x = 0, 10, and 15 at. %) metallic glasses were investigated. Magnetic field and composition tuned magnetoresistances changing from negative to positive values were observed at low temperature. It was suggested that these peculiar phenomena were caused by the tunable competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida interaction in Ce-Al metallic glass with the variation in Ce content and magnetic field. Further magnetization and Ce-2p3d resonant inelastic x-ray scattering spectroscopy measurements supported this scenario. These Ce-Al metallic glasses could provide an interesting model system for the investigation of 4f electron behaviors in complex condensed matter with tunable transport properties.

Application of a new composite cubic-boron nitride gasket assembly for high pressure inelastic x-ray scattering studies of carbon related materials.

Abstract: We have developed a new composite cubic-boron nitride (c-BN) gasket assembly for high pressurediamond anvil cell studies, and applied it to inelastic x-ray scattering (IXS) studies of carbon related materials in order to maintain a larger sample thickness and avoid the interference from the diamond anvils. The gap size between the two diamond anvils remained ∼80 μm at 48.0 GPa with this new composite c-BN gasket assembly. The sample can be located at the center of the gap, ∼20 μm away from the surface of both diamond anvils, which provides ample distance to separate the sample signal from the diamond anvils. The high pressure IXS of a solvated C60 sample was studied up to 48 GPa, and a pressure induced bonding transition from sp 2 to sp 3 was observed at 27 GPa.

Studying single nanocrystals under high pressure using an x-ray nanoprobe

Abstract: In this report, we demonstrate the feasibility of applying a 250-nm focused x-ray beam to study a single crystalline NbSe3 nanobelt under high-pressure conditions in a diamond anvil cell. With such a small probe, we not only resolved the distribution and morphology of each individual nanobelt in the x-ray fluorescence maps but also obtained the diffraction patterns from individual crystalline nanobelts with thicknesses of less than 50 nm. Single crystalline diffraction measurements on NbSe3 nanobelts were performed at pressures up to 20 GPa.

Compressional, temporal, and compositional behavior of H2-O2 compound formed by high pressure x-ray irradiation.

Abstract: X-ray irradiation was found to convert H2O at pressures above 2 GPa into a novel molecular H2-O2 compound. We used optical Raman spectroscopy to explore the behavior of x-ray irradiated H2O samples as a function of pressure, time, and composition. The compound was found to be stable over a period of two years, as long as high pressure conditions (>2 GPa) were maintained. The Raman shifts for the H2 and O2 vibrons behaved differently from pure H2 and O2 as pressure was increased on the compound up to 70 GPa, indicating that it remains a distinct, molecular compound. Based on spectra taken from different locations in a single sample, it appears that multiple forms of the H2-O2 compound exist. The structure and composition of the starting material plays an important role in compound formation, as we found that hydrogen-filled ice clathrate C2 (H2)H2O did not undergo the same dissociation as observed in ice VII upon x-ray irradiation until pressure was increased to above 10 GPa.

X-ray-induced dissociation of H.sub.2O and formation of an O.sub.2-H.sub.2 alloy at high pressure

Abstract: A novel molecular alloy of O.sub.2 and H.sub.2 and a method of producing such a molecular alloy are provided. When subjected to high pressure and extensive x-radiation, H.sub.2O molecules cleaved, forming O--O and H--H bonds. In the method of the present invention, the O and H framework in ice VII was converted into a molecular alloy of O.sub.2 and H.sub.2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrate that this crystalline solid differs from previously known phases.