Shibing Wang

Families of superhard crystalline carbon allotropes constructed via cold compression of graphite and nanotubes.

We report a general scheme to systematically construct two classes of structural families of superhard sp(3) carbon allotropes of cold-compressed graphite through the topological analysis of odd 5+7 or even 4+8 membered carbon rings stemmed from the stacking of zigzag and armchair chains. Our results show that the previously proposed M, bct-C(4), W and Z allotropes belong to our currently proposed families and that depending on the topological arrangement of the native carbon rings numerous other members are found that can help us understand the structural phase transformation of cold-compressed graphite and carbon nanotubes (CNTs). In particular, we predict the existence of two simple allotropes, R and P carbon, which match well the experimental x-ray diffraction patterns of cold-compressed graphite and CNTs, respectively, display a transparent wide-gap insulator ground state and possess a large Vickers hardness comparable to diamond.

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

Pressure induced metallization with absence of structural transition in layered molybdenum diselenide.

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.

High pressure chemistry in the H2-SiH4 system

Understanding the behavior of hydrogen-rich systems at extreme conditions has significance to both condensed matter physics, where it may provide insight into the metallization and superconductivity of element one, and also to applied research areas, where it can provide guidance for designing improved hydrogen storage materials for transportation applications. Here we report the high-pressure study of the SiH4-H2 binary system up to 6.5 GPa at 300 K in a diamond anvil cell. Raman measurements indicate significant intermolecular interactions between H2 and SiH4. We found that the H2 vibron frequency is softened by the presence of SiH4 by as much as 40 cm−1 for the fluid with 50 mol% H2 compared with pure H2 fluid at the same pressures. In contrast, the Si-H stretching modes of SiH4 shift to higher frequency in the mixed fluid compared with pure SiH4. Pressure-induced solidification of the H2-SiH4 fluid shows a binary eutectic point at 72(±2) mol% H2 and 6.1(±0.1) GPa, above which the fluid crystallizes into a mixture of two nearly end-member solids. Neither solid has a pure end-member composition, with the silane-rich solid containing 0.5–1.5 mol% H2 and the hydrogen-rich solid containing 0.5–1 mol% SiH4. These two crystalline phases can be regarded as doped hydrogen-dominant compounds. We were able to superpressurize the sample by 0.2–0.4 GPa above the eutectic before complete crystallization, indicating extended metastability.

Bonding and structural changes in siderite at high pressure

Abstract Understanding the physical and chemical properties of carbonate minerals at extreme conditions is important for modeling the deep carbon cycle, because they represent likely hosts for carbon in the lower mantle. Previous high-pressure studies have identified a structural and electronic phase transition in siderite using X-ray diffraction and X-ray emission spectroscopy. The Fe end-member of the carbonate group, siderite (FeCO3), exhibits unique high-pressure behavior that we investigated using a combination of in situ Raman spectroscopy, synchrotron X-ray diffraction, and theoretical methods. In this Raman spectroscopy study, we observed the appearance of a new CO3 symmetric stretching mode at 20 cm−1 lower frequency beginning at approximately 46 GPa. This softening is due to the lengthening of the C-O bonds as a result of a combination of rotation and volume shrinkage of the FeO6 octahedra while siderite undergoes the isostructural volume collapse and electronic spin transition.

Bonding in boranes and their interaction with molecular hydrogen at extreme conditions.

The effects of high pressure and temperature on the bonding in ammonia borane (AB), NH(3)BH(3) and decaborane (DB), B(10)H(14) and their interactions with molecular hydrogen (H(2)) were investigated using Raman spectroscopy in a diamond anvil cell. At 0.7 GPa, AB becomes amorphous between 120 and 127 degrees C, indicating a positive Clapeyron slope. Heated to 140 degrees C, AB begins to undergo decomposition to polyaminoborane. The amorphous and decomposed AB does not recrystallize back to AB during slow cooling to room temperature or upon application of high pressure up to 3 GPa, underscoring the challenge of rehydrogenation of decomposed AB. The molecular Raman modes broaden in the reacted phase, and the NH(3) modes show no pressure dependence. DB was studied at room temperature up to 11 GPa. The observed frequency dependence with pressure (dnu/dP) and mode Gruneisen parameters varied for different spectral groups, and a new transition was identified at approximately 3 GPa. In both DB and heated AB, we found that they could store additional H(2) with the application of pressure. We estimate that we can store approximately 3 wt % H(2) in heated AB at 3 GPa and 1 wt % H(2) in DB at 4.5 GPa.

Pressure-induced structural transitions and metallization in Ag2Te

High pressure in-situ synchrotron X-ray diffraction experiments were performed on Ag2Te up to 42.6 GPa at room temperature and four phases were identified. Phase I ({\beta}-Ag2Te) transformed into phase II at 2.4 GPa, and phase III and phase IV emerged at 2.8 GPa 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 semi metallic phase II with the perseverance of topologically non trivial nature, and then to bulk metallic phase III. Density of States (DOS) calculations indicate the contrasting transport behavior for Ag2-{\delta}Te and Ag2+{\delta}Te under compression. Our results highlight pressures dramatic role in tuning Ag2Tes electronic band structure, and its novel electrical and magneto transport behaviors.

Bonding and electronic changes in rhodochrosite at high pressure

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 (MnCO3) 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 MnCO3 at 50 GPa, which is not accompanied by any changes in the electronic spin state. These results highlight the unique behavior of MnCO3, which we found to be quite different from other common carbonates such as siderite, magnesite, and calcite.

Pressure-induced symmetry breaking in tetragonal CsAuI3

Results of in situ high pressure x-ray powder diffraction on the mixed valence compound Cs2Au(I)Au(III)I6 (CsAuI3) 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.

Experimental strategies for optical pump - soft x-ray probe experiments at the LCLS

Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.