Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Hc phase to hexagonal 2Ha phase, which accompanies the metallization at ∼37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occur at low temperature near the metallization.
Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.
Hydrazinium azide (HA) has been investigated at high pressures to 68 GPa using confocal micro-Raman spectroscopy and synchrotron powder x-ray diffraction. The results show that HA undergoes structural phase transitions from solid HA-I to HA-II at 13 GPa, associated with the strengthening of hydrogen bonding, and then to N at 40 GPa. The transformation of HA to recently predicted N (N≡N-N-N=N-N-N≡N) is evident by the emergence of new peaks at 2384 cm, 1665 cm, and 1165 cm, arising from the terminal N≡N stretching, the central N=N stretching, and the N-N stretching, respectively. However, upon decompression, N decomposes to ε-N below 25 GPa, but the remnant can be seen as low as 3 GPa.
We report the reversible photochemical transformation of sulfur–hydrogen mixture to (H2S)2H2, which occurs through an expanded state of photoactive a-S phase of sulfur at 4 GPa. Upon further compression, the photoproduct (H2S)2H2 undergoes a phase transition at 17 GPa, and 40 GPa. The pressure-induced Raman changes indicate that the phase transition from phase I to II at 17 GPa is associated with the proton-ordering process in (H2S)2H2, evident by the profound splitting of S–H and H–H vibrational modes, whereas the transition at 40 GPa is accompanied by the disappearance of all the S–H stretching and bending modes and partial dissociation to sulfur. With increase in pressure, the molar volume of (H2S)2H2 is substantially larger than that of S + H2 mixtures, suggesting the significance of photochemical effect in order to drive the reaction from S + H2 to (H2S)2H2. In addition, we have also provided the thermal- and pressure-induced effect in the mixtures using confocal Raman spectroscopy. From our results, it is clear that the effect of pressure and photochemistry can be coupled to drive the reaction at room temperature and lower pressure, rather than having to drive the reaction thermally or mechanically, underscoring the significance of the photochemical effect in understanding the path-dependent transformations of sulfur and sulfur-containing materials.
MoO3 has large electronic and mechanical anisotropy arising from diverse chemical bonding in its layered structure. Here, we report high-pressure structural and electronic transitions in MoO3 to 100 GPa, using confocal micro-Raman spectroscopy, synchrotron X-ray diffraction, and electric conductivity measurements at both ambient and low temperatures. The results indicate that MoO3-I (Pnma) undergoes a series of structural phase transitions, initially to MoO3-II (P21/m) at 11 GPa and then to MoO3-III (Pmma) at 60 GPa. The former transition occurs displacively within nearly a same lattice of a I = 2 c II, b I = b II, and c I = a II, whereas the latter transition occurs reconstructively with ∼3–5% volume collapse at the transition. Interestingly, MoO3-II is absent in hydrostatic helium pressure medium, underscoring the presence of helium resisting shear deformation of this layered structure. MoO3-I directly transforms to MoO3-III at ∼26 GPa. These structural phase transitions also accompany a strong modification of electronic structure from insulating MoO3-I to poor metallic MoO3-III. The refined crystal structure of MoO3-III indicates that Mo atoms occupy two different sites at the centers of octahedra and slightly distorted square planar resulting in the O 2p–Mo 4d hybridization. This unique arrangement of Mo atoms in MoO3-III gives rise to an interesting resistive anomaly with a T –1/4 dependence of log(ρ) between 256 and 67 K, showing a variable range hopping mechanism of a Mott insulator.
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