89 research outputs found
Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots
The highest superconducting temperature T observed in any elemental metal
(Li with T ~ 20 K at pressure P ~ 40 GPa) is shown to arise from critical
(formally divergent) electron-phonon coupling to the transverse T phonon
branch along intersections of Kohn anomaly surfaces with the Fermi surface.
First principles linear response calculations of the phonon spectrum and
spectral function reveal (harmonic) instability already at
25 GPa. Our results imply that the fcc phase is anharmonically stabilized in
the 25-38 GPa range.Comment: 4 pages, 3 embedded figure
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Pressure-Induced Antifluorite-to-Anticotunnite Phase Transition in Lithium Oxide
Using synchrotron angle-dispersive x-ray diffraction (ADXD) and Raman spectroscopy on samples of Li{sub 2}O pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite ({alpha}, Fm-3m) to orthorhombic anticotunnite ({beta}, Pnma) phase at 50({+-}5) GPa at ambient temperature. This transition is accompanied by a relatively large volume collapse of 5.4 ({+-}0.8)% and large hysteresis upon pressure reversal (P{sub down} at {approx} 25 GPa). Contrary to a recent study, our data suggest that the high-pressure {beta}-phase (B{sub o} = 188 {+-} 12 GPa) is substantially stiffer than the low-pressure {alpha}-phase (B{sub o} = 90 {+-} 1 GPa). A relatively strong and pressure-dependent preferred orientation in {beta}-Li{sub 2}O is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides
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Electronic Transitions in f-electron Metals at High Pressures:
This study was to investigate unusual phase transitions driven by electron correlation effects that occur in many f-band transition metals and are often accompanied by large volume changes: {approx}20% at the {delta}-{alpha} transition in Pu and 5-15% for analogous transitions in Ce, Pr, and Gd. The exact nature of these transitions has not been well understood, including the short-range correlation effects themselves, their relation to long-range crystalline order, the possible existence of remnants of the transitions in the liquid, the role of magnetic moments and order, the critical behavior, and dynamics of the transitions, among other issues. Many of these questions represent forefront physics challenges central to Stockpile materials and are also important in understanding the high-pressure behavior of other f- and d-band transition metal compounds including 3d-magnetic transition monoxide (TMO, TM=Mn, Fe, Co, Ni). The overarching goal of this study was, therefore, to understand the relationships between crystal structure and electronic structure of transition metals at high pressures, by using the nation's brightest third-generation synchrotron x-ray at the Advanced Photon Source (APS). Significant progresses have been made, including new discoveries of the Mott transition in MnO at 105 GPa and Kondo-like 4f-electron dehybridization and new developments of high-pressure resonance inelastic x-ray spectroscopy and x-ray emission spectroscopy. These scientific discoveries and technology developments provide new insights and enabling tools to understand scientific challenges in stockpile materials. The project has broader impacts in training two SEGRF graduate students and developing an university collaboration (funded through SSAAP)
Theoretical and experimental investigation of the equation of state of boron plasmas
We report a theoretical equation of state (EOS) table for boron across a wide
range of temperatures (5.110-5.210 K) and densities
(0.25-49 g/cm), and experimental shock Hugoniot data at unprecedented high
pressures (5608118 GPa). The calculations are performed with full,
first-principles methods combining path integral Monte Carlo (PIMC) at high
temperatures and density functional theory molecular dynamics (DFT-MD) methods
at lower temperatures. PIMC and DFT-MD cross-validate each other by providing
coherent EOS (difference 1.5 Hartree/boron in energy and 5% in pressure)
at 5.110 K. The Hugoniot measurement is conducted at the National
Ignition Facility using a planar shock platform. The pressure-density relation
found in our shock experiment is on top of the shock Hugoniot profile predicted
with our first-principles EOS and a semi-empirical EOS table (LEOS 50). We
investigate the self diffusivity and the effect of thermal and pressure-driven
ionization on the EOS and shock compression behavior in high pressure and
temperature conditions We study the performance sensitivity of a polar
direct-drive exploding pusher platform to pressure variations based on
comparison of the first-principles calculations with LEOS 50 via 1D
hydrodynamic simulations. The results are valuable for future theoretical and
experimental studies and engineering design in high energy density research.
(LLNL-JRNL-748227)Comment: 12 pages, 9 figures, 2 table
Femtosecond X-Ray Diffraction Studies of the Reversal of the Microstructural Effects of Plastic Deformation during Shock Release of Tantalum
We have used femtosecond x-ray diffraction (XRD) to study laser-shocked fiber-textured polycrystalline tantalum targets as the 37-253 GPa shock waves break out from the free surface. We extract the time and depth-dependent strain profiles within the Ta target as the rarefaction wave travels back into the bulk of the sample. In agreement with molecular dynamics (MD) simulations the lattice rotation and the twins that are formed under shock-compression are observed to be almost fully eliminated by the rarefaction process
Experimental observation of open structures in elemental magnesium at terapascal pressures
Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. Over the past decade, computational predictions have revealed that compression to terapascal pressures may bring about counter-intuitive changes in the structure and bonding of solids as quantum mechanical forces grow in influence1,2,3,4,5,6. Although this behaviour has been observed at modest pressures in the highly compressible light alkali metals7,8, it has not been established whether it is commonplace among high-pressure solids more broadly. We used shaped laser pulses at the National Ignition Facility to compress elemental Mg up to 1.3 TPa, which is approximately four times the pressure at the Earth’s core. By directly probing the crystal structure using nanosecond-duration X-ray diffraction, we found that Mg changes its crystal structure several times with non-close-packed phases emerging at the highest pressures. Our results demonstrate that phase transformations of extremely condensed matter, previously only accessible through theoretical calculations, can now be experimentally explored
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New cubic phase of lithium nitride to 200 GPa
We present a new cubic ({gamma}) Li{sub 3}N phase discovered above 40({+-}5) GPa. Structure and electronic bands are examined at high pressure with synchrotron x-ray diffraction and inelastic x-ray scattering in a diamond anvil cell, and also with first-principles calculations. We observe a dramatic band-gap widening and volume collapse at the phase transition. {gamma}-Li{sub 3}N remains extremely stable and ionic to 200 GPa, with predicted metallization near 8 TPa. The high structural stability, wide band-gap and simple electronic structure of {gamma}-Li{sub 3}N are analogous to that of such lower valence closed-shell solids as NaCl, MgO and Ne, meriting its use as a low-Z internal pressure standard
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