Choong-Shik Yoo

Spin Transition Zone in Earth's Lower Mantle

Mineral properties in Earths lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.

Phase Diagram of Iron by in Situ X-ray Diffraction: Implications for Earth's Core

The phase diagram of iron has been studied to 130 gigapascals (1 gigapascal = 104 atmospheres) and 3500 kelvin by a combined laser-heated diamond-anvil cell and x-ray diffraction technique that provides direct identification of the solid phases. Iron in the hexagonal close-packed (hcp) phase (ϵ-Fe) is stable from 50 to at least 110 gigapascals at high temperatures. The wide stability field of ϵ-Fe indicates that this polymorph should currently be considered the most relevant solid phase for Earths core. The triple point between the γ, ϵ, and liquid phases is located at 2500 ± 200 kelvin and 50 ± 10 gigapascals. There is evidence for a phase with a double hcp structure below 40 gigapascals and for another transition above 110 gigapascals and 3000 kelvin.

Equation of state, phase transition, decomposition of β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) at high pressures

Pressure-volume relations and vibrational Raman spectra of unreacted HMX (octahydro-1, 3,5,7-tetranitro-1,3,5,7-tetrazocine) have been obtained in both quasihydrostatic conditions to 45 GPa and nonhydrostatic conditions to 10 GPa by using diamond-anvil cell, angle-resolved synchrotron x-ray diffraction, and micro-Raman spectroscopy. The results show that the high-pressure behavior of HMX strongly depends on the stress conditions. HMX is more compressible in hydrostatic conditions (B0=12.4 GPa and B′=10.4) than in nonhydrostatic conditions (B0=14.4 GPa, B′=13.3). This discrepancy in HMX compressibility can be explained in terms of chemical reactions occurring in nonhydrostatic conditions. The static isotherm is in good agreement with the shock Hugoniot, suggesting little temperature effect on the pressure–volume relation. The hydrostatic data suggest that β(monoclinic)-HMX undergoes two phase transitions: (i) a conformational transition at 12 GPa with no apparent abrupt volume change and (ii) a discontinuo...

Phase Transformations in Carbon Fullerenes at High Shock Pressures

C60 powders were shock-compressed quasi-isentropically and quenched from pressures in the range 10 to 110 GPa (0.1 to 1.1 Mbar). Recovered specimens were analyzed by Raman spectroscopy and optical microscopy. C60 fullerenes are stable into the 13- to 17-GPa pressure range. The onset of a fast (∼0.5 �s) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 600�C is relatively well ordered with crystal planar domain size of about 100 �. Above 50 GPa a continuous transformation to an amorphous state is observed in recovered specimens. The fast transformation to graphite is proposed to occur by π-electron rehybridization which initiates breakup of the ball structure and formation of the graphite structure at high density.

Anisotropic shock sensitivity and detonation temperature of pentaerythritol tetranitrate single crystal

Shock temperatures of pentaerythritol tetranitrate (PETN) single crystals have been measured by using a nanosecond time-resolved spectropyrometric system operated at six discrete wavelengths between 350 and 700 nm. The results show that the shock sensitivity of PETN is strongly dependent on the crystal orientation: Sensitive along the shock propagation normal to the (110) plane, but highly insensitive normal to the (100) plane. The detonation temperature of PETN is, however, independent from the crystal orientation and is determined to be 4140 (±70) K. The time-resolved data yielding the detonation velocity 8.28 (±0.10) mm/μs can be interpreted in the context of a modified thermal explosion model.

Electronic structure and magnetism in compressed 3d transition metals

The authors present a systematic study of high-pressure effects on electronic structure and magnetism in 3d transition metals (Fe, Co, and Ni) based on x-ray magnetic circular dichroism measurements. The data show that the net magnetic moment in Fe vanishes above 18GPa upon the transition to hcp Fe, while both cobalt and nickel remain ferromagnetic to well over 100GPa. The authors estimate the total disappearance of moment in hcp Co at around 150GPa and predict a nonmagnetic Ni phase above 250GPa. The present data suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced broadening of the 3d valence bands.

Diamondlike metastable carbon phases from shock-compressed C60 films

Thin films of C60, 2.5‐μm, have been shocked isentropically to 69 GPa, about 2200 K, and thermally quenched at rates up to 1011 K/s. The recovered specimen is transparent with a crystallographic habit, or ‘‘tilelike’’ structure, but it slowly transforms to a black highly disordered carbon at the ambient condition. The selected area electron diffraction patterns suggest that the transparent carbon phase contains an amorphous cubic diamond and n‐diamond crystallites sized 50–350 A in diameter.

Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots

The highest superconducting temperature Tc observed in any elemental metal (Li with Tc approximately 18-20 K at pressure 35-48 GPa) is shown to arise from increasingly strong electron-phonon coupling concentrated along intersections of Kohn anomaly surfaces with the evolving Fermi surface. First-principles linear response calculations of the phonon spectrum and spectral function alpha2F(omega) reveal very strong Q- and phonon-polarization dependence of coupling strength, resulting in values of in the observed range. The sharp momentum dependence of the coupling even for the simple Li Fermi surface indicates more generally that a fine Q mesh is required for precise evaluation of lamda.

Dynamic diamond anvil cell (dDAC): A novel device for studying the dynamic-pressure properties of materials

We have developed a unique device, a dynamic diamond anvil cell (dDAC), which repetitively applies a time-dependent load/pressure profile to a sample. This capability allows studies of the kinetics of phase transitions and metastable phases at compression (strain) rates of up to 500 GPa/s (approximately 0.16 s(-1) for a metal). Our approach adapts electromechanical piezoelectric actuators to a conventional diamond anvil cell design, which enables precise specification and control of a time-dependent applied load/pressure. Existing DAC instrumentation and experimental techniques are easily adapted to the dDAC to measure the properties of a sample under the varying load/pressure conditions. This capability addresses the sparsely studied regime of dynamic phenomena between static research (diamond anvil cells and large volume presses) and dynamic shock-driven experiments (gas guns, explosive, and laser shock). We present an overview of a variety of experimental measurements that can be made with this device.

dhcp as a possible new ϵ′ phase of iron at high pressures and temperatures☆

Abstract Using in-situ X-ray diffraction and ab initio theoretical calculations, we have identified a possible new phase of iron, ϵ′-Fe, in the stability field of what was previously known for ϵ(hcp)- and γ(fcc)-Fe. The crystal structure of ϵ′-Fe is indexed to a dhcp structure with an ABAC stacking sequence. The ϵ′-phase exists at temperatures lower than the γ-phase and at pressures between 15 and 40 GPa, which differs from the stability field previously suggested for β-Fe above 40 GPa.