Hyunchae Cynn

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...

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.

Experimental method for in situ determination of material textures at simultaneous high pressure and high temperature by means of radial diffraction in the diamond anvil cell

We introduce the design and capabilities of a resistive heated diamond anvil cell that can be used for side diffraction at simultaneous high pressure and high temperature. The device can be used to study lattice-preferred orientations in polycrystalline samples up to temperatures of 1100 K and pressures of 36 GPa. Capabilities of the instrument are demonstrated with preliminary results on the development of textures in the bcc, fcc, and hcp polymorphs of iron during a nonhydrostatic compression experiment at simultaneous high pressure and high temperature.

Irreversible xenon insertion into a small-pore zeolite at moderate pressures and temperatures

Pressure drastically alters the chemical and physical properties of materials and allows structural phase transitions and chemical reactions to occur that defy much of our understanding gained under ambient conditions. Particularly exciting is the high-pressure chemistry of xenon, which is known to react with hydrogen and ice at high pressures and form stable compounds. Here, we show that Ag16Al16Si24O8·16H2O (Ag-natrolite) irreversibly inserts xenon into its micropores at 1.7 GPa and 250 °C, while Ag(+) is reduced to metallic Ag and possibly oxidized to Ag(2+). In contrast to krypton, xenon is retained within the pores of this zeolite after pressure release and requires heat to desorb. This irreversible insertion and trapping of xenon in Ag-natrolite under moderate conditions sheds new light on chemical reactions that could account for the xenon deficiency relative to argon observed in terrestrial and Martian atmospheres.

The phase diagram of cobalt at high pressure and temperature: the stability of -cobalt and new -cobalt

A metastable dhcp phase of cobalt, , has been discovered below 60 GPa by using in situ x-ray diffraction and a laser-heated diamond-anvil cell. The volume at 34 GPa is with = 3.190. First-principles theory shows that and are close in energy below 60 GPa, and that temperature and magnetic effects can make dhcp-Co more stable than hcp-Co. The -phase is found to be stable and quenchable over a wide range of pressure and temperature. New constraints for the phase diagram of Co are presented.

Distinct thermal behavior of GeO2 glass in tetrahedral, intermediate, and octahedral forms

One fascinating high-pressure behavior of tetrahedral glasses and melts is the local coordination change with increasing pressure, which provides a structural basis for understanding numerous anomalies in their high-pressure properties. Because the coordination change is often not retained upon decompression, studies must be conducted in situ. Previous in situ studies have revealed that the short-range order of tetrahedrally structured glasses and melts changes above a threshold pressure and gradually transforms to an octahedral form with further pressure increase. Here, we report a thermal effect associated with the coordination change at given pressures and show distinct thermal behaviors of GeO2 glass in tetrahedral, octahedral, and their intermediate forms. An unusual thermally induced densification, as large as 16%, was observed on a GeO2 glass at a pressure of 5.5 gigapascal (GPa), based on in situ density and x-ray diffraction measurements at simultaneously high pressures and high temperatures. The large thermal densification at high pressure was found to be associated with the 4- to 6-fold coordination increase. Experiments at other pressures show that the tetrahedral GeO2 glass displayed small thermal densification at 3.3 GPa arising from the relaxation of intermediate range structure, whereas the octahedral glass at 12.3 GPa did not display any detectable thermal effects.

High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase of KFe 2 As 2 under high pressure

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low-pressure regime, we show that the superconducting transition temperature Tc finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum Tc and reversal of pressure dependence at this pressure. Upon applying higher pressures, this Tc is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band structure calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe2As2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-Tc superconductivity in iron pnictide superconductors.

High pressure crystal structure of PrN

Compression of PrN yields a phase transformation to a tetragonal structure with ~8.8 % volume collapse at ~40 GPa at ambient temperature. A refinement reveals a distorted CsCl-like structure for the high pressure phase PrN(II), which is different from the high pressure phases seen among other lanthanide monopnictides. The space group of the new structure is P4/nmm (#129) with Pr in the 2c(0,1/2,0.3546) and N in the 2a(0,0,0) positions. PrN(II) persists to 85 GPa.

PHASE TRANSITION AND DECOMPOSITION OF 90% HYDROGEN PEROXIDE AT HIGH PRESSURES

Physical and chemical changes of 90 wt% hydrogen peroxide have been investigated to pressures of 12 GPa by using a diamond-anvil cell, synchrotron x-ray diffraction, and Raman spectroscopy. Hydrogen peroxide freezes at 1.5 GPa and ambient temperature into a tetragonal structure (P41212, Z=4, denoted as H2O2-I. This is the same transition that has previously been reported in this material at 253 K. The unit cell parameters at 6.3 GPa are a=3.759 A, c=7.397 A, and V=15.74 cm3/mol, representing 21% compression from that at ambient pressure. H2O2-I has been found to transform into a high pressure phase H2O2-II at 7.5 GPa, and it decomposes into water and oxygen at the onset of melting, which may be incongruent. In contrast to water, hydrogen peroxide exhibits a relatively simple polymorphism and a positive initial slope of the melting curve at high pressures.

Titanium Alloys at Extreme Pressure Conditions

The electronic structures of the early transition metals are characterised by the relationship that exists between the occupied narrow d bands and the broad sp bands. Under pressure, the sp bands rise faster in energy, causing electrons to be transferred to the d bands (Gupta et al., 2008). This process is known as the s-d transition and it governs the structural properties of the transition metals. At ambient conditions, pure Ti crystallizes in the 2-atom hcp, or  phase crystal structure (space group P63/mmc) and has an axial ratio (c/a) ~ 1.58. Under pressure, the  phase undergoes a martensitic transformation at room temperature (RT) into the 3-atom hexagonal, or  phase structure (space group P6/mmm). The appearance of the ω phase at high pressure raises a number of scientific and engineering issues mainly because the  phase appears to be fairly brittle compared with the  phase, and this may significantly limit the use of Ti in high pressure applications. Furthermore, after pressure treatment the ω phase appears to be fully, or at least, partially recoverable at ambient conditions, thus raising questions as to which is the lowest thermodynamically stable crystallographic phase of Ti at RT and pressure.