Articles

April 3, 2018 05:20 PM

Recent discoveries of several stable dense hydrous minerals at high pressure–temperature conditions have led to an important implication of a massive water reservoir in the Earth’s lower mantle. A research team used HPCAT facility and demonstrated that (Fe,Al)OOH can be stabilized in a hexagonal lattice at 107–136 GPa and 2,400 K. By combining powder X-ray-diffraction techniques with multigrain indexation, the team has determined this hexagonal hydrous phase with a = 10.5803(6) Å and c = 2.5897(3) Å at 110 GPa, which is stable under the deep lower mantle conditions and can transform to the cubic pyrite structure at low T with the same density. The hexagonal phase can be produced when δ-AlOOH in the subducting slabs incorporates FeOOH under the deep lower mantle conditions. Subducting along with the continuous slab penetration, the hexagonal phase might accumulate at the bottom of the lower mantle due to its ultrahigh density.

March 19, 2018 12:43 PM

By engineering molecules with mechanically heterogeneous components with a compressible (‘soft’) mechanophore and incompressible (‘hard’) ligands, a research team has created ‘molecular anvils’, resulting in isotropic stress that leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. X-ray experiments, including the use of x-ray absorption measurements at HPCAT, demonstrate hydrostatic-pressure-driven redox reactions in metal–organic chalcogenides, incorporating molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal–chalcogen bonds, leading to the formation of the elemental metal.

March 19, 2018 12:09 PM

Topological Kondo insulator possesses characteristics of both the strong electron correlations and the topological configurations, as shown in a mixed-valence material SmB6. YbB6 is a structural analog of SmB6, but remains controversial whether compressed YbB6 material is a topological insulator or a Kondo topological insulator. A research team used HPCAT facility and performed x-ray measurements on YbB6 samples. Both the high-pressure powder x-ray diffraction and optical Raman measurements show no trace of structural phase transitions in YbB6 up to 50 GPa. The x-ray absorption measurements reveal a gradual change of Yb valence from nonmagnetic Yb2+ to magnetic Yb3+ above 18 GPa concomitantly with the increase in resistivity.

February 13, 2018 08:07 PM

At temperatures (<319 K or <46 oC) where life is sustained, water is an abnormal liquid, having a number of anomalous properties. For instance, water displays minima of isobaric heat capacity at 308 K and isothermal compressibility at 319 K which are related to entropy and density fluctuations, respectively. It has been widely accepted that water’s anomalies are not a result of simple thermal fluctuation, but are connected to the formation of various structural aggregates in the hydrogen bonding network. To understand water’s anomalous behavior, a two-liquid model with a high-density liquid (HDL) and a low-density liquid (LDL) has been proposed from theoretical simulations. However, it has been experimentally challenging to probe the region of the phase diagram of H2O, often referred to as water’s no man’s land, where the LDL phase is expected to occur.

February 7, 2018 02:41 PM

Understanding the behavior of solids under shock compression, including transformations, their pathways, and kinetics, lies at the core of contemporary static and dynamic compression science. A team led by scientists from Sandia National Laboratories is leveraging the capabilities of two sectors of the APS, HPCAT and DCS, for real-time observations of the kinetics of a shock-driven phase transition in a simple ionic solid, CaF2, a model structure in high-pressure physics.

February 7, 2018 03:27 PM

Knowledge on the structure and properties of silicate magma under extreme pressure plays an important role in understanding the nature and evolution of Earth’s deep interior.  However, such information is scarce owing to experimental challenges.  Using a recently developed double-stage Paris-Edinburgh press, combined with the multi-angle energy dispersive X-ray diffraction, a team has measured structures of MgSiO3 glass up to 111 GPa. The results revealed direct experimental evidence of a structural change in this glass at >88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO6 structural motifs similar to those of the crystalline MgSiO3 postperovskite, with densely packed oxygen atoms. The pressure of the structural change is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO3.

January 29, 2018 07:26 PM

Single crystal diamond is the hardest known material and widely used in studies on materials under extreme conditions. Nanocrystalline diamond (NCD) possesses hardness comparable to that of single crystal diamond, while also demonstrating increased fracture toughness and yield strength. Thus, NCD’s is a candidate material as second stage anvils to extend the maximum pressure in static high pressure technology. A research team, using Microwave Plasma Chemical Vapor Deposition, has successfully grown NCD on single crystal diamond anvil surface (Figure). The team used HPCAT facility and is able to generate static pressure of 500 GPa (0.5 TPa) on a tungsten sample as measured by synchrotron x-ray diffraction using the grown NCD as second stage micro-anvils.

January 10, 2018 06:09 PM

At sufficiently high pressure, hydrogen is believed to become a monatomic metal with exotic electronic properties. Because of the very high pressures required to create such states, hydrogen-rich compounds have been considered alternative materials that could exhibit many of the properties of atomic metallic hydrogen, such as very high-temperature superconductivity, but at accessible pressures. With the help of theoretical predictions, a research team using the HPCAT facility has successfully synthesized superhydrides with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH10 having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure undergoes a rhombohedral distortion of the La sublattice.

January 10, 2018 05:53 PM

Nanotwinning is known as a highly effective approach for strengthening structural materials and impeding the degradation of mechanical properties. Recently a major breakthrough was realized when nanotwinned cubic-BN (nt-c-BN) and diamond (nt-diamond) were successfully synthesized from onion-like nanoparticle precursors under high pressure conditions. Understanding the microscopic origin of the twin boundaries, and the formation of such from onion-like precursors, are therefore critically important and can provide guidance to the production of nt-diamond at a larger scale. A research team has studied the nucleation mechanism of nt-diamond samples using multiple experimental and theoretical methods, including the synchrotron diffraction at HPCAT. By a direct high-pressure high-temperature synthesis of nanotwinned diamond from onion carbon without high-density defects, the team has obtained nanotwinned diamond possessing an exceptionally high Vickers hardness of 215 GPa at 4.9 N.