Research Highlight

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. The structural transformation from onion carbon to nt-diamond is shown to be a martensitic process, in which the high-density defects may not be necessary for the formation of nanotwinning, but they do play a role in lowering the onset of the transition pressure. Specifically, the appearance of {111} nanotwinned structure and stacking faults was determined by the characteristics of the onion shells, while the accumulation of the stress due to the sliding of the shells cause the crystal to re-align along the shear direction. These findings not only clarify the direct transformation mechanism from onion-like precursors to nanotwinned diamond, but also have broad implications for further exploration of new materials with exceptional properties. More in H. Tang, et al., “Revealing the formation mechanism of ultrahard nanotwinned diamond from onion carbon”, Carbon, 129, 2018.

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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. The superhydride phases consist of an atomic hydrogen sublattice with H−H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized. More in Z.M. Geballe, et al, “Synthesis and Stability of Lanthanum Superhydrides”, Angew. Chem., 688, 2018.

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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. Atomic force microscopy analysis after decompression from ultrahigh pressures showed that the detachment of the NCD stage occurred in the bulk of the SCD and not at the interface, suggesting significant adhesive bond strength between nanocrystalline and single crystal diamond anvil. More in S.L. Moore, et al., Sci. Rep., 8, 1402, 2018.

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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. Considering similarities in pressure-induced structural changes between silicate melts and glasses, a similar ultrahigh-pressure structural change may occur in MgSiO3 melts in the deep lower mantle. These results suggest that the ultrahigh-pressure structural change may occur in silicate melts above the CMB, with significant densification and potentially profound influence on the dynamics of melt storage and circulation in the Earth’s deep interior. More in Kono et al., ‘Pressure-induced structural change in MgSiO3 glass at pressures near the Earth’s core-mantle boundary’, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1716748115.

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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.

Traditionally, shock compression research infers phase transitions from continuum-level measurements and uses corresponding static compression experiments, shock-recovery studies, or calculations to deduce their response during a shock event. The advent of synchrotron facilities where shock compression is coupled with real-time monitoring using an x-ray beam now allows for uncovering what occurs during the nanosecond time-scale when a shock wave propagates inside of a solid.

At DCS, synchrotron x-ray diffraction is coupled with plate impact launchers and photonic Doppler velocimetry is used to follow, in real time, the unfolding of a phase transition in shock-compressed CaF2. The DCS results are compared with diamond anvil cell studies at HPCAT, where x-ray diffraction under static compression and high temperatures is designed to mimic the states achieved in shock compression. The results of this work give insight into the kinetic time scale of the fluorite-to-cotunnite phase transition in under shock compression, which is relevant to a number of isomorphic compounds. A direct comparison of unit cell volumes between dynamic and static loading points to measurable structural effects of temperature on increased shock loading. Such cross-platform comparisons provide understanding of phase transitions at different time scales that improves our capability to simulate materials at extreme conditions.

This work is a multidisciplinary collaboration led by scientists from Sandia National Laboratories in collaboration with Washington State University, the University of Nevada Las Vegas and the Carnegie Institution of Washington. More in P. Kalita et al., Phys. Rev. Lett. 119, 255701, 2017.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA-0003525.

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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. A team at HPCAT overcame the experimental challenge by adopting a new kinetic pathway to access the region of LDL via decompression of a high-pressure crystal. In contrast to the liquid water, the LDL is found to have the tetrahedral network fully developed. The relationship of LDA, LDL, and supercooled liquid can be used to explain water’s anomalous properties. At high temperatures above ∼319 K, water exhibits normal liquid behavior (dominated by HDL), in that both heat capacity and compressibility increase as it is heated. As water is cooled from 319 K down to supercooled temperatures, the structural fluctuations between LDL and HDL may cause the anomalous properties of water. The anomalous region with structural fluctuations may extend down to around the Widom line. At temperatures far below the Widom line, water may exhibit normal liquid again dominated by the fully developed tetrahedrally structured LDL observed in this study. More in C. Lin et al. Proc. Nat. Acad. Sci., doi.org/10.1073/pnas.1716310115, (2018)

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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 YbB6samples. 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. A complete transition to insulating state occurs at pressures above 30 GPa, accompanied by an increase in the shear stress and anomalies in the pressure dependence of the Raman T2g mode and in the B atomic position. The resistivity at high pressures can be described by a model taking into account coexisting insulating and metallic channels with the activation energy for the insulating channel about 30 meV. These results indicate that YbB6 may become a topological Kondo insulator at high pressures above 35 GPa. More in J. Ying et al., Phys Rev. B, 97, 121101(R), 2018.

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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. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis. More in H. Yan, et al, Nature, 554, 505-510, 2018.

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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. As a result, there could be a substantial quantity of water in the deep lower mantle. More in L. Zhang et al, Proc. Nat. Acad. Sci. 115, 12, 2908-2911

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When compressed above megabar pressures (>100 GPa), glasses may undergo structural transitions into more densely packed networks that differ from those at ambient pressure. Inelastic x-ray scattering (IXS, or X-ray Raman scattering), which can probe core electron excitation from glasses in a diamond anvil cell (DAC), has enabled exploration of the pressure-induced changes in atomic configurations of elements in oxide glasses under extreme compression. While IXS provides a rare opportunity to probe the pressure-induced bonding transitions, a decade of efforts to collect an IXS signal from any matters beyond 100 GPa have not been successful, because of the inherent challenges of IXS techniques (with a signal intensity several orders of magnitude smaller than that of elastic x-rays) and the increasing background signals from the gaskets and anvils.

While the signal reduction of incident and scattered photons is inevitable, a polycapillary post-sample collimator with improved x-ray optics provides a new opportunity to collect the signal primarily from the sample, with significantly reduced background signals. This provides the potential to explore details of structural transitions in glasses at megabar pressures. Using the enhanced capability at HPCAT, a research group measured IXS spectra for prototypical B2O3 glasses at high pressure up to ∼120 GPa, where it is found that only four-coordinated boron ([4]B) is prevalent. The reduction in the [4]B–O length up to 120 GPa is minor, indicating the extended stability of sp3-bonded [4]B. In contrast, a substantial decrease in the average O–O distance upon compression is revealed, suggesting that the densification in B2O3 glasses is primarily due to O–O distance reduction without the formation of [5]B. Together with earlier results with other archetypal oxide glasses, such as SiO2 and GeO2, these results confirm that the transition pressure of the formation of highly coordinated framework cations systematically increases with the decreasing atomic radius of the cations. These observations highlight a new opportunity to study the structure of oxide glass above megabar pressures, yielding the atomistic origins of densification in melts at extreme conditions such as at the Earth’s core– mantle boundary. More in Lee et al, PNAS,

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