A miniature multi-anvil apparatus using diamond as anvils-MDAC: Multi-axis diamond anvil cell.

The diamond anvil cell (DAC) has been widely used in high-pressure research. Despite significant progress over the past five decades, the opposed anvil geometry in the DAC inevitably leads to a disk-shaped sample configuration at high pressure. This intrinsic limitation is largely responsible for the large pressure and temperature gradients in the DAC, which often compromise precise experiments and their characterizations.

Imaging of the electronic bonding of diamond at pressures up to 2 million atmospheres.

Diamond shows unprecedented hardness. Because hardness is a measure of resistance of chemical bonds in a material to external indentation, the electronic bonding nature of diamond beyond several million atmospheres is key to understanding the origin of hardness. However, probing the electronic structures of diamond at such extreme pressure has not been experimentally possible.

Modulating the Extent of Anisotropic Cuprophilicity via High Pressure with Piezochromic Luminescence Sensitization.

Metallophilicity has been widely studied as a fundamental supramolecular interaction. However, the extent and directionality thereof remain controversial. A major obstacle lies in the difficulty to separately control the geometry and chemical composition.

Direct Observation of the Pressure-Induced Structural Variation in Gold Nanoclusters and the Correlated Optical Response.

The ability to gradually modify the atomic structures of nanomaterials and directly identify such structural variation is important in nanoscience research. Here, we present the first example of a high-pressure single-crystal X-ray diffraction analysis of atomically precise metal nanoclusters. The pressure-dependent, subangstrom structural evolution of an ultrasmall gold nanoparticle, AuS, has been directly identified.

Overview of HPCAT and capabilities for studying minerals and various other materials at high-pressure conditions.

High-Pressure Collaborative Access Team (HPCAT) is a synchrotron-based facility located at the Advanced Photon Source (APS). With four online experimental stations and various offline capabilities, HPCAT is focused on providing synchrotron x-ray capabilities for high pressure and temperature research and supporting a broad user community. Overall, the array of online/offline capabilities is described, including some of the recent developments for remote user support and the concomitant impact of the current pandemic.

Electron Density Changes across the Pressure-Induced Iron Spin Transition.

High-pressure single-crystal x-ray diffraction is used to experimentally map the electron-density distribution changes in (Fe,Mg)O as ferrous iron undergoes a pressure-induced transition from high- to low-spin states. As the bulk density and elasticity of magnesiowüstite-one of the dominant mineral phases of Earth's mantle-are affected by this electronic transition, our results have applications to geophysics as well as to validating first-principles calculations. The observed changes in diffraction intensities indicate a spin-transition-induced change in orbital occupancies of the Fe ion in general accord with crystal-field theory, illustrating the use of electron density measurements for characterizing high-pressure d-block chemistry and motivating further studies characterizing chemical bonding under pressure.

Pressure-Driven Changes in the Electronic Bonding Environment of GeO Glass above Megabar Pressures.

Noncrystalline oxides under pressure undergo gradual structural modifications, highlighted by the formation of a dense noncrystalline network topology. The nature of the densified networks and their electronic structures at high pressures may account for the mechanical hardening and the anomalous changes in electromagnetic properties. Despite its importance, direct probing of the electronic structures in amorphous oxides under compression above the Mbar pressure (>100 GPa) is currently lacking.

Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90 GPa.

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information in situ under high pressures in a diamond-anvil cell.

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures.

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques.

Structural Evolution of SiO_{2} Glass with Si Coordination Number Greater than 6.

Pair distribution function measurement of SiO_{2} glass up to 120 GPa reveals changes in the first-, second-, and third-neighbor distances associated with an increase in Si coordination number C_{Si} to >6 above 95 GPa. Packing fractions of Si and O determined from the first- and second-neighbor distances show marked changes accompanied with the structural evolution from C_{Si}=6 to >6. Structural constraints in terms of ionic radius ratio of Si and O, and ratio of nonbonded radius to bonded Si─O distance support the structural evolution of SiO_{2} glass with C_{Si}>6 at high pressures.

Structural Changes in Liquid Lithium under High Pressure.

We have experimentally studied the effect of compression on the structure of liquid lithium (Li) by multiangle energy dispersive X-ray diffraction in a large-volume cupped-Drickamer-Toroidal cell. The structure factors, (), of liquid Li have been successfully determined under an isothermal compression at 600 ± 30 K and at pressures up to 11.5 GPa.

Nitrogen in black phosphorus structure.

Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus-structured nitrogen at 146 GPa and 2200 K.

Oxygen Quadclusters in SiO_{2} Glass above Megabar Pressures up to 160 GPa Revealed by X-Ray Raman Scattering.

As oxygen may occupy a major volume of oxides, a densification of amorphous oxides under extreme compression is dominated by reorganization of oxygen during compression. X-ray Raman scattering (XRS) spectra for SiO_{2} glass up to 1.6 Mbar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the potential emergence of quadruply coordinated oxygen (oxygen quadcluster).

Observation of 9-Fold Coordinated Amorphous TiO at High Pressure.

Knowledge of the structure in amorphous dioxides is important in many fields of science and engineering. Here we report new experimental results of high-pressure polyamorphism in amorphous TiO (a-TiO). Our data show that the Ti coordination number (CN) increases from 7.

Real time study of grain enlargement in zirconium under room-temperature compression across the α to ω phase transition.

We report a synchrotron Laue diffraction study on the microstructure evolution in zirconium (Zr) as it undergoes a pressure-driven structural phase transformation, using a recently developed real time scanning x-ray microscopy technique. Time resolved characterizations of microstructure under high pressure show that Zr exhibits a grain enlargement across the α-Zr to ω-Zr structural phase transition at room-temperature, with nucleation and growth of ω-Zr crystals observed from initially a nano-crystalline aggregate of α-Zr. The observed grain enlargement is unusual since the enlargement processes typically require substantially high temperature to overcome the activation barriers for forming and moving of grain boundaries.

Ultrahigh-pressure isostructural electronic transitions in hydrogen.

High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal, which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate. Therefore, understanding such transitions remains an important objective in condensed matter physics. However, measurements of the crystal structure of solid hydrogen, which provides crucial information about the metallization of hydrogen under compression, are lacking for most high-pressure phases, owing to the considerable technical challenges involved in X-ray and neutron diffraction measurements under extreme conditions.

In situ x-ray diffraction study of polyamorphism in HO under isothermal compression and decompression.

Amorphous-amorphous transformations in HO have been studied under rapid isothermal compression and decompression in a diamond anvil cell together with in situ x-ray diffraction measurements using brilliant synchrotron radiation. The experimental pathways provide a density-driven approach for studying polyamorphic relations among low-, high-, and very high-density amorphs (LDA, HDA, VHDA) in a pressure range of 0-3.5 GPa at temperatures of 145-160 K.

Compressed glassy carbon maintaining graphite-like structure with linkage formation between graphene layers.

Amorphous diamond, formed by high-pressure compression of glassy carbon, is of interests for new carbon materials with unique properties such as high compressive strength. Previous studies attributed the ultrahigh strength of the compressed glassy carbon to structural transformation from graphite-like sp-bonded structure to diamond-like sp-bonded structure. However, there is no direct experimental determination of the bond structure of the compressed glassy carbon, because of experimental challenges.

Multimode scanning X-ray diffraction microscopy for diamond anvil cell experiments.

We have designed and implemented a new experimental system for fast mapping of crystal structures and other structural features of materials under high pressure at the High Pressure Collaborative Access Team, Sector 16 of the Advanced Photon Source. The system utilizes scanning X-ray diffraction microscopy (SXDM) and is optimized for use with diamond anvil cell devices. In SXDM, the X-ray diffraction (XRD) is collected in a forward scattering geometry from points on a two-dimensional grid by fly-scanning the sample with respect to a micro-focused X-ray beam.

Fly scan apparatus for high pressure research using diamond anvil cells.

The hardware and software used to execute fly scans at Sector 16 of the Advanced Photon Source are described. The system design and capabilities address dimensions and time scales relevant to samples in high pressure diamond anvil cells. The time required for routine sample positioning and centering is significantly reduced, and more importantly, the time savings associated with fly scanning make it feasible for users to routinely generate two-dimensional x-ray transmission and x-ray diffraction maps.

Pressure induced transformation and subsequent amorphization of monoclinic NbO and its effect on optical properties.

Pressure-induced phase transitions of monoclinic H-NbO have been studied by in situ synchrotron x-ray diffraction, pair distribution function (PDF) analysis, and Raman and optical transmission spectroscopy. The initial monoclinic phase is found to transform into an orthorhombic phase at ~9 GPa and then change to an amorphous form above 21.4 GPa.

A CO laser heating system for high pressure-temperature experiments at HPCAT.

We present a CO laser heating setup for synchrotron x-ray diffraction inside a diamond anvil cell, situated at HPCAT (Sector 16, Advanced Photon Source, Argonne National Lab, Illinois, USA), which is modular and portable between the HPCAT experiment hutches. The system allows direct laser heating of wide bandgap insulating materials to thousands of degrees at static high pressures up to the Mbar regime. Alignment of the focused CO laser spot is performed using a mid-infrared microscope, which addressed past difficulties with aligning the invisible radiation.

Amorphous boron oxide at megabar pressures via inelastic X-ray scattering.

Structural transition in amorphous oxides, including glasses, under extreme compression above megabar pressures (>1 million atmospheric pressure, 100 GPa) results in unique densification paths that differ from those in crystals. Experimentally verifying the atomistic origins of such densifications beyond 100 GPa remains unknown. Progress in inelastic X-ray scattering (IXS) provided insights into the pressure-induced bonding changes in oxide glasses; however, IXS has a signal intensity several orders of magnitude smaller than that of elastic X-rays, posing challenges for probing glass structures above 100 GPa near the Earth's core-mantle boundary.

Experimental evidence of low-density liquid water upon rapid decompression.

Water is an extraordinary liquid, having a number of anomalous properties which become strongly enhanced in the supercooled region. Due to rapid crystallization of supercooled water, there exists a region that has been experimentally inaccessible for studying deeply supercooled bulk water. Using a rapid decompression technique integrated with in situ X-ray diffraction, we show that a high-pressure ice phase transforms to a low-density noncrystalline (LDN) form upon rapid release of pressure at temperatures of 140-165 K.