equation of state of boron carbide.

We report the equation of state measurements of BC to 50 GPa and approximately 2500 K in laser-heated diamond anvil cells. We obtain an ambient temperature, third-order Birch-Murnaghan fit to the data that yields a bulk modulus of 221(2) GPa and derivative, (d/d) of 3.3(1).

White Laue and powder diffraction studies to reveal mechanisms of HCP-to-BCC phase transformation in single crystals of Mg under high pressure.

Mechanisms of hexagonal close-packed (HCP) to body-centered cubic (BCC) phase transformation in Mg single crystals are observed using a combination of polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques under quasi-hydrostatic pressures of up to 58 ± 2 GPa at ambient temperature. Although experiments were performed with both He and Ne pressure media, crystals inevitably undergo plastic deformation upon loading to 40-44 GPa. The plasticity is accommodated by dislocation glide causing local misorientations of up to 1°-2°.

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.

Evidence for superionic HO and diffusive He-HO at high temperature and high pressure.

We present the evidence of superionic phase formed in HO and, for the first time, diffusive HO-He phase, based on time-resolved x-ray diffraction experiments performed on ramp-laser-heated samples in diamond anvil cells. The diffraction results signify a similar bcc-like structure of superionic HO and diffusive He-HO, while following different transition dynamics. Based on time and temperature evolution of the lattice parameter, the superionic HO phase forms gradually in pure HO over the temperature range of 1350-1400 K at 23 GPa, but the diffusive He-HO phase forms abruptly at 1300 K at 26 GPa.

Solid-State Pathway Control via Reaction-Directing Heteroatoms: Ordered Pyridazine Nanothreads through Selective Cycloaddition.

Nanothreads are one-dimensional nanomaterials composed of a primarily sp hydrocarbon backbone, typically formed through the compression of small molecules to high pressures. Although nanothreads have been synthesized from a range of precursors, controlling reaction pathways to produce atomically precise materials remains a difficult challenge. Here, we show how heteroatoms within precursors can serve as "thread-directing" groups by selecting for specific cycloaddition reaction pathways.

X-ray diffraction and equation of state of the C-S-H room-temperature superconductor.

X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room-temperature superconductor is derived from previously established van der Waals compounds found in the HS-H and CH-H systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique, leading to the superconducting critical temperature T of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the AlCu-type determined for (HS)H and (CH)H that differs from those predicted and observed for the S-H system at these 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.

Helium-hydrogen immiscibility at high pressures.

Hydrogen and helium are the most abundant elements in the universe, and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells, we performed experiments on H-He 1:1 and D-He 1:10 compressed gas mixtures up to 100 GPa at 300 K.

Studies of Multiferroic Palladium Perovskites.

We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd and Pd substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices.

Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures.

Recent predictions and experimental observations of high T_{c} superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including four-probe electrical transport measurements that display significant drops in resistivity on cooling up to 260 K and 180-200 GPa, and resistivity transitions at both lower and higher temperatures in other experiments.

Synthesis and Stability of Lanthanum Superhydrides.

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. X-ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized 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 LaH having cages of thirty-two hydrogen atoms surrounding each La atom.

Persistence of Mixed and Non-intermediate Valence in the High-Pressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X-ray Diffraction (XRD), and Density Functional Theory (DFT) Study.

The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgAgO, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.

Structural, vibrational, and electronic properties of BaReH under pressure.

We present a study of the high-pressure behavior of BaReH, a novel hydrogen-rich compound, using optical, Raman, and infrared spectroscopy as well as synchrotron x-ray diffraction. The x-ray diffraction measurements demonstrate that BaReH retains its hexagonal structure on room temperature compression up to 40 GPa. Optical absorption shows the absence of a gap closure to 80 GPa.

Hydrogen-Stuffed, Quartz-like Water Ice.

Through use of in situ Raman spectroscopy and single-crystal/powder X-ray diffraction, we resolve the "C" phase structure discovered recently in the H + HO system. This phase forms at ∼400 MPa and 280 K with the nominal composition (HO)H and three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H molecules, and shows topological similarities with the mineral quartz.

Backbone NxH compounds at high pressures.

Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first-principles theoretical structure predictions to investigate mixtures of N2 and H2 up to 55 GPa. Our experiments show the formation of structurally complex van der Waals compounds [see also D. K.

Structure and stability of solid Xe(H2)n.

Mixtures of xenon and molecular hydrogen form a series of hexagonal, van der Waals compounds at high pressures and at 300 K. Synchrotron, x-ray, single crystal diffraction studies reveal that below 7.5 GPa, Xe(H2)8 crystallizes in a P3̄m1 structure that displays pressure-induced occupancy changes of two pairs of xenon atoms located on the 2c and 2d sites (while the third pair on yet another 2c site remains fully occupied).

Unexpected stable stoichiometries of sodium chlorides.

Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. As a result of the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding (with 1:1 stoichiometry dictated by charge balance) and B1-type crystal structure. By combining theoretical predictions and diamond anvil cell experiments, we found that new materials with different stoichiometries emerge at high pressures.

Novel cooperative interactions and structural ordering in H2S-H2.

Hydrogen sulfide (H(2)S) and hydrogen (H(2)) crystallize into a 'guest-host' structure at 3.5 GPa and, at the initial formation pressure, the rotationally disordered component molecules exhibit weak van der Waals-type interactions. With increasing pressure, hydrogen bonding develops and strengthens between neighboring H(2)S molecules, reflected in a pronounced drop in S-H vibrational stretching frequency and also observed in first-principles calculations.

Polymorphism of dense, hot oxygen.

The phase diagram and polymorphism of oxygen at high pressures and temperatures are of great interest to condensed matter and earth science. X-ray diffraction and Raman spectroscopy of oxygen using laser and resistively heated diamond anvil cells reveal that the molecular high-pressure phase ε-O(2), which consists of (O(2))(4) clusters, reversibly transforms in the pressure range of 44 to 90 GPa and temperatures near 1000 K to a new phase with higher symmetry. The data suggest that this new phase (η') is isostructural to a phase η reported previously at lower pressures and temperatures, but differs from it in the P-T range of stability and type of intermolecular association.

Brillouin scattering of H2O ice to megabar pressures.

The sound velocity in polycrystalline ice was measured as a function of pressure at room temperature to 100 GPa, through the phase field of ice VII and crossing the ice X transition, by Brillouin scattering in order to examine the elasticity, compression mechanism, and structural transitions in this pressure range. In particular, we focused on previously proposed phase transitions below 60 GPa. Throughout this pressure range, we find no evidence for anomalous changes in compressibility, and the sound velocities and elastic moduli do not exhibit measurable discontinuous shifts with pressure.

Bonding changes in hot fluid hydrogen at megabar pressures.

Raman spectroscopy in laser-heated diamond anvil cells has been employed to probe the bonding state and phase diagram of dense hydrogen up to 140 GPa and 1,500 K. The measurements were made possible as a result of the development of new techniques for containing and probing the hot, dense fluid, which is of fundamental importance in physics, planetary science, and astrophysics. A pronounced discontinuous softening of the molecular vibron was found at elevated temperatures along with a large broadening and decrease in intensity of the roton bands.

Convergent Raman features in high density amorphous ice, ice VII, and ice VIII under pressure.

The high-pressure behavior of H(2)O ice at temperatures below 100 K has been investigated to 20 GPa by Raman spectroscopy. Between 10 and 80 K, high density amorphous (hda) ice formed from ice I(h) undergoes a phase transition near 5 GPa to ice VII' and a transition at 14 GPa to a different phase. At 14 GPa, a new low-frequency band appears at ~150 cm(-1) that steeply increases in frequency with pressure.

Pressure-induced bonding and compound formation in xenon-hydrogen solids.

Closed electron shell systems, such as hydrogen, nitrogen or group 18 elements, can form weakly bound stoichiometric compounds at high pressures. An understanding of the stability of these van der Waals compounds is lacking, as is information on the nature of their interatomic interactions. We describe the formation of a stable compound in the Xe-H(2) binary system, revealed by a suite of X-ray diffraction and optical spectroscopy measurements.

Vibrational dynamics, intermolecular interactions, and compound formation in GeH4-H2 under pressure.

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH(4)) + hydrogen (H(2)). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH(4)(H(2))(2), when the constituents are compressed above 7.5 GPa.

Pressure-induced complexation of NH(3)BH(3)-H(2).

High pressure Raman spectroscopy of NH(3)BH(3)-H(2) mixtures up to 60 GPa reveals unusual pressure-induced complexation and intermolecular interactions. Stretching modes of H(2) in the complex arise at 6.7 and 10 GPa, increasing in frequency with pressure of up to 60 GPa with different pressure coefficients, and at approximately 40 GPa, the lower frequency mode approaches vibron frequency of bulk H(2).