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. General overview of work done at HPCAT and with a focus on some of the minerals relevant work and supporting capabilities is also discussed. With the impending APS-Upgrade (APS-U), there is a considerable effort within HPCAT to improve and add capabilities. These are summarized briefly for each of the end-stations.

Novel Valence Transition in Elemental Metal Europium around 80 GPa.

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Structural, Vibrational, and Electronic Properties of 1D-TlInTe2 under High Pressure: A Combined Experimental and Theoretical Study.

Analogous to 2D layered transition-metal dichalcogenides, the TlSe family of quasi-one dimensional chain materials with the Zintl-type structure exhibits novel phenomena under high pressure. In the present work, we have systematically investigated the high-pressure behavior of TlInTe2 using Raman spectroscopy, synchrotron X-ray diffraction (XRD), and transport measurements, in combination with first principles crystal structure prediction (CSP) based on evolutionary approach. We found that TlInTe2 undergoes a pressure-induced semiconductor-to-semimetal transition at 4 GPa, followed by a superconducting transition at 5.7 GPa (with Tc = 3.8 K). An unusual giant phonon mode (Ag) softening appears at ∼10-12 GPa as a result of the interaction of optical phonons with the conduction electrons. The high-pressure XRD and Raman spectroscopy studies reveal that there is no structural phase transitions observed up to the maximum pressure achieved (33.5 GPa), which is in agreement with our CSP calculations. In addition, our calculations predict two high-pressure phases above 35 GPa following the phase transition sequence as I4/mcm (B37) → Pbcm → Pm3̅m (B2). Electronic structure calculations suggest Lifshitz (L1 & L2-type) transitions near the superconducting transition pressure. Our findings on TlInTe2 open up a new avenue to study unexplored high-pressure novel phenomena in TlSe family induced by Lifshitz transition (electronic driven), giant phonon softening, and electron-phonon coupling.

Pressure-Induced Enhancement of Thermoelectric Figure of Merit and Structural Phase Transition in TiNiSn.

Half-Heusler thermoelectric materials are potential candidates for high thermoelectric efficiency. We report high-pressure thermoelectric and structural property measurements, density functional theory calculations on the half-Heusler material TiNiSn, and an increase of 15% in the relative dimensionless figure of merit, ZT, around 3 GPa. Thermal and electrical properties were measured utilizing a specialized sample cell assembly designed for the Paris-Edinburgh large-volume press to a maximum pressure of 3.5 GPa. High-pressure structural measurements performed up to 50 GPa in a diamond-anvil cell indicated the emergence of a new high-pressure phase around 20 GPa. A first-principles structure search performed using an ab initio random structure search approach identified the high-pressure phase as an orthorhombic type, in good agreement with the experimental results.

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, s(q), of liquid Li have been successfully determined under an isothermal compression at 600 ± 30 K and at pressures up to 11.5 GPa. The first peak position in s(q) is found to increase with increasing pressure and is showing an obvious slope change starting at ∼7.5 GPa. The slope change is interpreted as a structural change from bcc-like to fcc-like local ordering in liquid Li. At pressures above 8.7 GPa, the liquid Li becomes predominantly fcc-like up to the highest pressure of 11.5 GPa in this study. The observed structural changes in liquid Li are consistent with the recently determined melting curve of Li.

Novel Superstructure-Phase Two-Dimensional Material 1T-VSe2 at High Pressure.

A superstructure can elicit versatile new properties in materials by breaking their original geometrical symmetries. It is an important topic in the layered graphene-like two-dimensional transition metal dichalcogenides, but its origin remains unclear. Using diamond-anvil cell techniques, synchrotron X-ray diffraction, X-ray absorption, and first-principles calculations, we show that the evolution from weak van der Waals bonding to Heisenberg covalent bonding between layers induces an isostructural transition in quasi-two-dimensional 1T-type VSe2 at high pressure. Furthermore, our results show that high pressure induces a novel superstructure at 15.5 GPa rather than suppresses it as it would normally, which is unexpected. It is driven by Fermi-surface nesting, enhanced by pressure-induced distortion. The results suggest that the superstructure not only appears in the two-dimensional structure but also can emerge in the pressure-tuned three-dimensional structure with new symmetry and develop superconductivity.

Observation of 9-Fold Coordinated Amorphous TiO2 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 TiO2 (a-TiO2). Our data show that the Ti coordination number (CN) increases from 7.2 ± 0.3 at ∼16 GPa to 8.8 ± 0.3 at ∼70 GPa and finally reaches a plateau at 8.9 ± 0.3 at ≲86 GPa. The evolution of the structural changes under pressure is rationalized by the ratio (γ) of the ionic radius of Ti to that of O. It appears that the CN ≈ 9 plateau correlates with the two 9-fold coordinated polymorphs (cotunnite, Fe2P) with different γ values. This CN-γ relationship is compared with those of a-SiO2 and a-GeO2, displaying remarkably consistent behavior between CN and γ. The unified CN-γ relationship may be generally used to predict the compression behavior of amorphous AO2 compounds under extreme conditions.

In situ x-ray diffraction study of polyamorphism in H2O under isothermal compression and decompression.

Amorphous-amorphous transformations in H2O 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. Our approach using rapid (de)compression allows for studying the polyamorphic transformations at higher temperatures than the conditions previously studied under slow (de)compression or isobaric annealing. Multiple compression-decompression cycles can be integrated with in situ x-ray measurements, thus facilitating the study of repeatability and reversibility of the polyamorphic transformations. Fast in situ x-ray diffraction measurements allow for obtaining detailed insight into the structural changes across polyamorphic transformations regarding the (dis)continuity, reversibility, and possible intermediate forms. As demonstrated at isothermal conditions of 145 K and 155 K, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA') appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and HDA' and a continuous change within HDA' involving structural reconfigurations and finally converging to VHDA.

Tunable multiferroic order parameters in Sr1- x Ba x Mn1- y Ti y O3.

Responding to the rapidly increasing demand for efficient energy usage and increased speed and functionality of electronic and spintronic devices, multiferroic oxides have recently emerged as key materials capable of tackling this multifaceted challenge. In this paper, we describe the development of single-site manganese-based multiferroic perovskite materials with modest amounts of nonmagnetic Ti substituted at the magnetic Mn site in Sr1- x Ba x Mn1- y Ti y O3 (SBMTO). Significantly enhanced properties were achieved with ferroelectric-type structural transition temperatures boosted to ∼430K. Ferroelectric distortions with large spontaneous polarization values of ∼30µC/cm2, derived from a point charge model, are similar in magnitude to those of the prototypical nonmagnetic BaTiO3. Temperature dependence of the system's properties was investigated by synchrotron x-ray powder diffraction and neutron powder diffraction at ambient and high pressures. Various relationships were determined between the structural and magnetic properties, Ba and Ti contents, and T N and T C. Most importantly, our results demonstrate the large coupling between the magnetic and ferroelectric order parameters and the wide tunability of this coupling by slight variations of the material's stoichiometry.

Pressure-induced structural change in MgSiO3 glass at pressures near the Earth's core-mantle boundary.

Knowledge of 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. Here we report the structure of MgSiO3 glass, considered an analog of silicate melts, up to 111 GPa. The first (r1) and second (r2) neighbor distances in the pair distribution function change rapidly, with r1 increasing and r2 decreasing with pressure. At 53-62 GPa, the observed r1 and r2 distances are similar to the Si-O and Si-Si distances, respectively, of crystalline MgSiO3 akimotoite with edge-sharing SiO6 structural motifs. Above 62 GPa, r1 decreases, and r2 remains constant, with increasing pressure until 88 GPa. Above this pressure, r1 remains more or less constant, and r2 begins decreasing again. These observations suggest an ultrahigh-pressure structural change around 88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO6 structural motifs similar to those of the crystalline 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 These results suggest that a structural change may occur in MgSiO3 melt under pressure conditions corresponding to the deep lower mantle.

Giant Pressure-Induced Enhancement of Seebeck Coefficient and Thermoelectric Efficiency in SnTe.

The thermoelectric properties of polycrystalline SnTe have been measured up to 4.5 GPa at 330 K. SnTe shows an enormous enhancement in Seebeck coefficient, greater than 200 % after 3 GPa, which correlates to a known pressure-induced structural phase transition that is observed through simultaneous in situ X-ray diffraction measurement. Electrical resistance and relative changes to the thermal conductivity were also measured, enabling the determination of relative changes in the dimensionless figure of merit (ZT), which increases dramatically after 3 GPa, reaching 350 % of the lowest pressure ZT value. The results demonstrate a fundamental relationship between structure and thermoelectric behaviours and suggest that pressure is an effective tool to control them.

A metastable liquid melted from a crystalline solid under decompression.

A metastable liquid may exist under supercooling, sustaining the liquid below the melting point such as supercooled water and silicon. It may also exist as a transient state in solid-solid transitions, as demonstrated in recent studies of colloidal particles and glass-forming metallic systems. One important question is whether a crystalline solid may directly melt into a sustainable metastable liquid. By thermal heating, a crystalline solid will always melt into a liquid above the melting point. Here we report that a high-pressure crystalline phase of bismuth can melt into a metastable liquid below the melting line through a decompression process. The decompression-induced metastable liquid can be maintained for hours in static conditions, and transform to crystalline phases when external perturbations, such as heating and cooling, are applied. It occurs in the pressure-temperature region similar to where the supercooled liquid Bi is observed. Akin to supercooled liquid, the pressure-induced metastable liquid may be more ubiquitous than we thought.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice.

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ∼1 Pa, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

High-pressure Seebeck coefficients and thermoelectric behaviors of Bi and PbTe measured using a Paris-Edinburgh cell.

A new sample cell assembly design for the Paris-Edinburgh type large-volume press for simultaneous measurements of X-ray diffraction, electrical resistance, Seebeck coefficient and relative changes in the thermal conductance at high pressures has been developed. The feasibility of performing in situ measurements of the Seebeck coefficient and thermal measurements is demonstrated by observing well known solid-solid phase transitions of bismuth (Bi) up to 3 GPa and 450 K. A reversible polarity flip has been observed in the Seebeck coefficient across the Bi-I to Bi-II phase boundary. Also, successful Seebeck coefficient measurements have been performed for the classical high-temperature thermoelectric material PbTe under high pressure and temperature conditions. In addition, the relative change in the thermal conductivity was measured and a relative change in ZT, the dimensionless figure of merit, is described. This new capability enables pressure-induced structural changes to be directly correlated to electrical and thermal properties.

Ultrahigh-pressure polyamorphism in GeO2 glass with coordination number >6.

Knowledge of pressure-induced structural changes in glasses is important in various scientific fields as well as in engineering and industry. However, polyamorphism in glasses under high pressure remains poorly understood because of experimental challenges. Here we report new experimental findings of ultrahigh-pressure polyamorphism in GeO2 glass, investigated using a newly developed double-stage large-volume cell. The Ge-O coordination number (CN) is found to remain constant at ∼6 between 22.6 and 37.9 GPa. At higher pressures, CN begins to increase rapidly and reaches 7.4 at 91.7 GPa. This transformation begins when the oxygen-packing fraction in GeO2 glass is close to the maximal dense-packing state (the Kepler conjecture = ∼0.74), which provides new insights into structural changes in network-forming glasses and liquids with CN higher than 6 at ultrahigh-pressure conditions.

General 2.5 power law of metallic glasses.

Metallic glass (MG) is an important new category of materials, but very few rigorous laws are currently known for defining its "disordered" structure. Recently we found that under compression, the volume (V) of an MG changes precisely to the 2.5 power of its principal diffraction peak position (1/q1). In the present study, we find that this 2.5 power law holds even through the first-order polyamorphic transition of a Ce68Al10Cu20Co2 MG. This transition is, in effect, the equivalent of a continuous "composition" change of 4f-localized "big Ce" to 4f-itinerant "small Ce," indicating the 2.5 power law is general for tuning with composition. The exactness and universality imply that the 2.5 power law may be a general rule defining the structure of MGs.

New developments in micro-X-ray diffraction and X-ray absorption spectroscopy for high-pressure research at 16-BM-D at the Advanced Photon Source.

The monochromator and focusing mirrors of the 16-BM-D beamline, which is dedicated to high-pressure research with micro-X-ray diffraction (micro-XRD) and X-ray absorption near edge structure (XANES) (6-45 keV) spectroscopy, have been recently upgraded. Monochromatic X-rays are selected by a Si (111) double-crystal monochromator operated in an artificial channel-cut mode and focused to 5 µm × 5 µm (FWHM) by table-top Kirkpatrick-Baez type mirrors located near the sample stage. The typical X-ray flux is ∼5 × 10(8) photons/s at 30 keV. The instrumental resolution, Δq/qmax, reaches to 2 × 10(-3) and is tunable through adjustments of the detector distance and X-ray energy. The setup is stable and reproducible, which allows versatile application to various types of experiments including resistive heating and cryogenic cooling as well as ambient temperature compression. Transmission XANES is readily combined with micro-XRD utilizing the fixed-exit feature of the monochromator, which allows combined XRD-XANES measurements at a given sample condition.

X-ray imaging for studying behavior of liquids at high pressures and high temperatures using Paris-Edinburgh press.

Several X-ray techniques for studying structure, elastic properties, viscosity, and immiscibility of liquids at high pressures have been integrated using a Paris-Edinburgh press at the 16-BM-B beamline of the Advanced Photon Source. Here, we report the development of X-ray imaging techniques suitable for studying behavior of liquids at high pressures and high temperatures. White X-ray radiography allows for imaging phase separation and immiscibility of melts at high pressures, identified not only by density contrast but also by phase contrast imaging in particular for low density contrast liquids such as silicate and carbonate melts. In addition, ultrafast X-ray imaging, at frame rates up to ∼10(5) frames/second (fps) in air and up to ∼10(4) fps in Paris-Edinburgh press, enables us to investigate dynamics of liquids at high pressures. Very low viscosities of melts similar to that of water can be reliably measured. These high-pressure X-ray imaging techniques provide useful tools for understanding behavior of liquids or melts at high pressures and high temperatures.