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Mao, Ho-kwang

Publications and source records attributed to Mao, Ho-kwang.

17 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Atomistic evidence of nucleation mechanism for the direct graphite-to-diamond transformation

The direct graphite-to-diamond transformation mechanism has been a subject of intense study and remains debated concerning the initial stages of the conversion, the intermediate phases, and their transformation pathways. Here, we successfully recover samples at the early conversion stage by tuning high-pressure/high-temperature conditions and reveal direct evidence supporting the nucleation-growth mechanism. Atomistic observations show that intermediate orthorhombic graphite phase mediates the growth of diamond nuclei. Furthermore, we observe that quenchable orthorhombic and rhombohedra graphite are stabilized in buckled graphite at lower temperatures. These intermediate phases are further converted into hexagonal and cubic diamond at higher temperatures following energetically favorable pathways in the order: graphite → orthorhombic graphite → hexagonal diamond, graphite → orthorhombic graphite → cubic diamond, graphite → rhombohedra graphite → cubic diamond. Furthermore, these results significantly improve our understanding of the transformation mechanism, enabling the synthesis of different high-quality forms of diamond from graphite.

Graphite-diamond phase transformation↗
DOE OSTI2024

Evolution of Superconducting-Transition Temperature with Superfluid Density and Conductivity in Pressurized Cuprate Superconductors

What factors fundamentally determine the value of superconducting transition temperature T c in high temperature superconductors has been the subject of intense debate. Following the establishment of an empirical law known as Homes' law, there is a growing consensus in the community that the T c value of the cuprate superconductors is closely linked to the superfluid density (ρ s ) of its ground state and the conductivity (σ) of its normal state. However, all the data supporting this empirical law (ρ s = AσT c ) have been obtained from the ambient-pressure superconductors. In this study, we present the first high-pressure results about the connection of the quantities of ρ s and σ with T c , through the studies on the Bi 1.74 Pb 0.38 Sr 1.88 CuO 6+δ and Bi 2 Sr 2 CaCu 2 O 8+δ , in which the value of their high-pressure resistivity (ρ = 1/σ) is achieved by adopting our newly established method, while the quantity of ρs is extracted using Homes' law. In conclusion, we highlight that the T c values are strongly linked to the joint response factors of magnetic field and electric field, i.e., ρ s and σ, respectively, implying that the physics determining T c is governed by the intrinsic electromagnetic fields of the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

The stability of FeH x and hydrogen transport at Earth’s core mantle boundary

Iron hydride in Earth’s interior can be formed by the reaction between hydrous minerals (water) and iron. Studying iron hydride improves our understanding of hydrogen transportation in Earth’s interior. Our high-pressure experiments found that face-centered cubic (fcc) FeH x (x≤1) is stable up to 165 GPa, and our ab initio molecular dynamics simulations predicted that fcc FeH x transforms to a superionic state under lower mantle conditions. In the superionic state, H-ions in fcc FeH become highly diffusive-like fluids with a high diffusion coefficient of ~3.7 × 10 -4 cm 2 s -1 , which is comparable to that in the liquid Fe-H phase. The densities and melting temperatures of fcc FeH x were systematically calculated. Similar to superionic ice, the extra entropy of diffusive H-ions increases the melting temperature of fcc FeH. Further, the wide stability field of fcc FeH enables hydrogen transport into the outer core to create a potential hydrogen reservoir in Earth’s interior, leaving oxygen-rich patches (ORP) above the core mantle boundary (CMB).

58 GEOSCIENCES↗
DOE OSTI2023

Pressure-induced nonmonotonic cross-over of steady relaxation dynamics in a metallic glass

Relaxation dynamics, as a key to understand glass formation and glassy properties, remains an elusive and challenging issue in condensed matter physics. In this work, in situ high-pressure synchrotron high-energy X-ray photon correlation spectroscopy has been developed to probe the atomic-scale relaxation dynamics of a cerium-based metallic glass during compression. Although the sample density continuously increases, the collective atomic motion initially slows down as generally expected and then counterintuitively accelerates with further compression (density increase), showing an unusual nonmonotonic pressure-induced steady relaxation dynamics cross-over at ~3 GPa. Furthermore, by combining in situ high-pressure synchrotron X-ray diffraction, the relaxation dynamics anomaly is evidenced to closely correlate with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either sample density or overall stress level. In conclusion, these findings could provide insight into relaxation dynamics and their relationship with local atomic structures of glasses.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Preservation of high-pressure volatiles in nanostructured diamond capsules

High pressure induces dramatic changes and novel phenomena in condensed volatiles(1,2) that are usually not preserved after recovery from pressure vessels. In this study we report a process that pressurizes volatiles into nanopores of type 1 glassy carbon precursors, converts glassy carbon into nanocrystalline diamond by heating and synthesizes free-standing nanostructured diamond capsules (NDCs) capable of permanently preserving volatiles at high pressures, even after release back to ambient conditions for various vacuum-based diagnostic probes including electron microscopy. As a demonstration, we perform a comprehensive study of a high-pressure argon sample preserved in NDCs. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy show nanometre-sized argon crystals at around 22.0 gigapascals embedded in nanocrystalline diamond, energy-dispersive X-ray spectroscopy provides quantitative compositional analysis and electron energy-loss spectroscopy details the chemical bonding nature of high-pressure argon. The preserved pressure of the argon sample inside NDCs can be tuned by controlling NDC synthesis pressure. To test the general applicability of the NDC process, we show that high-pressure neon can also be trapped in NDCs and that type 2 glassy carbon can be used as the precursor container material. Additional experiments on other volatiles and carbon allotropes open the possibility of bringing high-pressure explorations on a par with mainstream condensed-matter investigations and applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Novel Valence Transition in Elemental Metal Europium around 80 GPa

We report 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.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Structural, Vibrational, and Electronic Properties of 1D-TlInTe 2 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 TlInTe 2 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 T c = 3.8 K). An unusual giant phonon mode (A g ) 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 → Pm$\bar{3}$m (B2). Electronic structure calculations suggest Lifshitz (L1 & L2-type) transitions near the superconducting transition pressure. Furthermore, our findings on TlInTe 2 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Pressure-stabilized divalent ozonide CaO 3 and its impact on Earth's oxygen cycles

High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO 3 by reaction of CaO and O 2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with laser heating. High-pressure x-ray diffraction data show that CaO 3 crystal forms at 35 GPa and persists down to 20 GPa on decompression. Analysis of charge states reveals a formal oxidation state of -2 for ozone anions in CaO 3 . These findings unravel the ozonide chemistry at high pressure and offer insights for elucidating prominent seismic anomalies and oxygen cycles in Earth’s interior. We further predict multiple reactions producing CaO 3 by geologically abundant mineral precursors at various depths in Earth’s mantle.

58 GEOSCIENCES↗
DOE OSTI2020

Aberrant electronic and structural alterations in pressure tuned perovskite NaOsO 3

The perovskite NaOsO 3 has a metal–insulator transition at temperature 410 K, which is delicate, intriguing, and provokes a lot of debate on its nature. Our combined electrical resistance, Raman, and synchrotron x-ray diffraction experiments show that the insulating ground state in this osmate endures under high pressure up to at least 35 GPa. In this pressure range, compression reveals hidden hysteretic resistance properties with a transient metallic state near 200 K, manifested three electronic character anomalies (at 1.7, 9.0, and 25.5 GPa), and a structural transition to the singular polar phase (at ~18 GPa). We distinguish NaOsO 3 from the regular crystallographic behavior of perovskites, though the electrical specificities resemble iridates and nickelates. The theoretical first-principle band structure and lattice dynamics calculations demonstrate that the magnetically itinerant Lifshitz-type mechanism with spin–orbit and spin–phonon interactions is responsible for these pressure-induced changes. Our findings provide another new playground for the emergence of new states in 5d materials by using high-pressure methods.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Highly tunable properties in pressure-treated two-dimensional Dion–Jacobson perovskites

Significance Two-dimensional Dion–Jacobson (2D D-J) lead-halide perovskite is a cutting-edge material paradigm that has realized high-efficiency photovoltaic applications and synergetic chemical stability. With the addition of the pressure dimension, this material provides a crucial platform for studying both pressure-tailored physical properties and pressure-driven novel phenomena. Here, we demonstrate that 2D D-J perovskites experience various transitions under pressure, such as crystalline-amorphous and 2D–three-dimensional structural transformation, and the probable metallization are strongly suggested. Moreover, the fundamental changes in the material properties are observed at ambient conditions after pressure treatment, which is crucial for achieving the desired characteristics for viable applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Key problems of the four-dimensional Earth system

Compelling evidence indicates that the solid Earth consists of two physicochemically distinct zones separated radially in the middle of the lower mantle at ~1800 km depth. The inner zone is governed by pressure-induced physics and chemistry dramatically different from the conventional behavior in the outer zone. These differences generate large physical and chemical potentials between the two zones that provide fundamental driving forces for triggering major events in Earth’s history. One of the main chemical carriers between the two zones is H 2 O in hydrous minerals that subducts into the inner zone, releases hydrogen, and leaves oxygen to create superoxides and form oxygen-rich piles at the core–mantle boundary, resulting in localized net oxygen gain in the inner zone. Accumulation of oxygen-rich piles at the base of the mantle could eventually reach a supercritical level that triggers eruptions, injecting materials that cause chemical mantle convection, superplumes, large igneous provinces, extreme climate changes, atmospheric oxygen fluctuations, and mass extinctions. Interdisciplinary research will be the key for advancing a unified theory of the four-dimensional Earth system.

58 GEOSCIENCES↗
DOE OSTI2020

Superconductivity in La and Y hydrides: Remaining questions to experiment and theory

Recent reports of the superconductivity in hydrides of two different families (covalent lattice, as in SH3 and clathrate-type H-cages containing La and Y atoms, as in LaH10 and YH6) have revealed new families of high-Tc materials with Tc’s near room temperature values. These findings confirm earlier expectations that hydrides may have very high Tc’s due to the fact that light H atoms have very high vibrational frequencies, leading to high Tc values within the conventional Bardeen–Cooper–Schrieffer phonon mechanism of superconductivity. However, as is pointed out by Ashcroft, it is important to have the metallic hydrogen “alloyed” with the elements added to it. This concept of a metallic alloy containing a high concentration of metal-like hydrogen atoms has been instrumental in finding new high-Tc superhydrides. These new superhydride “room-temperature” superconductors are stabilized only at very high pressures above 100 GPa, making the experimental search for their superconducting properties very difficult. We will review the current experimental and theoretical results for LaH10−x and YH6−x superhydrides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗