Megabar Pressures Research Articles

High-pressure synthesis of rhenium carbide Re3C under megabar compression

The rhenium carbide Re3C was predicted to be stable under high pressure and expected to have high hardness and low compressibility. In this study, we realise the synthesis of Re3C at megabar pressures of 105(3) and 140(5) GPa in laser-heated diamond anvil cells and characterise its structure using synchrotron single-crystal X-ray diffraction. The structure of Re3C has the monoclinic space group C2/m and is built of CRe7 capped octahedra. Our combined ab initio calculations and quantitative topological analysis support experimental structural data and further deepen the understanding of the chemical bonding in the newly synthesized compound.

Imaging magnetic transition of magnetite to megabar pressures using quantum sensors in diamond anvil cell

High-pressure diamond anvil cells have been widely used to create novel states of matter. Nevertheless, the lack of universal in-situ magnetic measurement techniques at megabar pressures makes it difficult to understand the underlying physics of materials’ behavior at extreme conditions, such as high-temperature superconductivity of hydrides and the formation or destruction of the local magnetic moments in magnetic systems. Here, we break through the limitations of pressure on quantum sensors by modulating the uniaxial stress along the nitrogen-vacancy axis and develop the in-situ magnetic detection technique at megabar pressures with high sensitivity (~1μT/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1{{{\\rm{\\mu }}}}{{{\\rm{T}}}}/\\sqrt{{{{\\rm{Hz}}}}}$$\\end{document}) and sub-microscale spatial resolution. By directly imaging the magnetic field and the evolution of magnetic domains, we observe the macroscopic magnetic transition of Fe3O4 in the megabar pressure range from ferrimagnetic (α-Fe3O4) to weak ferromagnetic (β-Fe3O4) and finally to paramagnetic (γ-Fe3O4). The scenarios for magnetic changes in Fe3O4 characterized here shed light on the direct magnetic microstructure observation in bulk materials at high pressure and contribute to understanding magnetism evolution in the presence of numerous complex factors such as spin crossover, altered magnetic interactions and structural phase transitions.

Structural phase transition in NH₄F under extreme pressure conditions.

Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH₄F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.

All-Single Bonds Fused N18 Macro-Rings and N8 Cagelike Building Blocks Stabilized in Lanthanum Supernitrides.

Single-bonded polymeric nitrogen has gained tremendous research interest because of its unique physical properties and great potential applications. Despite much progress in theoretical predictions, it is still challenging to experimentally synthesize polynitrogen compounds with novel all-single-bonded units. Herein, we have synthesized two brand-new lanthanum supernitrides LaN8, through a direct reaction between La and N2 in laser-heated diamond anvil cells at megabar pressures. Our experiments and calculations revealed that two LaN8 phases had the R-3 and P4/n symmetry characterized by a unique 2D network with N18 macro-rings and cagelike N8 building blocks, respectively. Differing from known polynitrogen structures, these two polymers were composed of single-bonded nitrogen atoms belonging to sp3 and sp2 hybridizations. In particular, P4/n LaN8 possessed the longest N-N bond length among all of the experimentally reported metal nitrides, potentially being a high-energy-density material. The present study opens a fresh, promising avenue for the rational design and discovery of new supernitrides with unique nitrogen structures via the high-pressure treatment.

Tunneling Spectroscopy at Megabar Pressures: Determination of the Superconducting Gap in Sulfur.

The recent discovery of high-temperature, high-pressure superconductors, such as hydrides and nickelates, has opened exciting avenues in studying high-temperature superconductivity. The primary superconducting properties of these materials are well characterized by measuring various electrical and magnetic properties, despite the challenges posed by the high-pressure environment. Experimental microscopic insight into the pairing mechanism of these superconductors is even more challenging, due to the lack of direct probes of the superconducting gap structures at high pressure conditions. Here, we have developed a planar tunnel junction technique for diamond anvil cells and present ground-breaking tunneling spectroscopy measurements at megabar pressures. We determined the superconducting gap of elemental sulfur at 160GPa, a key constituent of the high-temperature superconductor H_{3}S. High quality tunneling spectra indicate that β-Po phase sulfur is a type II superconductor with a single s-wave gap with a gap value 2Δ(0)=5.6 meV. This technique is compatible with superconducting compounds synthesized in diamond anvil cells and provides insight into the pairing mechanism in novel superconductors under high-pressure conditions.

Predicted hot superconductivity in LaSc2H24 under pressure

The recent theory-driven discovery of a class of clathrate hydrides (e.g., CaH6, YH6, YH9, and LaH10) with superconducting critical temperatures (Tc) well above 200 K has opened the prospects for "hot" superconductivity above room temperature under pressure. Recent efforts focus on the search for superconductors among ternary hydrides that accommodate more diverse material types and configurations compared to binary hydrides. Through extensive computational searches, we report the prediction of a unique class of thermodynamically stable clathrate hydrides structures consisting of two previously unreported H24 and H30 hydrogen clathrate cages at megabar pressures. Among these phases, LaSc2H24 shows potential hot superconductivity at the thermodynamically stable pressure range of 167 to 300 GPa, with calculated Tcs up to 331 K at 250 GPa and 316 K at 167 GPa when the important effects of anharmonicity are included. The very high critical temperatures are attributed to an unusually large hydrogen-derived density of states at the Fermi level arising from the newly reported peculiar H30 as well as H24 cages in the structure. Our predicted introduction of Sc in the La-H system is expected to facilitate future design and realization of hot superconductors in ternary clathrate superhydrides.

Electronic bonding transitions in oxide glass above two megabar pressures

Inelastic x-ray scattering (IXS) of B2O3 glass up to ∼2.2 Mbar reveals electronic bonding transitions in oxide glasses. B -edge IXS identifies the high-energy feature above ∼1.4 Mbar and a gradual increase in its intensity toward ∼2.2 Mbar, indicating the formation of hypervalent boron via electron polarization to oxygen atoms. The pressure-driven high energy shifts in O -edge IXS indicate pronounced electronic dispersion that increases upon densification of amorphous oxides above ∼2 Mbar. The extent of the energy shifts and enhanced polarization correlate with increasing atomic radius of cation in oxide glass, establishing the role of cation radius in electronic structures of amorphous oxides under compression. The results elucidate the electronic mechanisms behind the structural transformation in low- oxide glasses, where transitions to highly coordinated cations are hindered well above 1 Mbar, providing the origin of incompressibility of low- amorphous oxide under multi-Mbar compression. Published by the American Physical Society 2024

Nitrogen sequestration in the core at megabar pressure and implications for terrestrial accretion

Nitrogen (N) is the most abundant element in Earth’s atmosphere, but is extremely depleted in the silicate Earth. However, it is not clear whether core sequestration or early atmospheric loss was responsible for this depletion. Here we study the effect of core formation on the inventory of nitrogen using laser-heated diamond anvil cells. We find that, due to the simultaneous dissolution of oxygen in the metal, N becomes much less siderophile (iron-loving) at pressures and temperatures up to 104 GPa and 5000 K, a thermodynamic condition relevant to the bottom of the magma ocean in the aftermath of the moon-forming giant impact. Using a core–mantle–atmosphere coevolution model, we show that the impact-induced processes (core formation and/or atmospheric loss) are unlikely to account for the observed N anomaly, which is instead best explained by the accretion of mainly N-poor impactors. The terrestrial volatile pattern requires severe N depletion on precursor bodies, prior to their accretion to the proto-Earth.

Leveraging oxide reactive sputtering for thermal insulation in laser-heated diamond anvil cell

ABSTRACT We have developed an oxide reactive sputtering technique aiming at rapidly sputtering high-quality oxide films on a diamond surface, providing excellent thermal insulation in high-pressure and temperature experiments using a laser-heated diamond anvil cell (LH-DAC). We identified conditions for rapid deposition of an alumina layer on diamond anvils, showing deposition rates as high as 0.67 um/hr. We also investigated the deposition conditions of zirconia, which have lower thermal conductivity than alumina. Laser-heating tests were performed at high pressures and temperatures to evaluate the thermal insulation of the oxide film deposited on the diamond anvils. The heating efficiency of zirconia-deposited anvils was higher than that of the alumina-deposited ones. Our zirconia-sputtered anvils were capable of generating up to 2580 K at around megabar pressure without additional thermal insulation, demonstrating the potential of this technique for ultra-high temperature generation.

Imaging the Meissner effect in hydride superconductors using quantum sensors.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena1. The megabar regime represents an interesting frontier, in which recent discoveries include high-temperature superconductors, as well as structural and valence phase transitions2-6. However, at such high pressures, many conventional measurement techniques fail. Here we demonstrate the ability to perform local magnetometry inside a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach uses a shallow layer of nitrogen-vacancy colour centres implanted directly within the anvil7-9; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the nitrogen-vacancy centre to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9 (ref. 10). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed-loop optimization of superhydride materials synthesis.

The current status and future development of high-temperature conventional superconductivity.

The robust evidence and reproducibility of high-temperature superconductivity in hydrogen-rich materials under challenging experimental conditions of megabar pressures is presented.

Equations of State of Solid CO2 Phases at Megabar Pressures

Equations of State of Solid CO2 Phases at Megabar Pressures

Trapped flux in pure and Mn-substituted CaKFe4As4 and MgB2 superconducting single crystals

Measurements of temperature dependent magnetization associated with trapped magnetic flux in single crystals of CaKFe4As4, CaK(Fe0.983Mn0.017)4As4 and MgB2 using zero-field-cooled and field-cooled protocols are presented. The results allow for the determination of the values of superconducting transition temperature, lower critical field and self-field critical current density. These are compared with the literature data. Possible experimental concerns are briefly outlined. Our results, on these known superconductors at ambient pressure, are qualitatively similar to those recently measured on superhydrides at megabar pressures (Minkov et al 2023 Nat. Phys. https://doi.org/10.1038/s41567-023-02089-1) and, as such, hopefully serve as a baseline for the interpretation of high pressure, trapped flux measurements.

Superconductivity induced by gate-driven hydrogen intercalation in the charge-density-wave compound 1T-TiSe2

Hydrogen (H) plays a key role in the near-to-room temperature superconductivity of hydrides at megabar pressures. This suggests that H doping could have similar effects on the electronic and phononic spectra of materials at ambient pressure as well. Here, we demonstrate the non-volatile control of the electronic ground state of titanium diselenide (1T-TiSe2) via ionic liquid gating-driven H intercalation. This protonation induces a superconducting phase, observed together with a charge-density wave through most of the phase diagram, with nearly doping-independent transition temperatures. The H-induced superconducting phase is possibly gapless-like and multi-band in nature, in contrast with those induced in TiSe2 via copper, lithium, and electrostatic doping. This unique behavior is supported by ab initio calculations showing that high concentrations of H dopants induce a full reconstruction of the bandstructure, although with little coupling between electrons and high-frequency H phonons. Our findings provide a promising approach for engineering the ground state of transition metal dichalcogenides and other layered materials via gate-controlled protonation.

Extremely Narrow Superconducting Band with Crystal Spin 32ℏ in LaH10

We show that the high-temperature superconductor LaH10 possesses an unusual superconducting band in its band structure. We examine this band by group-theoretical methods and report evidence that the special features of the band support the characteristics of the superconducting state in LaH10 at megabar pressures, i.e., the strong electron–phonon coupling and the high superconducting transition temperature.

Structure and equation of state of Bi2Sr2Can−1CunO2n+4+δ from x-ray diffraction to megabar pressures

Pressure is a unique tuning parameter for probing the properties of materials, and it has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasihydrostatic compression produced by a neon pressure medium on the structures of bismuth-based high-Tc cuprate superconductors with the nominal composition Bi2Sr2Can−1CunO2n+4+δ (n=1,2,3) up to 155 GPa. The structures of all three compositions obtained by synchrotron x-ray diffraction can be described as pseudotetragonal over the entire pressure range studied. We show that previously reported pressure-induced distortions and structural changes arise from the large strains that can be induced in these layered materials by nonhydrostatic stresses. The pressure-volume equations of state (EOS) measured under these quasihydrostatic conditions cannot be fit to single phenomenological formulation over the pressure ranges studied, starting below 20 GPa. This intrinsic anomalous compression as well as the sensitivity of Bi2Sr2Can−1CunO2n+4+δ to deviatoric stresses provide explanations for the numerous inconsistencies in reported EOS parameters for these materials. We conclude that the anomalous compressional behavior of all three compositions is a manifestation of the changes in electronic properties that are also responsible for the remarkable nonmonotonic dependence of Tc with pressure, including the increase in Tc at the highest pressures studied so far for each. Transport and spectroscopic measurements up to megabar pressures are needed to fully characterize these cuprates and explore higher possible critical temperatures in these materials.1 MoreReceived 15 December 2022Accepted 5 May 2023DOI:https://doi.org/10.1103/PhysRevMaterials.7.064803©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEquations of stateHigh-pressure studiesPhysical SystemsCupratesTechniquesX-ray diffractionCondensed Matter, Materials & Applied Physics

Colloquium : Room temperature superconductivity: The roles of theory and materials design

For half a century after the discovery of superconductivity, materials exploration for better superconductors proceeded without knowledge of the underlying mechanism. The 1957 BCS theory cleared that up: the superconducting state occurs due to pairing of electrons over the Fermi surface. Over the following half century higher critical temperature T$_c$ was achieved only serendipitously as new materials were synthesized. Meanwhile the formal theory of phonon-coupled superconductivity at the material-dependent level became highly developed: given a known compound, its value of T$_c$, the superconducting gap function, and several other properties of the superconducting state became available independent of further experimental input. More recently, density functional theory based computational materials design has progressed to a predictive level -- new materials can be predicted on the basis of various numerical algorithms. Taken together, these capabilities enable theoretical prediction of new superconductors. Here the process that resulted in three new highest temperature superconductors, predicted numerically, confirmed experimentally -- SH$_3$, LaH$_{10}$, and YH$_9$ -- is recounted. These hydrides have T$_c$ in the 200-280K range at megabar pressures, and here the development will be chronicled. Current activities and challenges are discussed, together with Regularities in compressed hydrides that can guide further exploration.

Advancing neutron diffraction for accurate structural measurement of light elements at megabar pressures

Over the last 60 years, the diamond anvil cell (DAC) has emerged as the tool of choice in high pressure science because materials can be studied at megabar pressures using X-ray and spectroscopic probes. In contrast, the pressure range for neutron diffraction has been limited due to low neutron flux even at the strongest sources and the resulting large sample sizes. Here, we introduce a neutron DAC that enables break-out of the previously limited pressure range. Key elements are ball-bearing guides for improved mechanical stability, gem-quality synthetic diamonds with novel anvil support and improved in-seat collimation. We demonstrate a pressure record of 1.15 Mbar and crystallographic analysis at 1 Mbar on the example of nickel. Additionally, insights into the phase behavior of graphite to 0.5 Mbar are described. These technical and analytical developments will further allow structural studies on low-Z materials that are difficult to characterize by X-rays.

Magnesium oxide-water compounds at megabar pressure and implications on planetary interiors

Magnesium Oxide (MgO) and water (H2O) are abundant in the interior of planets. Their properties, and in particular their interaction, significantly affect the planet interior structure and thermal evolution. Here, using crystal structure predictions and ab initio molecular dynamics simulations, we find that MgO and H2O can react again at ultrahigh pressure, although Mg(OH)2 decomposes at low pressure. The reemergent MgO-H2O compounds are: Mg2O3H2 above 400 GPa, MgO3H4 above 600 GPa, and MgO4H6 in the pressure range of 270–600 GPa. Importantly, MgO4H6 contains 57.3 wt % of water, which is a much higher water content than any reported hydrous mineral. Our results suggest that a substantial amount of water can be stored in MgO rock in the deep interiors of Earth to Neptune mass planets. Based on molecular dynamics simulations we show that these three compounds exhibit superionic behavior at the pressure-temperature conditions as in the interiors of Uranus and Neptune. Moreover, the water-rich compound MgO4H6 could be stable inside the early Earth and therefore may serve as a possible early Earth water reservoir. Our findings, in the poorly explored megabar pressure regime, provide constraints for interior and evolution models of wet planets in our solar system and beyond.

Superconducting Be2SH3 with kagome hydrogen at high pressure

Hydrogen atoms in Be2SH3 form an intriguing kagome lattice, which is energetically stable and exhibits superconductivity below megabar pressures.