Science at Extreme Pressures

The synthesis and characterization of new crystalline-amorphous hybrid materials is challenging. Here, the authors report the preparation of a nested order-disorder framework by applying high pressure to a nested copper chalcogenide Cu₁₂Sb₄S₁₃.

Identifying a universal magnetic ground state across the iron-based superconductor family may help to propose a universal pairing glue responsible for unconventional superconductivity in these intriguing materials. Here, the authors discover an antiferromagnetic stripe order that appears as a precursor to superconductivity in pressured BaFe2Se3, suggesting that the presence of magnetic fluctuations from the stripe order may hold the key to the superconducting pairing of the iron-based superconductors.

The discovery of superconductivity in hydrides at critical temperature (T_c) near room temperature receives intensive attentions. Here the authors report experimental synthesis and discovery of superconductivity with T_c above 210 K in calcium superhydrides at 160–190 GPa.

High-pressure synthesis is used to stabilize superconducting (Ba,K)SbO₃, whose properties provide a fresh perspective on the origin of superconductivity in these types of materials.

Mastering thermal conductivities of materials under pressure is extremely important for managing thermal processes, understanding the thermal transport mechanisms and for potential technological applications. This Review surveys the progresses in technique developments, research results and scientific implications in this field.

Observation of a high-pressure insulating state in cuprate superconductors provides a fresh challenge for understanding the mechanism of superconductivity in these materials.

Measurements of the phase diagram of water reveal first-order phase transitions to face- and body-centred cubic superionic ice phases. The former is suggested to be present in the interior of ice giant planets.

The discovery of high temperature superconductivity in hydrogen-rich compounds stimulates further extensive studies. Here, the authors report superconductivity in pressurized yttrium-hydrogen system with highest predicted T_c among binary compounds.

Alkali metals at high pressures have a liquid–liquid transition that is difficult to study in detail. Numerical calculations now suggest that the higher-pressure state is an electride liquid, in which electrons behave like localized anions.

Methane is abundant in the Universe, is an important energy carrier and a model system for fundamental studies. Here, the authors measure the self-diffusion coefficient of supercritical methane at ambient temperature up to the freezing pressure, and find a different behavior than expected based on previous models.

High-pressure diamond anvil cell experiments reveal that compression strengthening of nanocrystalline nickel increases as its grain sizes decrease to 3 nanometres, owing to dislocation hardening and suppression of grain boundary plasticity.

Meteoritic diamonds and synthesized diamond-related materials contain a wide variety of complex nanostructures. This Comment highlights and classifies this structural complexity by a systematic hierarchical approach, and discusses the perspectives on nanostructure and properties engineering of diamond-related materials.

Solid hydrogen has increasingly hindered rotation under high pressure, but the effect on spin isomer populations had not been directly probed. Here the authors measure NMR spectra of solid hydrogen up to the megabar, and observe the crossover to a spin 1/2 dipolar system above 70 GPa where distinction between ortho and para spin isomers is lost.

The probable transition of hydrogen to its metal state near 425 GPa is observed, with the required high pressures created using a toroidal diamond anvil cell.

While metallic glasses are expected to have tunable structures, these have rarely been demonstrated. Here, the authors combine temperature and pressure to show a two-way structural tuning in rare earth-based metallic glasses beyond the nearest-neighbor atomic shells.

By applying a pressure of 2.8 GPa using a diamond anvil cell, a topological phase transition is found to occur in Cr-doped PbSe. This enables a thermoelectric figure of merit ZT of 1.7 at room temperature.

X-ray diffraction measurements of solid hydrogen provide crystallographic information for high-pressure phases of hydrogen and transitions between them, suggesting a series of isostructural transitions under compression before band closure and metallization.

The predicted metallization of hydrogen has long fascinated high-pressure physicists. Conductivity and spectroscopic measurements now reveal that above pressures of 350 GPa, hydrogen starts to conduct in a manner akin to a semimetal.

Superionic states of matter simultaneously exhibit some of the properties of a liquid and of a solid. Detailed numerical simulations predict two superionic phases in mixtures of helium and water.

High-entropy alloys establish a new conceptual framework for alloy design and can exhibit outstanding properties attractive for technological applications. The authors investigate the pressure induced magnetovolume effect in the high-entropy alloy CoCrFeAl and find its origin in two progressive, experimentally tunable magnetic transitions.

Up to now, all iron-based high-T_c superconductors contain a square iron lattice. Here, Wang et al. report the observation of superconductivity in an iron honeycomb lattice accompanied with pressure-driven spin-crossover, in-plane lattice collapse and insulator-metal transition.

Nitrogen is a model system still presenting unknown behaviors at the pressures and temperatures typical of deep planets’ interiors. Here the authors explore, by pulsed laser heating in a diamond anvil cell and optical measurements, the metallization and non-molecular states of nitrogen in a previously unexplored domain above 1 Mbar and at 2000-7000K.

Pressures of up to 900 gigapascals (9 million atmospheres) are achieved in a laser-heated double-stage diamond cell, enabling the synthesis of Re₇N₃, and materials characterization is performed in situ using single-crystal X-ray diffraction.

The chemical stability of alkali halides has caused them to be exploited as inert media in high-pressure, high-temperature experiments. Here, NaCl and KCl are unexpectedly found to react with yttrium, dysprosium and iron oxide in a laser-heated diamond anvil cell, producing Y₂Cl, DyCl, Y₂ClC and Dy₂ClC at ~40 GPa and 2000 K and FeCl₂ at ~160 GPa and 2100 K.

Numerical studies have predicted that solids at extremely high pressures should exhibit changes in structure driven by quantum mechanical effects. These predictions have now been verified in magnesium.

The discovery of graphite–diamond hybrid carbon, Gradia, which consists of graphite and diamond nanodomains interlocked through coherent interfaces, clarifies the long-standing mystery of how graphite turns into diamond.

The authors use in-situ high pressure nuclear magnetic resonance spectroscopy in diamond anvil cells to show that at all observed H-bond environments undergo a distinct maximum in hydrogen mobility regardless of the structure of the compounds.

The planar hexazine dianion ring (N₆^2–), which had previously been predicted to exist, has now been synthesized from potassium azide (KN₃) under laser heating in a diamond anvil cell above 45 GPa; it remains metastable down to 20 GPa. By contrast, at 30 GPa an unusual N₂-containing compound with the formula K₃(N₂)₄ was produced.

A study describes the synthesis, structural characterization and formation mechanism of a paracrystalline state of diamond, adding an unusual form of diamond to the family of carbon-based materials.

The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase.

Chemical elements at high pressure may behave more consistently with their periodic properties than they do at ambient conditions. The authors report the synthesis of PH₃ from black phosphorous and hydrogen, and the crystallization of the van der Waals compound (PH₃)₂H₂ which fills a gap in the chemistry of adjacent elements in the periodic table.

Porous molecular crystals are easy to fabricate but thought to have limited stability as they are bound by non-covalent interactions. Here, a porous crystal composed of C₆₀ and phthalocyanine is demonstrated with stability to heat, acid, base and high pressures.

The simultaneous high-pressure and high-temperature phase diagram of two MOFs, ZIF-4 and ZIF-62, is mapped. Crystalline, pressure- and temperature-amorphous, and liquid states are found, while melting temperature is found to decrease with pressure.

Molecular systems are predicted to transform into atomic solids and be metallic at high pressure; this was observed for the diatomic elements iodine and bromine. Here the authors access the higher pressures needed to observe the dissociation in chlorine, through an incommensurate phase, and provide evidence for metallization.

Hydrogen-rich superhydrides are promising high-temperature superconductors which have been observed only at pressures above 170 GPa. Here the authors show that CeH₉ can be synthesized at 80-100 GPa with laser heating, and is characterized by a clathrate structure with a dense 3-dimensional atomic hydrogen sublattice.

Owing to the energetic nature of N–N bonds, poly-nitrogen compounds are considered promising high energy density materials. Here, the authors synthesize three iron–nitrogen compounds at high pressure, including FeN₄, which features polymeric nitrogen chains of [N₄²⁻]_n units.

Molecular dynamics simulations show that the light elements hydrogen, oxygen and carbon become highly diffusive like liquid in solid iron under the inner-core conditions, leading to a reduction in the seismic velocities.

Most of our knowledge about the chemical composition of the Earth’s interior is primarily retrieved by indirect observations, experiments and calculations that are limited to simple compositions. Here, the authors present the investigation of inclusions trapped in super deep diamonds as an alternative source of a wealth of information on the chemical state of the Earth’s interior through time.

The density and velocity of the inner core deduced from seismic observations can be explained by a two-phase mixture of ordered body-centred cubic and hexagonal close-packed phases, according to high pressure and temperature experiments

Although the presence of ‘light’ elements (such as S, Si, O, C and H) can explain the core’s density deficit, the exact composition of the Earth’s core remains unknown. This Review explores the likely range of outer and inner core compositions and their implications.

A carbon content in Earth’s outer core between 0.3 and 2.0 % by weight, along with at least two other light elements, is compatible with observational constraints, according to molecular dynamics simulations, and could make the core Earth’s largest carbon reservoir.

Hydrogen and helium mixtures can be compressed to the extreme temperature and pressure conditions found in the interior of Jupiter and Saturn, and the immiscibility revealed supports models of Jupiter that invoke a layered interior.

Through platinum metal-silicate partitioning coefficient measurements, the authors here show that platinum partitioning into metal is lowered at high pressure–temperature conditions. This finding implies that the Earth’s mantle was likely enriched in platinum immediately following the core-mantle differentiation.

Under the pressure of a watery ocean, rock-forming minerals might dissolve at a planet’s rock–water interface, generating a denser-than-water layer that should be incorporated into models. The experimental data for MgO presented here are relevant to water-rich Earth-sized planets such as TRAPPIST-1 c and f, and to Uranus.

Under conditions of Earth’s deep lower mantle, hydrogen ions diffuse freely through the FeOOH lattice framework and electrical conductivity increases rapidly, according to electrical conductivity experiments and first-principles simulations.

Calcium carbonate transported by subducting slabs could explain elevated ferric iron content in the upper mantle through redox reactions with iron-rich garnet and graphite as products, according to high-pressure, high-temperature experiments

The interior structure and rheology of large terrestrial exoplanets is strongly affected by the phase transition of iron-oxide, according to dynamic compression and X-ray diffraction FeO experiments up to 700 GPa and calculation of the binary MgO–FeO phase diagram.

Calcium and oxygen are abundant elements in the Earth’s mantle, largely present as calcium oxide. Here the authors show, by experiments and computations, that calcium ozonide (CaO₃) is stabilized at the high pressures and temperatures characteristic of the lower mantle, with implications for the deep Earth’s chemistry.

Seismology provides information on the structure and composition of the Earth’s deep mantle. Without accurate constraints on the elastic properties of deep-mantle minerals, however, the seismic dataset cannot be fully interpreted. This Technical Review outlines the current techniques that are used to investigate the elasticity of typical mantle minerals.

Synchrotron Mössbauer source spectroscopy is used to reveal that haematite remains magnetic in cold subducting slabs at the depth of the transition zone in the Earth’s mantle, with implications for the locations of magnetic poles during inversions of the Earth’s magnetic field.

Subduction of oceanic crust introduces huge amounts of carbonates into Earth’s mantle, contributing to the global carbon cycle. Here, based on high-pressure-temperature experiments, the authors present a reversible temperature-induced transition from aragonite to amorphous CaCO₃.

Iron oxides prevail in the deep Earth, at extreme pressures and temperatures, with different stoichiometries than in ambient conditions. Here, high-pressure synchrotron X-ray spectroscopic measurements reveal the oxidation states of Fe and O in iron superoxide, shedding light on the puzzling chemistry of iron and oxygen in the deep Earth

Iron has been ramp compressed to the pressures it would experience in the core of a 3–4 Earth-mass terrestrial exoplanet, providing experimental constraints on the mass–radius relationship for a hypothetical pure iron planet.

At temperatures and pressures typical of the Earth’s lower mantle, cubic CaSiO₃ perovskite is found to have lower strength and viscosity compared to bridgmanite and ferropericlase, providing clues to its role in subduction regions.

Experiments show that calcium solubility in bridgmanite increases with depth in Earth’s lower mantle, resulting in the disappearance of CaSiO₃ perovskite and indicating a transition from a two-perovskite to a single-perovskite domain.

The nanostructured diamond capsule process with the inert gases solid argon and neon is demonstrated, where the trapped volatile gases could sustain their high-pressure states without confinement of conventional high-pressure vessels, opening up the possibility of in-depth investigations of high-pressure phenomena.

Exploration of metastable phases of a given elemental composition is a data-intensive task. Here the authors integrate first-principles atomistic simulations with machine learning and high-performance computing to allow a rapid exploration of the metastable phases of carbon.

X-ray diffraction measurements of solid carbon compressed to pressures of about two terapascals (approximately twenty million atmospheres) find that carbon retains a diamond structure even under these extreme conditions.

Hydrogen has multiple molecular phases which are challenging to explore computationally. The authors develop a machine-learning approach, learning from reference ab initio molecular dynamics simulations, to derive a transferable hierarchical force model that provides insight into high pressure phases and the melting line of H₂.

Static pressures exceeding 4 million atmospheres are extremely challenging to achieve, but are necessary for the study of matter that exists under these conditions in natural environments. Here, diamonds anvils with a toroidal design are demonstrated to sustain over 6 million atmospheres in a diamond anvil cell.

Extreme static pressures exceeding a million atmospheres exist in a variety of natural environments, but obtaining such pressures in a laboratory is still a challenge. Here, the authors develop a toroidal diamond anvil design that allows for the generation of 600 GPa (6 million atmospheres) in routinely used diamond anvil cells.

Hydrogen atoms in water ices, under pressures at which they might exist in ocean exoplanets and icy moons, exhibit dynamics that are still poorly understood. Here, ¹H-NMR experiments approaching the Mbar range shed light on the symmetrisation of hydrogen bonds preceding and accompanying the transformation of ice VII into ice X.

Although predicted to occur in planetary interiors, superionic water ice has proved elusive to identify experimentally. Laser-driven shock-compression experiments on water ice VII now verify its existence.