The low-pressure stabilization of superconducting hydrides with high critical temperatures (T_cs) remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition-metal complex hydrides (hydrido complexes)-typically regarded as hydrogen storage materials-exhibit a large range of compounds stabilized at low pressure with recent predictions for high-T_c superconductivity. Motivated by this class of materials, we investigated complex hydride formation in the Mg-Pt-H system, which has no known ternary hydride compounds. Guided by ab initio structural predictions, we successfully synthesized a novel complex transition-metal hydride, Mg_4Pt_3H_6, using laser-heated diamond anvil cells. The compound forms in a body-centered cubic structural prototype at moderate pressures between 8-25 GPa. Unlike the majority of known hydrido complexes, Mg_4Pt_3H_6 is metallic, with formal charge described as 4[Mg]^2+.3[PtH_2]^2-. X-ray diffraction (XRD) measurements obtained during decompression reveal that Mg_4Pt_3H_6 remains stable upon quenching to ambient conditions. Magnetic-field and temperature-dependent electrical transport measurements indicate ambient-pressure superconductivity with T_c (50 reasonable agreement with theoretical calculations. These findings clarify the phase behavior in the Mg-Pt-H system and provide valuable insights for transition-metal complex hydrides as a new class of hydrogen-rich superconductors.

The behavior of iron carbonates at high pressures is relevant for geological processes occurring in Earth interiors. Here, cubic iron sp3-carbonate Fe2[C4O10] was synthesized in diamond anvil cell by reacting Fe2O3 and CO2 at 65(4) GPa and 3000(±500) K, simulating the environment of localized thermal anomalies in the mantle. The crystal structure, determined by in situ single-crystal X-ray diffraction, features pyramidal [C4O10]4- anions. The experimental crystal structure corresponds to a structural model from density functional theory calculations. Experimentally determined values for zero-pressure volume V0 and bulk modulus K0 are: V0 = 1059(17) Å3, K0 = 160(18) GPa, The DFT-calculated Raman spectrum, modeled with zinc substituting iron, matches the experimental one, supporting the structural model's accuracy. Fe2[C4O10] remained stable upon decompression down to 25 GPa, below which it amorphized. DFT calculations also reveal a spin crossover of Fe2+ cations at 95 GPa, which is significantly higher than in other Fe2+-containing carbonates.

CoSb3 belongs to the skutterudite family of compounds and serves as a crucial platform for the exploration of thermoelectric materials, however, its importance is equally high for studies of strong correlations at high pressures. Under compression it undergoes a 'self-insertion' isostructural transition resulting in a peculiar redistribution of large Sb atoms between different crystallographic sites. We conducted a comprehensive investigation of the structural phase stability of CoSb3 up to 70 GPa using single crystal samples characterized employing conventional single crystal X-ray diffraction and X-ray scattering focused on measuring Bragg peak at high resolution (including elements of Bragg Coherent Diffraction Imaging). We explore the compression behavior of CoSb3 in three different pressure transmitting media (PTM) and address several important, but previously unexplored topics: the influence of various PTMs and nonhydrostatic stresses on the strongly correlated system of CoSb3, including the 'self-insertion' crossover, the phase stability of CoSb3, the compound's polymorphism, its crystal chemistry, and its peculiar evolution under pressure at ambient temperature. Among other important observations, we track the population of Sb atoms within the dodecahedral sites of CoSb3 on compression, during the process of 'self-insertion', and on decompression. We detect that 'self-insertion' may not only reduce the solid's compressibility, but also make it negative. Finally, but not least, we report that the 'self- insertion' crossover is an important step preceding a previously unknown phase transformation from cubic Im3 CoSb3 into trigonal R3 occurring above 40 GPa, and discuss the distinctive behavior of CoSb3 phases and their structural frameworks.

Polar metals have attracted growing interest due to both their significance in fundamental science and their potential functionalities. Here, we report the discovery of a novel polar metal, magnesium chloride Mg3Cl7, in which the metallicity of the polar structure is uniquely driven by attractive halogen interactions. Mg3Cl7 was synthesized in laser-heated diamond anvil cells and observed at pressures of 28(2)-93(3) GPa. Synchrotron single-crystal X-ray diffraction revealed that the structure of the new compound has polar hexagonal space group P63mc, representing an example of a previously unknown anti-Th7Fe3 structure type. Measurements of the physical properties have shown that the material is a metallic conductor capable of emitting second-harmonic generation light. Ab initio calculations support experimental findings and reveal complex halogen-halogen interactions, anionic metallicity, anisotropic electronic structure, and the presence of Dirac and Weyl points at the Fermi level. Our findings broaden the family of polar metals, provide new insight into halogen bonding under extreme conditions, and offer a platform for further exploration of materials' unconventional electronic behavior.

Two novel ternary compounds, yttrium borate oC20-YBO3 and yttrium orthocarbonate hR39-Y3(CO4)2, were synthesized at 90 and 120 GPa upon heating to about 3500 K in a laser-heated diamond anvil cell. Their crystal structures were solved and refined using in situ high-pressure synchrotron single-crystal X-ray diffraction. The crystal structure of oC20-YBO3 features previously unobserved infinite unbranched zigzag chains of corner-sharing BO4 tetrahedra. The compound hR39-Y3(CO4)2 consists of isolated [CO4]4- orthocarbonate anions and represents the first rare-earth orthocarbonate. Density functional theory calculations reproduce the structures of the new compounds and provide further insight into their stability and physical properties. Our results enrich the chemistry of metal borates and carbonates.

Large ocean worlds in the solar system and in other planetary systems likely contain significant fraction of volatile and organic compounds, which condition their habitability potential. Some of these compounds are known to combine with water molecules to form clathrate hydrates under pressure. Methanol is a volatile of a particular interest due to its expected presence in various icy bodies of the outer solar system and its disputable role in clathrate hydrates formation. However, the clathration process of methanol in complex compositions and under high pressures representative of these ocean worlds still remain unexplored. Herein, using high-pressure - low-temperature in situ single crystal X-ray diffraction, we demonstrate that high pressure facilitates incorporation of methanol in clathrate structure in presence of a promoter, with formation of mixed methanol-bearing clathrate hydrates. The preferential trapping of methanol at high-pressure offers an explanation for the nondetection of methanol on surfaces of large ocean worlds, while more common on small icy worlds. Moreover, the hydrogenolysis of enclathrated primordial methanol into methane presents a novel hypothesis for the deepseated endogenic source of methane in Titan and potentially for other ocean worlds.

Melting experiments of Fe3C were conducted to 85 GPa in laser-heated diamond anvil cells with in situ X-ray diffraction and post-experiment textural observation. From the determined pressure-temperature conditions of the melting curve for Fe3C, together with literature data on the melting point of diamond and eutectic point of the system Fe-Fe3C/Fe7C3 under high pressures, we established a self-consistent thermodynamic model for high-pressure melting of the system Fe-C including the mixing parameters for liquids. The results show that mixing of Fe and C liquids is negatively nonideal from 1 bar to the pressure at the center of the Earth. The departure from ideal mixing becomes progressively larger with increasing pressure, which leads to greatly stabilized liquids under core pressures. The modeled carbon content in eutectic melts under core pressures is 3.3-4.4 wt%. From the Gibbs free energy, we derived an internally consistent parameters for Fe-C outer cores which included the crystallizing points at their bottoms, isentropic thermal profiles, and densities and longitudinal seismic wave speeds (Vp). While the addition of carbon in excess of the eutectic melt composition effectively reduces the density of iron liquid, the Vp of iron liquid is not greatly changed. Therefore, the low density and high Vp of PREM relative to pure iron cannot be reconciled by an Fe-C liquid. Therefore, the Earth's core cannot be approximated by the system Fe-C and should include another light element. Plain Language Summary The presence or absence of carbon in the Earth's central core has long been discussed in order to account for the observed core density lower than iron because the identification of such light elements in the core will provide critical information about the origin and evolution of the solid Earth. The relevance of carbon in the Earth's core is testable by comparing the density and seismic wave velocities of candidate Fe-C materials with observations. Here we present a new experiment-based thermodynamic model for the system Fe-C. We first determined the melting points of Fe3C to 85 GPa. We then assessed thermodynamic parameters of Fe-C liquids, from the constrained pressure-temperature conditions of the melting curve for Fe3C, together with literature data. From the established model, we consistently computed the properties of hypothetical Fe-C liquid outer cores including the density and longitudinal seismic wave velocity. While a carbon concentration greater than 4 wt% effectively reduces the density of iron liquid, the seismic velocity is not greatly changed, and therefore carbon cannot reconcile the low density and high velocity nature of the Earth's outer core. Therefore, the Earth's core cannot be approximated by the system Fe-C and should include another light element.

Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-T-c phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg2IrH6 may help overcome this challenge with calculations of high-T-c superconductivity (65K < T-c < 170K) in a material that is stable at atmospheric pressure. In this paper, the synthesis of Mg2IrH6 was targeted over a broad range of P-T conditions, and the resulting products were characterized using x-ray diffraction (XRD) and vibrational spectroscopy, in concert with first-principles calculations. The results indicate that the charge-balanced complex hydride Mg2IrH5 is more stable over all conditions tested up to approximately 28 GPa. The resulting hydride is isostructural with the predicted superconducting Mg2IrH6 phase except for a single hydrogen vacancy, which shows a favorable replacement barrier upon insertion of hydrogen into the lattice. Bulk Mg2IrH5 is readily accessible at mild P-T conditions and may thus represent a convenient platform to access superconducting Mg2IrH6 via nonequilibrium processing methods. Finally, the critical factors influencing the calculated range of superconducting transition temperatures for this material are discussed.

Abstract Seismic anisotropy is a powerful tool to map deformation processes in the deep Earth. Below 660 km, however, observations are scarce and conflicting. In addition, the underlying crystal scale mechanisms, leading to microstructures and crystal orientations, remain poorly constrained. Here, we use multigrain X‐ray diffraction in the laser‐heated diamond anvil cell to investigate the orientations of hundreds of grains in pyrolite, a model composition of the Earth's mantle, at in situ pressure and temperature. Bridgmanite in pyrolite exhibits three regimes of microstructures, due to transformation and deformation at low and high pressure. These microstructures result in predictions of 1.5%–2% shear wave splitting between 660 and 2,000 km with reversals in fast S‐wave polarization direction at about 1,300 km depth. Anisotropy can develop in pyrolite at lower mantle conditions, but pressure has a significant impact on the plastic behavior of bridgmanite, and hence seismic observations, which may explain conflicting anisotropy observations.

AbstractStructural studies of pyrene have been limited to below 2 GPa. Here, we report on investigations of pyrene up to ~35 GPa using in situ single-crystal synchrotron X-ray diffraction in diamond anvil cells and ab initio calculations. They reveal the phase transitions from pyrene-I to pyrene-II (0.7 GPa), and to the previously unreported pyrene-IV (2.7 GPa), and pyrene-V (7.3 GPa). The structure and bonding analysis shows that gradual compression results in continuous compaction of molecular packing, eventually leading to curvature of molecules, which has never been observed before. Large organic molecules exhibit unexpectedly high conformational flexibility preserving pyrene-V up to 35 GPa. Ab initio calculations suggest that the phases we found are thermodynamically metastable compared to pyrene-III previously reported at 0.3 and 0.5 GPa. Our study contributes to the fundamental understanding of the polymorphism of polycyclic aromatic hydrocarbons and calls for further theoretical exploration of their structure–property relationships.

Following long-standing predictions associated with hydrogen, high-temperature superconductivity has recently been observed in several hydride-based materials. Nevertheless, these high-T_c phases only exist at extremely high pressures, and achieving high transition temperatures at ambient pressure remains a major challenge. Recent predictions of the complex hydride Mg_2IrH_6 may help overcome this challenge with calculations of high-T_c superconductivity (65 K< T_c < 170 K) in a material that is stable at atmospheric pressure. In this work, the synthesis of Mg_2IrH_6 was targeted over a broad range of P-T conditions, and the resulting products were characterized using X-ray diffraction (XRD) and vibrational spectroscopy, in concert with first-principles calculations. The results indicate that the charge-balanced complex hydride Mg_2IrH_5 is more stable over all conditions tested up to ca 28 GPa. The resulting hydride is isostructural with the predicted superconducting Mg_2IrH_6 phase except for a single hydrogen vacancy, which shows a favorable replacement barrier upon insertion of hydrogen into the lattice. Bulk Mg_2IrH_5 is readily accessible at mild P-T conditions and may thus represent a convenient platform to access superconducting Mg_2IrH_6 via non-equilibrium processing methods.

Abstract Fe1-x O, although chemically simple, possesses a complex structural and magnetic phase diagram. The crystal structures of Fe1-x O and its magnetic properties at extreme conditions are still a matter of debate. Here, we performed a systematic investigation on Fe0.94O up to 94 GPa and 1700 K using synchrotron X-ray diffraction and synchrotron Mössbauer source spectroscopy. We observe a transition of Fe0.94O to the monoclinic phases above 40 GPa and at high temperatures and use the group theory analysis of the observed phases to discuss their properties and their relation to the ambient pressure phases. The Mössbauer spectra of the rhombohedral and the room temperature monoclinic phase contain a component attributed to Fe2.5+, caused by the electron exchange between the Fe3+ defect and neighboring Fe2+ atoms. Our results present a structural and magnetic transitional pressure-temperature diagram of Fe1-x O and show the complex physicochemical properties of simple Fe1-x O binary oxide under extreme conditions.

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 N-2 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 N-18 macro-rings and cagelike N-8 building blocks, respectively. Differing from known polynitrogen structures, these two polymers were composed of single-bonded nitrogen atoms belonging to sp(3) and sp(2) 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.

In this work, an in situ characterization of the high-pressure and high-temperature phase diagram of ruthenium was carried out. This experiment was performed using a combination of laser-heated diamond anvil cell (LH-DAC) and X-ray diffraction (XRD) techniques at the beamline ID27 of the European Synchrotron Radiation Facility (ESRF). XRD patterns were collected in the range of 15 GPa to 110 GPa and from ambient temperature up to 6600 K. While the hcp-fcc transition, predicted to occur at elevated temperatures was not observed, this study has produced the first experimental observation of a solid–liquid phase transition of Ru at HP conditions. A P-V-T equation of state valid up to 100 GPa and 3000 K is reported.

Polytypism of incommensurately modulated structures was hitherto unobserved. Here, we found the phenomenon in simple halogen systems of bromine and iodine upon molecular dissociation in the solids under pressure. Single-crystal synchrotron X-ray diffraction in laser heated diamond anvil cells pressurised up to 112 GPa revealed a number of allotropes of bromine and iodine including polytypes of Br-III{\gamma} (Fmmm(00{\gamma})s00) with {\gamma} varying within 0.18 to 0.3.

The recent revolution in the superconductivity field stems from hydride superconductors. Multicomponent hydrides provide a crucial platform for tracking high-temperature superconductors. Besides high superconducting transition temperature (Tc), achieving both giant upper critical magnetic field [μ0Hc2(0)] and high critical current density [Jc(0)] is also key to the latent potential of the application for hydride superconductors. In this work, we have successfully synthesized quaternary La-Y-Ce hydrides with excellent properties under moderate pressure by using the concept of "entropy engineering." The obtained temperature dependence of the resistance provides evidence for the superconductivity of Fm3m-(La,Y,Ce)H10, with the maximum Tc ∼ 190 K (at 112 GPa). Notably, Fm3m-(La,Y,Ce)H10 boasts exceptional properties: μ0Hc2(0) reaching 292 T and Jc(0) surpassing 4.61 × 107 A/cm2. Compared with the binary LaH10/YH10, we find that the Fm3m structure in (La,Y,Ce)H10 can be stable at relatively low pressures (112 GPa). These results indicate that multicomponent hydrides can significantly enhance the superconducting properties and regulate stabilizing pressure through the application of "entropy engineering." This work stimulates the experimental exploration of multihydride superconductors and also provides a reference for the search of room-temperature superconductors in more diversified hydride materials in the future.

The yttrium-hydrogen system has gained attention because of near-ambient temperature superconductivity reports in yttrium hydrides at high pressures. We conducted a study using synchrotron single-crystal x-ray diffraction (SCXRD) at 87 to 171 GPa, resulting in the discovery of known (two YH3 phases) and five previously unknown yttrium hydrides. These were synthesized in diamond anvil cells by laser heating yttrium with hydrogen-rich precursors-ammonia borane or paraffin oil. The arrangements of yttrium atoms in the crystal structures of new phases were determined on the basis of SCXRD, and the hydrogen content estimations based on empirical relations and ab initio calculations revealed the following compounds: Y3H11, Y2H9, Y4H23, Y13H75, and Y4H25. The study also uncovered a carbide (YC2) and two yttrium allotropes. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature, as revealed by ab initio calculations, underline the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials.

The thermodynamic parameter pressure is ideal for producing novel ultraincompressible and superhard materials as it promotes the formation of polymeric frameworks and higher atomic coordination. In this regard, carbon and nitrogen are particularly attractive elements as they can produce extended arrangements of strong covalent bonds. In this study, a previously unobserved C3N4 polymorph, denoted as oP28-C3N4 (Pnnm, #58), is synthesized at pressures between 73 and 104 GPa in laser-heated diamond anvil cells and found recoverable to ambient conditions and stable in air. The crystal structure of oP28-C3N4, comprised of corner-sharing CN4 tetrahedra, is solved and refined using synchrotron single-crystal X-ray diffraction. With a bulk modulus of 334(3) GPa deduced from experimental data, the compound is highly incompressible. Based on macroscopic and microscopic calculations, its hardness may achieve 47.5 or 79.7 GPa, respectively, making it a superhard material. Incompressibility of CN4 tetrahedra in all experimentally observed C3N4 polymorphs is found to be greater than that of the CC4 and BN4 tetrahedra forming the structures of diamond and cubic boron nitride. Density functional theory calculations provide further insight into the electronic, vibrational, and mechanical properties of oP28-C3N4, as well as their stability relative to other C & horbar;N phases.

In this overview article, we present the main features of the upgraded ID27 beamline which is fully optimised to match the exceptional characteristics of the new Extremely Bright Source (EBS) of the European Synchrotron Radiation Facility (ESRF). The ID27 beamline has been converted to a 120-m-long instrument and has undergone a major refurbishment of all its critical components including both hardware and software environments. We will successively introduce the ID27 undulator X-ray source, optical scheme and the main components of the experimental hutch. We will illustrate the potential of this new instrument through some selected research examples.