Magnesium chloride hydrates show a rich diversity of structural forms, with different hydration numbers ranging from 1 to 12. We show that this structural versatility is also present within the same hydration numbers, with the formation of a new phase of magnesium chloride decahydrate (MgCl210H2O-II) stable at the conditions of 0.03-0.40(14) GPa at 234(11) K and up to 2.11(3) GPa at 220(2) K, as determined by in situ single-crystal X-ray diffraction. The hydrogen bonding interactions of MgCl210H2O-II are identified and the bulk modulus determined at B 0 = 18.9(12) GPa at 220(2) K. We discuss the implication of this high-pressure MgCl2 hydrate for icy moon compositional investigations and showcase the structural and density changes between the various hydrates in MgCl2nH2O.

Low-dimensional polytellurides are of broad interest because their flexible bonding gives rise to unusual electronic states, structural instabilities, and transport phenomena, yet their average crystal structures can obscure the local tellurium motifs that actually control these properties. The electronic character of LnCu x Te2 (Ln = lanthanide) remains unresolved because electronic structure calculations based on the average crystal structure predict metallic behavior, whereas experiments consistently show semiconducting transport. Here we examine this discrepancy in LaCu x Te2 by combining low-temperature single-crystal X-ray diffraction, total-scattering analysis, electrical transport measurements, high-pressure studies, and first-principles calculations. Single-crystal diffraction at 100 K reveals an incommensurately modulated structure that is absent at room temperature, and X-ray total scattering indicates local unit-cell tripling along the direction of the apparent linear Te chains. These results show that the crystallographic infinite chain is not a real chemical entity but an average motif that masks locally distorted Te arrangements. In this picture, the anomalous transport of LaCu x Te2 is linked to chain distortion and structural disorder rather than to an idealized delocalized Te chain electronic state. Consistent with this interpretation, LaCu x Te2 exhibits semiconducting behavior over the full temperature range studied, variable-range hopping at low temperature, and quasi-linear nonsaturating magnetoresistance attributable to spatially inhomogeneous carrier mobility. Under pressure, the room-temperature structure remains unchanged and the charge transport activation energy decreases, yet metallic behavior does not emerge. Together, these results reconcile the apparent conflict between ideal-chain calculations and experiment, demonstrating that hidden local distortion within an average one-dimensional tellurium motif governs the electronic behavior of LaCu x Te2.

Accurate pressure calibration is fundamental to quantitative high-pressure science, yet inconsistencies among widely used secondary pressure scales persist. We report simultaneous synchrotron x-ray diffraction meatogether in diamond anvil cells to 140 GPa. This simultaneous-measurement strategy minimizes transitive errors and run-to-run inconsistencies inherent to paired-measurement approaches, yielding direct experimental volume-volume (V-V) relations among the calibrants. These V-V relations link the compression state of each phase to that of every other phase measured in the same experiment. By anchoring these V-V relations to the reduced 300 K equation of state (EOS) of copper derived from ramp-compression measurements, we derive an internally consistent set of Cu-referenced pressure scales via Vinet EOS fits. Because the primary experimental result is the V-V dataset, it provides an EOS-independent constraint that can be rereferenced if primary standards are revised. The dataset further enables a direct experimental cross-check of consistency between the adopted reduced 300 K Cu EOS and the corresponding reduced 300 K ramp-derived EOSs for Pt, Au, Ta, and hcp-Fe under a fixed V-V constraint. The residuals quantify small, material-dependent mismatches between each phase EOS and the Cu reference EOS.

Superconductivity has been a vigorously researched topic since its discovery in 1911. Raising the superconducting transition temperature (Tc) has been the main driving force behind such long-sustained efforts due to its potential for impacting humanity and the fundamental knowledge gained from understanding this macroscopic coherent quantum state at high temperatures. The successful development of high-Tc superconductivity will make possible extraordinarily efficient generation, delivery, and utilization of energy and could also enable the development of controlled fusion while impacting other burgeoning fields like quantum computation and quantum electronics. However, progress has been hindered by a longstanding plateau in the record ambient-pressure Tc, unchanged since 1993. Subsequent significant advancements in Tc have been achieved only under high pressures, preventing the realization of superconductivity's full potential. To directly address this challenge, we developed a pressure-quench protocol (PQP) to stabilize pressure-induced/-enhanced superconducting states at ambient pressure. Here, we achieve a record ambient-pressure Tc of 151 K in the cuprate HgBa2Ca2Cu3O8+δ via PQP. The experimental results are further supported by synchrotron X-ray diffraction measurements and phonon and electronic structure calculations. This breakthrough opens avenues for stabilizing and exploring ambient-pressure high-Tc superconducting states and other quantum states that have been previously only accessible under pressure, paving the way for deeper understanding and practical applications of high-Tc superconductivity and beyond.

Structure-property relations of the recently discovered germanium allotrope oP32-Ge are examined using a variety of experimental high-pressure techniques and compared with the results of density functional theory (DFT) calculations. High-pressure single-crystal x-ray diffraction shows that the unit-cell parameters and atomic positions of oP32-Ge change continuously up to 6 GPa, while peak shape and diffuse-scattering features remain essentially unchanged. X-ray diffraction reveals a sluggish transformation of oP32-Ge to the beta-Sn Ge-II polymorph, i.e., the transition begins around 8 GPa and is complete by 12 GPa. The reconstructive transition is associated with the breakdown of oP32-Ge crystals into polycrystalline aggregates, which were examined up to 32 GPa. Synchrotron infrared absorption measurements indicate an increase in the band gap of oP32-Ge prior to the transition at 8 GPa to the higher-pressure metallic phase, which is characterized by high optical reflectivity. The DFT calculations of the structural and electronic properties are in good agreement with the measurements. Our experimental work conclusively shows that, under room-temperature quasihydrostatic compression, oP32-Ge remains in its semiconducting phase up to approximately 8 GPa, transforming into the Ge-II structure only by 12 GPa, a stability interval comparable to that of diamond-structured Ge (Ge-I). Crucially, we observe no anomalous structural or bonding changes over the range of conditions explored, indicating that the previously reported low-pressure superconductivity (near 2 GPa) must stem from subtle structural or stress-induced modifications accessible under the nonhydrostatic, low-temperature conditions of those experiments.

Synchrotron X-ray diffraction has been used to investigate the structure and equation of state (EOS) of hydrazine (N2H4) up to 54.3 GPa at 298 K. The diffraction patterns could be fit to a monoclinic unit-cell structure and put strong constraints on previously reported phase transitions documented by vibrational spectroscopy over this pressure range. Pressure-volume (P-V) data were fit using a Vinet EOS, yielding parameters: V 0 = 45.2 & Aring;3/molecule (fixed), K 0 = 11.8(7) GPa, and K 0 ' = 6.5(2). Previously measured high-pressure vibrational frequency shifts were used to estimate the vibrational free energy and model P-V-T isotherms from 0 to 1200 K. The results of the P-V-T isotherms are compared to existing shock Hugoniot data on hydrazine and 298 K isotherms for assemblages of possible decomposition products. This comparison suggests dissociation at high density under shock loading. Good correspondence was found between the static lattice EOS as calculated by the model and the previously reported EOS as calculated by density functional theory. These results resolve existing uncertainties about the EOS and crystal symmetry of hydrazine at high pressure and provide valuable baseline information on this important energetic material.

The large lattice dynamics of lithium, driven by its low atomic mass, results in energetically similar structures and significant isotope effects under pressure, posing challenges to current theoretical models. Above 20 GPa and at low temperatures, lithium's electronic properties deviate from simple metallic behavior, with superconductivity emerging in a complex, pressure-dependent manner, alongside an unusual isotope effect. The structural phases of 7Li reported under these conditions are inconsistent across studies, and the structures of 6Li remain unexamined. These gaps limit our understanding of the effects of pressure on lithium's electronic properties and the role of quantum lattice effects on its structural behavior under pressure. Here, we integrate experimental and theoretical approaches to investigate the low-temperature structural phase boundaries in lithium isotopes. We map the structural phase diagram of 7Li from 5 to 55 GPa and 15-75 K, identifying the sequence fcc-* hR1-* cI 16. A pronounced isotope effect is observed, with 6Li shifting the fcc-* hR1 phase boundary to lower pressures at 15 K. Density functional theory calculations further clarify how these structural changes affect superconducting properties, particularly emphasizing the role of the fcc-* hR1 transition in lithium's superconductivity. Our findings offer insights into the unique behavior of lithium isotopes under pressure.

Rare earth superhydrides exhibit high temperature superconductivity but are difficult to characterize and use in applications due to their high formation and stability pressures, which are typically in excess of 100 GPa. We studied how modification of the rare earth precursor improves hydrogen reactivity and hydrogen uptake for forming such metal hydrides at lower pressures. An elemental lanthanum precursor was milled at liquid nitrogen temperatures for different time intervals. After exposure to gaseous hydrogen at 380 C and 100 bar, we found a systematic enhancement of hydrogen absorption with increasing ball milling time for forming the LaHx, x=2-3 phase. Exposing the precursor to pressures up to 60 GPa with an ammonia borane (BNH6) hydrogen source resulted in a hypervalent LaH4 phase. This LaH4 phase is associated with the suppression of a rhombohedral distortion of the Fm3-m cubic structure after cryomilling the precursor.

We report on the structural verification of metastable ice VII solidifying in the phase space of ice VI at 1.80 GPa at room temperature. Using time-resolved (TR) x-ray diffraction and TR ruby luminescence paired with high-speed microphotography utilizing a dynamic diamond anvil cell, an initial compression rate range from 0.12 to 95.84 GPa/s was explored. The solidification pressure of metastable ice VII has a potential sigmoidal dependence upon compression rate with a turnover compression rate of similar to 80 GPa/s. The preferred crystallization of ice VII in the stability field of ice VI is due to the increased nucleation rate of ice VII over ice VI at 1.77 GPa that is driven by the surface energy difference between the liquid and solid phases along with the change in Gibbs free energy of solidification. The dynamic pressure-volume-compression behaviors of ice phases (VI and VII) show a lattice stiffening in both phases, especially during the compression loading. It is also found that the compression rate greatly affects the solid-solid phase transition between ice VI and VII but does not affect the liquid-solid transition between water and ice VI as much. Lastly, a third phase transition was found to occur after metastable ice VII transforms into high-density amorphous (HDA) ice, which could be a disordered hydrogenbonded network configuration of ice VII forming out of HDA ice facilitated by the decoupling of the oxygen movement and reorientation of the H2O molecule. These results demonstrate the complexity of a seemingly simple molecule H2O, how it can readily change its static properties with the modification of (de)compression rate, and highlight the need to use multiple TR structural and spectroscopic probes at higher time resolutions to realize the most comprehensive understanding.

The high pressure equation of state for the W W0.75 W0.75Re W0.75Re0.25 alloy is experimentally determined up to 183 GPa with synchrotron angle-dispersive powder x-ray diffraction in the diamond-anvil cell and to ' '925 GPa with density-functional theory. W-Re alloys are used in many industrial high-temperature applications and as a confining gasket material in high-pressure diamond-anvil cell research. The inclusion of 25 wt. % Re achieves the highest performance in terms of strength and ductility while also maintaining the body-centered-cubic (bcc) crystal structure, yet to date there has been no investigation into its elastic behavior at high pressure. We present the experimentally and theoretically determined volumetric and elastic pressure response and systematically compare these results to other W-Re alloys, finding that the bulk modulus of W-Re alloys varies nonlinearly with Re content and W W0.75 W0.75Re W0.75Re0.25 becomes less incompressible than W at 85 GPa.

The pressure-induced polymorphism of binary octect compounds has long been considered a settled problem although the possible atomic disordering of some phases remains a puzzling observation. Taking GaP as a case study, we conclude, through x-ray microdiffraction and first-principles calculations, that its high-pressure phase II (previously reported as being disordered) adopts in fact an ordered base-centered monoclinic structure previously unknown in this class of compounds. The formation of layered patterns with variable degrees of interlayer dimerization, as observed in GaP, marks a paradigm shift of our understanding of ordering in octect high-pressure phases which calls for a more extensive re-examination. A rich polymorphism with fine tuning of chemical and physical properties can be envisioned.

Nuclear materials often evolve into two-phase systems comprising a bulk matrix with dispersed inert-gas bubbles. The presence of these bubbles can have consequences to the thermomechanical response of materials and is a key life-limiting factor in some nuclear fuel forms. Understanding the behavior of these two-phase, bubble-matrix systems is, thus, important to improved predictive models and frameworks for many nuclear materials applications. While temperature excursions of these two-phase systems have been characterized, fewer studies have focused on the evolution of inert-gas bubbles under pressure. In this paper, we use x-ray tools to interrogate a He-implanted gold foil to determine the pressure-dependent evolution of the individual components (Au matrix + bubbles), and we compare that total pressure dependence to theoretical equation-of-state descriptions based on mixing rules.

High-pressure β-Sn germanium may transform into diverse metastable allotropes with distinctive nanostructures and unique physical properties via multiple pathways under decompression. However, the mechanism and transition kinetics remain poorly understood. Here, we investigate the formation of metastable phases and nanostructures in germanium via controllable transition pathways of β-Sn Ge under rapid decompression at different rates. High-resolution transmission electron microscopy reveals three distinct metastable phases with the distinctive nanostructures: an almost perfect st12 Ge crystal, nanosized bc8/r8 structures with amorphous boundaries, and amorphous Ge with nanosized clusters (0.8–2.5 nm). Fast in situ x-ray diffraction and x-ray absorption measurements indicate that these nanostructured products form in certain pressure regions via distinct kinetic pathways and are strongly correlated with nucleation rates and electronic transitions mediated by compression rate, temperature, and stress. This work provides deep insight into the controllable synthesis of metastable materials with unique crystal symmetries and nanostructures for potential applications.

We have measured the luminescence shift of the ruby's R1-line corresponding to the transition pressure of bismuth (Bi) I-II. The Bi sample was loaded in a diamond anvil cell using neon as pressure medium, with small ruby spheres positioned near the Bi sample. Fine compression and decompression were controlled by a double-sided membrane system. Synchrotron x-ray diffraction measurements on Bi and luminescence measurements on ruby were simultaneously collected using an in-line ruby system at the 16-ID-B beamline at the Advanced Photon Source. The Bi I-II transformation is signified by the distinct diffraction patterns, associated with discontinuities in the luminescence shift as a function of time. The determined ruby's R1-shift is 0.9311 +/- 0.007 nm at the Bi I-II transition at 25.8 +/- 0.1 degrees C. Together with the previously determined R1-shift relative to the melting pressure of mercury, the calibrations constrain the initial slope of ruby gauge to 1860 +/- 3 GPa, compared to 1870 +/- 10 GPa in the Ruby2020 gauge.

We evaluate pressure consistency of equations of state (EOS) for NaCl and Au at 300 K. The simultaneous measurements of unit-cell volumes (V) with ruby R1 line shifts (Delta lambda) in a helium (He) loaded diamond cell effectively remove potential systematic errors. Compression and decompression data were automatically collected at 1 sec interval, yielding a dense dataset with >8,000 (V, Delta lambda) pairs each for NaCl and Au. Solidification of He has noticeable effects on both V and Delta lambda (hence P). Only data up to similar to 14 GPa, or 6000 (V, P) pairs, can be considered hydrostatic within the resolution. The P-V data are fitted to the Rydberg-Vinet and the 3(rd) order Birch-Murnaghan EOS. Predicted pressures of these EOSs agree with those given by the Ruby2020 ruby scale to within +/- 0.05 GPa. Overall, pressures predicted by the Rydberg-Vinet EOS are in better agreement with the average of commonly used NaCl and Au pressure scales.

Retraction of 'Carbon content drives high temperature superconductivity in a carbonaceous sulfur hydride below 100 GPa' by G. Alexander Smith et al., Chem. Commun., 2022, 58, 9064-9067, https://doi.org/10.1039/D2CC03170A.

Pressure-induced phase transformations (PTs) in Si, the most important electronic material, have been broadly studied. However, strain-induced PTs in Si were never studied in situ . Here, we revealed in situ various important plastic strain-induced PT phenomena. A correlation between the particle size's direct and inverse Hall-Petch effect on yield strength and pressure for strain-induced PT is found. For 100 nm particles, strain-induced PT Si-I→Si-II initiates at 0.3 GPa versus 16.2 GPa under hydrostatic conditions; Si-I→Si-III PT starts at 0.6 GPa and does not occur under hydrostatic pressure. Pressure in small Si-II and Si-III regions is ~5-7 GPa higher than in Si-I. Retaining Si-II and single-phase Si-III at ambient pressure and obtaining reverse Si-II→Si-I PT demonstrates the possibilities of manipulating different synthetic paths. The obtained results corroborate the elaborated dislocation pileup-based mechanism and have numerous applications for developing economic defect-induced synthesis of nanostructured materials, surface treatment (polishing, turning, etc.), and friction.

Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI$_{2}$ is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI$_{2}$. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector $\mathbf{q}$ of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI$_{2}$.

Synchrotron X-ray diffraction (XRD) and Raman spectroscopy in laser heated diamond anvil cells and first principles molecular dynamics (FPMD) calculations have been used to investigate the reactivity of calcite and molecular hydrogen (H 2 ) at high pressures up to 120 GPa. We find that hydrogen reacts with calcite starting below 0.5 GPa at room temperature forming chemical bonds with carbon and oxygen. This results in the unit cell volume expansion; the hydrogenation level is much higher for powdered samples. Single-crystal XRD measurements at 8 - 24 GPa reveal the presence of previously reported III, IIIb, and VI calcite phases; some crystallites show up to 4% expansion, which is consistent with the incorporation of <= 1 hydrogen atom per formula unit. At 40 - 102 GPa XRD patterns of hydrogenated calcite demonstrate broadened features consistent with the calcite VI structure with incorporated hydrogen atoms. Above 80 GPa, the C - O stretching mode of calcite splits suggesting a change in the coordination of C - O bonds. Laser heating at 110 GPa results in the formation of C - C bonds manifested in the crystallization of diamond recorded by in situ XRD at 300 K and 110 GPa and by Raman spectroscopy on recovered samples commenced with C 13 calcite. We explored several theoretical models, which show that incorporation of atomic hydrogen results in local distortions of CO 3 groups, formation of corner-shared C - O polyhedra, and chemical bonding of H to C and O, which leads to the lattice expansion and vibrational features consistent with the experiments. The experimental and theoretical results support recent reports on tetrahedral C coordination in high-pressure carbonate glasses and suggest a possible source of the origin of ultradeep diamonds.