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Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C 6 ] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC 2 polymorph and a Ca 3 C 7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C 6 ] rings can exist even at conditions of deep planetary interiors ( ~ 140 GPa and ~3300 K). Hydrolysis of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe., (© 2024. The Author(s).)

Nitrogen catenation under high pressure leads to the formation of polynitrogen compounds with potentially unique properties. The exploration of the entire spectrum of poly- and oligo-nitrogen moieties is still in its earliest stages. Here, we report on four novel scandium nitrides, Sc 2 N 6 , Sc 2 N 8 , ScN 5, and Sc 4 N 3 , synthesized by direct reaction between yttrium and nitrogen at 78-125 GPa and 2500 K in laser-heated diamond anvil cells. High-pressure synchrotron single-crystal X-ray diffraction reveals that in the crystal structures of the nitrogen-rich Sc 2 N 6 , Sc 2 N 8, and ScN 5 phases nitrogen is catenated forming previously unknown N 6 6 - and N 8 6 - units and ∞ 2 ( N 5 3 - ) anionic corrugated 2D-polynitrogen layers consisting of fused N 12 rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc 2 N 6 and Sc 2 N 8 possess an anion-driven metallicity, while ScN 5 is an indirect semiconductor. Sc 2 N 6 , Sc 2 N 8 , and ScN 5 solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT., (© 2024. The Author(s).)

Chemical reactions between dysprosium and carbon were studied in laser-heated diamond anvil cells at pressures of 19, 55, and 58 GPa and temperatures of ∼2500 K. In situ single-crystal synchrotron X-ray diffraction analysis of the reaction products revealed the formation of novel dysprosium carbides, Dy 4 C 3 and Dy 3 C 2 , and dysprosium sesquicarbide Dy 2 C 3 previously known only at ambient conditions. The structure of Dy 4 C 3 was found to be closely related to that of dysprosium sesquicarbide Dy 2 C 3 with the Pu 2 C 3 -type structure. Ab initio calculations reproduce well crystal structures of all synthesized phases and predict their compressional behavior in agreement with our experimental data. Our work gives evidence that high-pressure synthesis conditions enrich the chemistry of rare earth metal carbides., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Akbar, Aslandukova, Aslandukov, Yin, Trybel, Khandarkhaeva, Fedotenko, Laniel, Bykov, Bykova, Dubrovinskaia and Dubrovinsky.)

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures ( tP 10-Na 2 Cl 3 , hP 18-Na 4 Cl 5 , and hP 18-Na 4 Br 5 ), a series of isostructural cubic cP 8-AX 3 halides (NaCl 3 , KCl 3 , NaBr 3 , and KBr 3 ), and a trigonal potassium chloride ( hP 24-KCl 3 ). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl 3 - anion in hP 24-KCl 3 and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl] ∞ n - and [Br] ∞ n - , in the structures of cP 8-AX 3 compounds and in hP 18-Na 4 Cl 5 and hP 18-Na 4 Br 5 . In Na 4 Cl 5 and Na 4 Br 5 , we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth's lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93. Here we use Synchrotron Mössbauer Source (SMS) spectroscopy and single-crystal X-ray diffraction to determine the iron oxidation state and structure of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. Inclusion Mg#s vary between 16.1 and 84.5. Ferropericlase inclusions contain no ferric iron within the detection limit of SMS, while both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite ((Mg,Fe)Fe 3+ 2 O 4 ) with an estimated 47-53 wt% Fe 2 O 3 . We argue that the wide range of Fe concentrations observed in (Mg,Fe)O inclusions in diamonds and the appearance of magnesioferrite result from oxidation of ferropericlase triggered by the introduction of subducted material into sublithospheric mantle., (© 2022. The Author(s).)

The lanthanum-hydrogen system has attracted significant attention following the report of superconductivity in LaH 10 at near-ambient temperatures and high pressures. Phases other than LaH 10 are suspected to be synthesized based on both powder X-ray diffraction and resistivity data, although they have not yet been identified. Here, we present the results of our single-crystal X-ray diffraction studies on this system, supported by density functional theory calculations, which reveal an unexpected chemical and structural diversity of lanthanum hydrides synthesized in the range of 50 to 180 GPa. Seven lanthanum hydrides were produced, LaH 3 , LaH ~4 , LaH 4+δ , La 4 H 23 , LaH 6+δ , LaH 9+δ , and LaH 10+δ , and the atomic coordinates of lanthanum in their structures determined. The regularities in rare-earth element hydrides unveiled here provide clues to guide the search for other synthesizable hydrides and candidate high-temperature superconductors. The hydrogen content variability in lanthanum hydrides and the samples' phase heterogeneity underline the challenges related to assessing potentially superconducting phases and the nature of electronic transitions in high-pressure hydrides., (© 2022. The Author(s).)

A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for X-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe 0.79 Si 0.07 B 0.14 in a number of high-pressure X-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either axial or radial geometry with X-ray beams of micrometre to sub-micrometre size. We report that Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets offer a stable sample environment under compression exceeding 1 Mbar in all XRD experiments described here, even in those involving small-molecule gases (e.g. Ne, H 2 ) used as pressure-transmitting media or in those with laser heating in a DAC. Our results emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. They indicate that the application of Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets in XRD experiments for both axial and radial geometries substantially improves various aspects of megabar experiments and, in particular, the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals., (open access.)

The experimental study of hydrogen-bonds and their symmetrization under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrization has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution in-situ 1 H-NMR experiments in diamond anvil cells investigating a range of systems containing linear O-H ⋯  O units at pressure ranges of up to 90 GPa covering their respective H-bond symmetrization. We found pronounced minima in the pressure dependence of the NMR resonance line-widths associated with a maximum in hydrogen mobility, precursor to a localisation of hydrogen atoms. These minima, independent of the chemical environment of the O-H ⋯  O unit, can be found in a narrow range of oxygen oxygen distances between 2.44 and 2.45 Å, leading to an average critical oxygen-oxygen distance of [Formula: see text] Å., (© 2022. The Author(s).)

Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions 1,2 . Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell 3 , producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re 7 N 3 -which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime., (© 2022. The Author(s).)

One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H 2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa (Meier, Annu. Rep. NMR Spectrosc. 94:1-74, 2017; Meier, Prog. Nucl. Magn. Reson. Spectrosc. 106-107:26-36, 2018) and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of ortho-hydrogen at low pressures, above 70 GPa, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids.

Three binary fcc -structured alloys ( fcc -Ir 0.50 Pt 0.50 , fcc -Rh 0.66 Pt 0.33 and fcc -Rh 0.50 Pd 0.50 ) were prepared from [Ir(NH 3 ) 5 Cl][PtCl 6 ], [Ir(NH 3 ) 5 Cl][PtBr 6 ], [Rh(NH 3 ) 5 Cl] 2 [PtCl 6 ]Cl 2 and [Rh(NH 3 ) 5 Cl][PdCl 4 ]·H 2 O, respectively, as single-source precursors. All alloys were prepared by thermal decomposition in gaseous hydrogen flow below 800 °C. Fcc -Ir 0.50 Pt 0.50 and fcc -Rh 0.50 Pd 0.50 correspond to miscibility gaps on binary metallic phase diagrams and can be considered as metastable alloys. Detailed comparison of [Ir(NH 3 ) 5 Cl][PtCl 6 ] and [Ir(NH 3 ) 5 Cl][PtBr 6 ] crystal structures suggests that two isoformular salts are not isostructural. In [Ir(NH 3 ) 5 Cl][PtBr 6 ], specific Br…Br interactions are responsible for a crystal structure arrangement. Room temperature compressibility of fcc -Ir 0.50 Pt 0.50 , fcc -Rh 0.66 Pt 0.33 and fcc -Rh 0.50 Pd 0.50 has been investigated up to 50 GPa in diamond anvil cells. All investigated fcc -structured binary alloys are stable under compression. Atomic volumes and bulk moduli show good agreement with ideal solutions model. For fcc -Ir 0.50 Pt 0.50 , V 0 / Z = 14.597(6) Å 3 ·atom -1 , B 0 = 321(6) GPa and B 0 ' = 6(1); for fcc -Rh 0.66 Pt 0.33 , V 0 / Z = 14.211(3) Å 3 ·atom -1 , B 0 =259(1) GPa and B 0 ' = 6.66(9) and for fcc -Rh 0.50 Pd 0.50 , V 0 / Z = 14.18(2) Å 3 ·atom -1 , B 0 =223(4) GPa and B 0 ' = 5.0(3).

Iron tetra-nitride, FeN 4 , was synthesized from the elements in a laser-heated diamond anvil cell at 180 (5) GPa and 2700 (200) K. Its crystal structure was determined based on single-crystal X-ray diffraction data collected from a submicron-sized grain at the synchrotron beamline ID11 of ESRF. The compound crystallizes in the triclinic space group P . In the asymmetric unit, the Fe atom occupies an inversion centre (Wyckoff position 1 d ), while two N atoms occupy general positions (2 i ). The structure is made up from edge-sharing [FeN 6 ] octa-hedra forming chains along [100] and being inter-connected through N-N bridges. N atoms form catena -poly[tetraz-1-ene-1,4-di-yl] anions [-N=N-N-N-] ∞ 2- running along [001]. In comparison with the previously reported structure of FeN 4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9 , 2756], the crystal structure of FeN 4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the bond lengths and angles.

Hydrogen bond symmetrisations in H-bonded systems triggered by pressure-induced nuclear quantum effects (NQEs) is a long-known concept but experimental evidence in high-pressure ices has remained elusive with conventional methods. Theoretical works predicted quantum-mechanical tunneling of protons within water ices to occur at pressures above 30 GPa, and the H-bond symmetrisation transition to occur above 60 GPa. Here we used 1 H-NMR on high-pressure ice up to 97 GPa, and demonstrate that NQEs govern the behavior of the hydrogen bonded protons in ice VII already at significantly lower pressures than previously expected. A pronounced tunneling mode was found to be present up to the highest pressures of 97 GPa, well into the stability field of ice X, where NQEs are not anticipated in a fully symmetrised H-bond network. We found two distinct transitions in the NMR shift data at about 20 GPa and 75 GPa attributed to the step-wise symmetrisation of the H-bond.