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We show, by single-crystal diffraction studies in laser-heated diamond-anvil cells, that Ca2CO4 orthocarbonate, which contains CO44− tetrahedra, can be formed already at ~20 GPa at ~1830 K, i.e., at much lower pressures than other carbonates with sp3-hybridized carbon. Ca2CO4 can also be formed at ~89 GPa and ~2500 K. This very broad P-T range suggests the possible existence of Ca2CO4 in the Earth’s transition zone and in most of the lower mantle. Raman spectroscopy shows the typical bands associated with tetrahedral CO44−-groups. DFT-theory based calculations reproduce the experimental Raman spectra and indicate that at least in the athermal limit the phase assemblage of Ca2CO4 + 2SiO2 is more stable than 2CaSiO3 + CO2 at high pressures.

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

Carbonates containing CO4 groups as building blocks have recently been discovered. A new orthocarbonate, Sr2CO4 is synthesized at 92 GPa and at a temperature of 2500 K. Its crystal structure was determined by in situ synchrotron single‐crystal X‐ray diffraction, selecting a grain from a polycrystalline sample. Strontium orthocarbonate crystallizes in the orthorhombic crystal system (space group Pnma) with CO4, SrO9 and SrO11 polyhedra as the main building blocks. It is isostructural to Ca2CO4. DFT calculations reproduce the experimental findings very well and have, therefore, been used to predict the equation of state, Raman and IR spectra, and to assist in the discussion of bonding in this compound., A new orthocarbonate, Sr2CO4, was synthesized under extreme pressure and temperature conditions of 92 GPa and 2500 K, respectively. The crystal structure of the compound s fully characterized in situ by synchrotron single‐crystal X‐ray diffraction and DFT calculations were employed to provide insight into its equation of state, Raman and IR spectra, and bonding. image

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf4N20 center dot ...

Acta crystallographica / Section E 76(5), 715 - 719 (2020). doi:10.1107/S2056989020005411, The crystal structure of MgCO$_3$-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp$^2$ to the sp$^3$ type of bonding. We have now determined the crystal structure of iron-bearing MgCO$_3$-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg$_{2.53}$Fe$_{0.47}$)C$_3$O$_9$ in the space group C2/m that contains tetra­hedrally coordinated carbon, where CO$_4$$^{4-}$ tetra­hedra are linked by corner-sharing oxygen atoms to form three-membered C$_3$O$_3$$^{6-}$ ring anions. The crystal structure of (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles., Published by National Center for Biotechnology Information, U.S. National Library of Medicine15046, Bethesda, MD

Nature 605(7909), 274 - 278 (2022). doi:10.1038/s41586-022-04550-2, Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions1,2. 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 cell3, 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., Published by Nature Publ. Group, London [u.a.]

Review of scientific instruments 93(3), 033904 (2022). doi:10.1063/5.0071786, The success of high-pressure research relies on the inventive design of pressure-generating instruments and materials used for their construction. In this study, the anvils of conical frustum or disk shapes with flat or modified culet profiles (toroidal or beveled) were prepared by milling an Ia-type diamond plate made of a (100)-oriented single crystal using the focused ion beam. Raman spectroscopy and synchrotron x-ray diffraction were applied to evaluate the efficiency of the anvils for pressure multiplication in different modes of operation: as single indenters forced against the primary anvil in diamond anvil cells (DACs) or as pairs of anvils forced together in double-stage DACs (dsDACs). All types of secondary anvils performed well up to about 250 GPa. The pressure multiplication factor of single indenters appeared to be insignificantly dependent on the shape of the anvils and their culets’ profiles. The enhanced pressure multiplication factor found for pairs of toroidally shaped secondary anvils makes this design very promising for ultrahigh-pressure experiments in dsDACs., Published by American Institute of Physics, [S.l.]

Changes in the bonding of carbon under high pressure leads to unusual crystal chemistry and can dramatically alter the properties of transition metal carbides. In this work, the new orthorhombic polymorph of yttrium carbide, $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Y}}_{4}{\mathrm{C}}_{5}$, was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at $\ensuremath{\sim}50\text{ }\text{ }\mathrm{GPa}$. The structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Y}}_{4}{\mathrm{C}}_{5}$ was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: [${\mathrm{C}}_{2}$] dimers and nonlinear [${\mathrm{C}}_{3}$] trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges (${[{\mathrm{C}}_{2}]}^{5.2\ensuremath{-}}$ and ${[{\mathrm{C}}_{3}]}^{6.8\ensuremath{-}}$) and unique bond orders ($l1.5$). Our results extend the list of possible carbon states at extreme conditions.

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

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe2O3 and the appearance of FeO2. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites. © 2021 American Physical Society. We acknowledge the Deutsches Elektronen-Synchrotron (DESY, PETRA III), the European Synchrotron Radiation Facility (ESRF), and the Advance Photon Source (APS) for provision of beamtime. N. D. and L. D. thank the Federal Ministry of Education and Research, Germany (BMBF, Grant No. 05K19WC1) and the Deutsche Forschungsgemeinschaft (DFG Projects No. DU 954–11/1, No. DU 393–9/2, and No. DU 393–13/1) for financial support. N. D. and I. A. A. thank the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). Electronic structure calculations were supported by the Russian Science Foundation (Project No. 18-12-00492). Theoretical analysis of chemical bonding was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” (No. K2-2019-001) implemented by a governmental decree, No. 211. Support from the Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant No. KAW-2018.0194), the Swedish Government Strategic Research Areas and SeRC, and the Swedish Research Council (VR) Grant No. 2019-05600 is gratefully acknowledged.

Minerals 11(5), 516 (2021). doi:10.3390/min11050516, We report the high-pressure synthesis and the equation of state (EOS) of a novel nickel carbide (Ni$_3$C). It was synthesized in a diamond anvil cell at 184(5) GPa through a direct reaction of a nickel powder with carbon from the diamond anvils upon heating at 3500 (200) K. Ni$_3$C has the cementite-type structure (Pnma space group, a = 4.519(2) Å, b = 5.801(2) Å, c = 4.009(3) Å), which was solved and refined based on in-situ synchrotron single-crystal X-ray diffraction. The pressure-volume data of Ni$_3$C was obtained on decompression at room temperature and fitted to the 3rd order Burch-Murnaghan equation of state with the following parameters: V$_0$ = 147.7(8) Å$^3$, K$_0$ = 157(10) GPa, and K$_0$ ' = 7.8(6). Our results contribute to the understanding of the phase composition and properties of Earth's outer core., Published by MDPI, Basel

Physical review letters 124(21), 216001 (2020). doi:10.1103/PhysRevLett.124.216001, Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal x-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitrogen phases and provides insights into polynitrogen arrangements at extreme densities., Published by APS, College Park, Md.

High pressure Nuclear Magnetic Resonance (NMR) is known to uncover behavior of matter at extreme conditions. However, significant maintenance demands, space requirements and high costs of superconducting magnets render its application unfeasible for regular modern high pressure laboratories. Here, we present a table-top NMR system based on permanent Halbach magnet arrays with dimensions of 25 cm diameter and 4 cm height. At the highest field of 1013 mT, 1H-NMR spectra of Ice VII have been recorded at 25 GPa and ambient temperature. The table-top NMR system can be used together with double sided laser heating set-ups. Feasibility of high-pressure high-temperature NMR was demonstrated by collecting 1H-NMR spectra of H2O at 25 GPa and 1063(50) K. We found that the change in signal intensity in laser-heated NMR diamond anvil cell yields a convenient way for temperature measurements.

This study is devoted to the detailed in situ Raman spectroscopy investigation of propane C3H8 in laser-heated diamond anvil cells in the range of pressures from 3 to 22 GPa and temperatures from 900 to 3000 K. We show that propane, while being exposed to particular thermobaric conditions, could react, leading to the formation of hydrocarbons, both saturated and unsaturated as well as soot. Our results suggest that propane could be a precursor of heavy hydrocarbons and will produce more than just sooty material when subjected to extreme conditions. These results could clarify the issue of the presence of heavy hydrocarbons in the Earth’s upper mantle.

Physical review / B 102(13), 134109 (1-8) (2020). doi:10.1103/PhysRevB.102.134109, The sulfur-hydrogen system is the first one in which superconductivity at temperatures over 200 K has been reported, albeit at high pressure. The particular phases causing the measured T$_c$ and their structures are not yet firmly identified. Here, synchrotron single-crystal x-ray diffraction studies of S-H samples were performed up to 150 GPa and revealed two previously unobserved and unpredicted sulfur-hydrogen phases—H$_{6±x}$S$_5$ with $x$∼0.4, and H$_{2.85±y}$S$_2$ with $y$∼0.35. The crystallographic data obtained in this work, both for the new phases and for the previously identified H$_3$S polymorphs, provide an unambiguous experimental proof of the chemical richness of the S-H system and the structural diversity of compounds forming at high pressures and high temperatures. Our results have profound implications for the interpretation of the resistance, superconductivity, and other physical properties measurements on the complex S-H system., Published by Inst., Woodbury, NY

Thermal decomposition of (NH4)2[OsxPt1-xCl6] as single-source precursors for Os–Pt binary alloys has been investigated under ambient and high pressure up to 40 GPa. Thermal decomposition of mixed-metal (NH4)2[OsxPt1-xCl6] precursors in hydrogen atmosphere (reductive environment) under ambient pressure results in formation of β-trans-[Pt(NH3)2Cl2] and α-trans-[Pt(NH3)2Cl2] crystalline intermediates as well as single and two-phase Os–Pt binary alloys. For the first time, direct thermal decomposition of coordination compound under pressure has been investigated. A formation of pure metallic alloys from single-source precursors under pressure has been shown. Miscibility between fcc- and hcp-structured alloys has been probed up to 50 GPa by in situ high-pressure X-ray diffraction. Miscibility gap between fcc- and hcp-structured alloys does not change its positions with pressure up to at least 50 GPa.

Physical review / B 102(16), 165109 (1-8) (2020). doi:10.1103/PhysRevB.102.165109, The atomic and electronic structures of Cu$_2$H and CuH have been investigated by high-pressure nuclear magnetic resonance spectroscopy up to 96 GPa, X-ray diffraction up to 160 GPa, and density functional theory-based calculations. Metallic Cu$_2$H was synthesized at a pressure of 40 GPa, and semimetallic CuH at 90 GPa, found stable up to 160 GPa. For Cu$_2$H, experiments and computations show an anomalous increase in the electronic density of state at the Fermi level for the hydrogen 1s states and the formation of a hydrogen network in the pressure range 43–58 GPa, together with high 1H mobility of ∼10$^{−7}$cm$^2$/s. A comparison of these observations with results on FeH suggests that they could be common features in metal hydrides., Published by Inst., Woodbury, NY

Minerals 10(6), 542 (2020). doi:10.3390/min10060542, It is widely accepted that the lower mantle consists of mainly three major minerals—ferropericlase, bridgmanite and calcium silicate perovskite. Ferropericlase ((Mg,Fe)O) is the second most abundant of the three, comprising approximately 16–20 wt% of the lower mantle. The stability of ferropericlase at conditions of the lowermost mantle has been highly investigated, with controversial results. Amongst other reasons, the experimental conditions during laser heating (such as duration and achieved temperature) have been suggested as a possible explanation for the discrepancy. In this study, we investigate the effect of pulsed laser heating on the stability of ferropericlase, with a geochemically relevant composition of Mg$_{0.76}$Fe$_{0.24}$O (Fp24) at pressure conditions corresponding to the upper part of the lower mantle and at a wide temperature range. We report on the decomposition of Fp24 with the formation of a high-pressure (Mg,Fe)$_3$O$_4$ phase with CaTi$_2$O$_4$-type structure, as well as the dissociation of Fp24 into Fe-rich and Mg-rich phases induced by pulsed laser heating. Our results provide further arguments that the chemical composition of the lower mantle is more complex than initially thought, and that the compositional inhomogeneity is not only a characteristic of the lowermost part, but includes depths as shallow as below the transition zone., Published by MDPI, Basel

Angewandte Chemie / International edition 57(29), 9048 - 9053 (2018). doi:10.1002/anie.201805152, A nitrogen‐rich compound, $ReN_{8}·xN_{2}$, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser‐heated diamond anvil cell. Single‐crystal X‐ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular‐shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, $ReN_{8}·xN_{2}$ is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [−N=N−]$_∞$ that constitute the framework have not been previously observed in any compound. Ab initio calculations on $ReN_{8}·xN_{2}$ provide strong support for the experimental results and conclusions., Published by Wiley-VCH, Weinheim

Knowledge of the behavior of hydrogen in metal hydrides is the key for understanding their electronic properties. Here, we present an 1H−NMR study of cubic FeH up to 202 GPa. We observe a distinct deviation from the ideal metallic behavior between 64 and 110 GPa that suggests pressure-induced H-H interactions. Accompanying ab initio calculations support this result, as they reveal the formation of an intercalating sublattice of electron density, which enhances the hydrogen contribution to the electronic density of states at the Fermi level. This study shows that pressure-induced H-H interactions can occur in metal hydrides at much lower compression and larger H-H distances than previously thought and stimulates an alternative pathway in the search for novel high-temperature superconductors., Physical Review X, 9 (3), ISSN:2160-3308

The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials. Attesting to the intrinsic difficulties related to their formation, only three polynitrogen ions, bulk stabilized as salts, are known. Here, magnesium and molecular nitrogen are compressed to about 50 GPa and laser-heated, producing two chemically simple salts of polynitrogen anions, MgN4 and Mg2N4. Single-crystal X-ray diffraction reveals infinite anionic polythiazyl-like 1D N-N chains in the crystal structure of MgN4 and cis-tetranitrogen N44− units in the two isosymmetric polymorphs of Mg2N4. The cis-tetranitrogen units are found to be recoverable at atmospheric pressure. Our results respond to the quest for polynitrogen entities stable at ambient conditions, reveal the potential of employing high pressures in their synthesis and enrich the nitrogen chemistry through the discovery of other nitrogen species, which provides further possibilities to design improved polynitrogen arrangements., Polynitrogen compounds are potentially promising high energy density materials, but are difficult to synthesize due to their instability. Here, the authors observe the formation, under high pressure, of a Mg2N4 magnesium–tetranitrogen salt which remains stable at ambient conditions.

Sulfur and nitrogen represent one of the most studied inorganic binary systems at ambient pressure on account of their large wealth of metastable exotic ring-like compounds. Under high pressure conditions, however, their behavior is unknown. Here, sulfur and nitrogen were compressed in a diamond anvil cell up to about 120 GPa and laser-heated at regular pressure intervals in an attempt to stabilize novel sulfur nitrogen compounds. Above 64 GPa, an orthorhombic (space group Pnnm) SN2 compound was synthesized and characterized by single-crystal and powder X-ray diffraction as well as Raman spectroscopy. It is shown to adopt a CaCl2-type structure hence it is isostructural, isomassic, and isoelectronic to CaCl2-type SiO2 comprised of SN6 octahedra. Complementary theoretical calculations were performed to provide further insight into the physicochemical properties of SN2, notably its equation of state, the bonding type between its constitutive elements, and its electronic density of states. This new solid is shown to be metastable down to about 20 GPa, after which it spontaneously decomposes into S and N-2. This investigation shows that despite the many metastable S N compounds existing at ambient conditions, none of them are formed by pressure. Funding Agencies|Alexander von Humboldt Foundation; Federal Ministry of Education and Research, Germany (BMBF) [5K16WC1]; Deutsche Forschungsgemeinschaft (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; Russian Science Foundation [18-12-00492]; Swedish Research Council (VR) [2015-04391]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Journal of alloys and compounds 788, 212 - 218 (2019). doi:10.1016/j.jallcom.2019.02.206, Knowledge of the compressional and thermal behaviour of metals and alloys is of a high fundamental and applied value. In this work, we studied the behaviour of Ir, Rh, and their fcc-structured alloys, Ir$_{0.42}$Rh$_{0.58}>$ and Ir$_{0.26}$Os$_{0.05}$Pt$_{0.31}$Rh$_{0.23}$Ru$_{0.15}$, up to 70 GPa using the diamond anvil cell technique with synchrotron X-ray diffraction. We found that all these materials are structurally stable upon room-temperature hydrostatic compression in the whole pressure interval, as well as upon heating to 2273 K both at ambient and high pressure. Rh, Ir$_{0.42}$Rh$_{0.58}$ and Ir$_{0.26}$Os$_{0.05}$Pt$_{0.31}$Rh$_{0.23}$Ru$_{0.15}$ were investigated under static compression for the first time. According to our data, the compressibility of Ir, Rh, fcc–Ir$_{0.42}$Rh$_{0.58}$, and fcc–Ir$_{0.26}$Os$_{0.05}$Pt$_{0.31}$Rh$_{0.23}$Ru$_{0.15}$, can be described with the 3rd order Birch-Murnaghan equation of state with the following parameters: V$_0$ = 14.14(6) Å$^3$·atom$^1 {−1}$, B$_0$ = 341(10) GPa, and B0' = 4.7(3); V$_0$ = 13.73(7) Å$^3$·atom$^{−1}$, B$_0$ = 301(9) GPa, and B$_0$' = 3.1(2); V$_0$ = 13.90(8) Å$^3$·atom$^{−1}$, B$_0$ = 317(17) GPa, and B$_0$' = 6.0(5); V$_0$ = 14.16(9) Å$^3$·atom$^{−1}$, B$_0$ = 300(22) GPa, B$_0$' = 6(1), where V$_0$ is the unit cell volume, B$_0$ and B$_0$' – are the bulk modulus and its pressure derivative., Published by ScienceDirect, Amsterdam [u.a.]

Journal of applied physics 125(9), 095901 (2019). doi:10.1063/1.5067268, Laser heating in a diamond anvil cell (DAC) is a common method for studying material behavior at high-pressure and high-temperature conditions. It has been previously proven that during continuous wave (CW) laser heating of a sample, carbon of the diamond anvils is mobilized, and its diffusion into the sample can lead to undesirable chemical reactions, which, if not detected, may cause misinterpretations of the results of the experiment. Minimizing the heating time with the use of a pulsed laser (PL) is thought to reduce the risk of possible carbon contamination of the sample; however, this has not been proven experimentally. Here, we report the results of our comparative study of the effect of pulsed and continuous wave (CW) laser heating on the mobilization of carbon and its chemical interaction with iron in a diamond anvil cell. Using X-ray absorption near edge structure spectroscopy, Synchrotron Mössbauer Source spectroscopy, and Synchrotron X-ray diffraction, we examined iron samples that were laser heated in DACs in various pressure transmitting media (neon, argon, and potassium chloride). According to our results, the use of the PL heating does not prevent the sample from carbon contamination. A reaction between carbon and iron happens within a few seconds even at moderate temperatures. We found that one analytical technique was generally insufficient to fully characterize the phase composition of the laser-heated samples., Published by American Inst. of Physics, Melville, NY

The diamond anvil cell (DAC) technique is a powerful method for the synthesis and studying of novel materials at extreme conditions. In this work we report on high-pressure high-temperature (HPHT) synthesis of palladium carbides (PdCx) and palladium hydride (PdH) in a laser heated diamond anvil cell. Formation of PdCx with a face-centered cubic (fcc) structure resulted from a chemical reaction of Pd with carbon from the diamond anvils at a pressure of about 50 GPa and temperature of 2500–3000 K. The samples were analyzed in situ using synchrotron X-ray diffraction. The compressional behavior of PdC0.19(3) and PdC0.17(3), was studied on decompression. The fit of the pressure-volume data using the 3rd order Birch-Murnaghan equation of state gave the following parameters: V0 = 65.1 (1) A3, K 0 = 241 (9) GPa and K 0 ’ = 2.1 (3) for PdC0.19(3), and V0 = 64.51 (5) A3, K 0 = 189 (8) GPa and K 0 ’ = 4.5 (4) for PdC0.17(3). The palladium hydride PdH was synthesized at P = 39 (2) GPa and T = 1500 (200) K through a direct reaction of Pd with paraffin oil.

The refined crystal structure model of iron tetra­nitride, FeN4, at 180 GPa is similar to that at 135 GPa but shows improved structural parameters in terms of bond lengths and angles., Iron tetra­nitride, FeN4, 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 1d), while two N atoms occupy general positions (2i). The structure is made up from edge-sharing [FeN6] octa­hedra forming chains along [100] and being inter­connected through N—N bridges. N atoms form catena-poly[tetraz-1-ene-1,4-di­yl] anions [–N=N—N—N–]∞ 2− running along [001]. In comparison with the previously reported structure of FeN4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9, 2756], the crystal structure of FeN4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the bond lengths and angles.