Extending carbon chemistry at high-pressure by synthesis of CaC2 and Ca3C7 with deprotonated polyacene- and para-poly(indenoindene)-like nanoribbons

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, [C6] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC2 polymorph and a Ca3C7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C6] 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.

Stabilization of N6 and N8 anionic units and 2D polynitrogen layers in high-pressure scandium polynitrides

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, Sc2N6, Sc2N8, ScN5, and Sc4N3, 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 Sc2N6, Sc2N8, and ScN5 phases nitrogen is catenated forming previously unknown N66- and N86- units and ∞ 2 ( N 5 3 - ) anionic corrugated 2D-polynitrogen layers consisting of fused N12 rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc2N6 and Sc2N8 possess an anion-driven metallicity, while ScN5 is an indirect semiconductor. Sc2N6, Sc2N8, and ScN5 solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.

Simple Molecules under High-Pressure and High-Temperature Conditions: Synthesis and Characterization of α- and β-C(NH)2 with Fully sp3 -Hybridized Carbon

The elements hydrogen, carbon, and nitrogen are among the most abundant in the solar system. Still, little is known about the ternary compounds these elements can form under the high-pressure and high-temperature conditions found in the outer planets' interiors. These materials are also of significant research interest since they are predicted to feature many desirable properties such as high thermal conductivity and hardness due to strong covalent bonding networks. In this study, the high-pressure high-temperature reaction behavior of malononitrile H2 C(CN)2 , dicyandiamide (H2 N)2 C=NCN, and melamine (C3 N3 )(NH2 )3 was investigated in laser-heated diamond anvil cells. Two previously unknown compounds, namely α-C(NH)2 and β-C(NH)2 , have been synthesized and found to have fully sp3 -hybridized carbon atoms. α-C(NH)2 crystallizes in a distorted β-cristobalite structure, while β-C(NH)2 is built from previously unknown imide-bridged 2,4,6,8,9,10-hexaazaadamantane units, which form two independent interpenetrating diamond-like networks. Their stability domains and compressibility were studied, for which supporting density functional theory calculations were performed.

Synthesis of Ultra-Incompressible and Recoverable Carbon Nitrides Featuring CN4 Tetrahedra

Carbon nitrides featuring three-dimensional frameworks of CN4 tetrahedra are one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, the high-pressure high-temperature synthesis of three carbon-nitrogen compounds, tI14-C3 N4 , hP126-C3 N4 , and tI24-CN2 , in laser-heated diamond anvil cells, is reported. Their structures are solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations show that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions.

Stabilization Of The CN3 5- Anion In Recoverable High-pressure Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates

A series of isostructural Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln3 O2 (CN3 ) solids are composed of the hitherto unknown CN3 5- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln3 O2 (CN3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln3 O2 (CN3 ) compounds are recoverable to ambient conditions. The stabilization of the CN3 5- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.

High-pressure reactions between the pnictogens: the rediscovery of BiN

We explore chemical reactions within pnictogens with an example of bismuth and nitrogen under extreme conditions. Understanding chemical reactions between Bi and N, elements representing the first and the last stable elements of the nitrogen group, and the physical properties of their compounds under ambient and high pressure is far from being complete. Here, we report the high-pressure high-temperature synthesis of orthorhombic Pbcn BiN (S.G. #60) from Bi and N2 precursors at pressures above 40 GPa. Using synchrotron single-crystal X-ray diffraction on the polycrystalline sample, we solved and refined the compound's structure and studied its behavior and compressibility on decompression to ambient pressure. We confirm the stability of Pbcn BiN to pressures as low as 12.5(4) GPa. Below that pressure value, a group-subgroup phase transformation occurs, resulting in the formation of a non-centrosymmetric BiN solid with a space group Pca21 (S.G. #29). We use ab initio calculations to characterize the polymorphs of BiN. They also provide support and explanation for our experimental observations, in particular those corresponding to peculiar Bi-N bond evolution under pressure, resulting in a change in the coordination numbers of Bi and N as a function of pressure within the explored stability field of Pbcn BiN.

Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na2Cl3 and Na4Cl5 and Bromide Na4Br5

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 (tP10-Na₂Cl₃, hP18-Na₄Cl₅, and hP18-Na₄Br₅), a series of isostructural cubic cP8-AX₃ halides (NaCl₃, KCl₃, NaBr₃, and KBr₃), and a trigonal potassium chloride (hP24-KCl₃). 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 Cl₃^- anion in hP24-KCl₃ and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl]_∞^n- and [Br]_∞^n-, in the structures of cP8-AX₃ compounds and in hP18-Na₄Cl₅ and hP18-Na₄Br₅. In Na₄Cl₅ and Na₄Br₅, 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.

High-pressure synthesis of dysprosium carbides

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, Dy₄C₃ and Dy₃C₂, and dysprosium sesquicarbide Dy₂C₃ previously known only at ambient conditions. The structure of Dy₄C₃ was found to be closely related to that of dysprosium sesquicarbide Dy₂C₃ with the Pu₂C₃-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.

Aromatic hexazine [N6]4- anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56

The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N₅]^- anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N₆ ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N₆]^4-. Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K₉N₅₆ formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN₃ in a laser-heated diamond anvil cell. The complex structure of K₉N₅₆-composed of 520 atoms per unit cell-was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N₆]^4- is planar and proposed to be aromatic.

Subduction-related oxidation of the sublithospheric mantle evidenced by ferropericlase and magnesiowüstite diamond inclusions

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)Fe³⁺₂O₄) with an estimated 47-53 wt% Fe₂O₃. 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.

Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α'-P3 N5, δ-P3 N5 and PN2

Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. These properties mainly rely on maximizing the number of strong covalent bonds, with crosslinked XN₆ octahedra frameworks being particularly attractive. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells, and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. δ-P₃ N₅ and PN₂ were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN₆ units. The two compounds are ultra-incompressible, having a bulk modulus of K₀ =322 GPa for δ-P₃ N₅ and 339 GPa for PN₂ . Upon decompression below 7 GPa, δ-P₃ N₅ undergoes a transformation into a novel α'-P₃ N₅ solid, stable at ambient conditions, that has a unique structure type based on PN₄ tetrahedra. The formation of α'-P₃ N₅ underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.

Fe0.79Si0.07B0.14 metallic glass gaskets for high-pressure research beyond 1 Mbar

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_0.79Si_0.07B_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_0.79Si_0.07B_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, H₂) 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_0.79Si_0.07B_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.

Materials synthesis at terapascal static pressures

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 cell³, producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re₇N₃-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.

Sub-micrometer focusing setup for high-pressure crystallography at the Extreme Conditions beamline at PETRA III

Scientific tasks aimed at decoding and characterizing complex systems and processes at high pressures set new challenges for modern X-ray diffraction instrumentation in terms of X-ray flux, focal spot size and sample positioning. Presented here are new developments at the Extreme Conditions beamline (P02.2, PETRA III, DESY, Germany) that enable considerable improvements in data collection at very high pressures and small scattering volumes. In particular, the focusing of the X-ray beam to the sub-micrometer level is described, and control of the aberrations of the focusing compound refractive lenses is made possible with the implementation of a correcting phase plate. This device provides a significant enhancement of the signal-to-noise ratio by conditioning the beam shape profile at the focal spot. A new sample alignment system with a small sphere of confusion enables single-crystal data collection from grains of micrometer to sub-micrometer dimensions subjected to pressures as high as 200 GPa. The combination of the technical development of the optical path and the sample alignment system contributes to research and gives benefits on various levels, including rapid and accurate diffraction mapping of samples with sub-micrometer resolution at multimegabar pressures.

Testing the performance of secondary anvils shaped with focused ion beam from the single-crystal diamond for use in double-stage diamond anvil cells

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.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions

Magnetite, Fe₃O₄, is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe₃O₄ polymorphs, γ-Fe₃O₄ with the orthorhombic Yb₃S₄-type structure and δ-Fe₃O₄ with the modified Th₃P₄-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe₃O₄ at HPHT conditions was found to result in the formation of exotic phases, Fe₅O₇ and Fe₂₅O₃₂, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe³⁺ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

Novel High-Pressure Yttrium Carbide γ-Y_{4}C_{5} Containing [C_{2}] and Nonlinear [C_{3}] Units with Unusually Large Formal Charges

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, γ-Y_{4}C_{5}, was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at ∼50  GPa. The structure of γ-Y_{4}C_{5} was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: [C_{2}] dimers and nonlinear [C_{3}] trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges ([C_{2}]^{5.2-} and [C_{3}]^{6.8-}) and unique bond orders (<1.5). Our results extend the list of possible carbon states at extreme conditions.

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties

In contrast to the corundum-type A₂X₃ structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX₃ ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe²⁺Ti⁴⁺O₃ ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn₂O₃) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn²⁺Mn⁴⁺O₃ (ε-Mn₂O₃) is an n-type semiconductor with an indirect narrow band gap of E_g = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn₂O₃ and previously discovered quadruple perovskite ζ-Mn₂O₃ phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn₂O₃ below 210 K. The synthesis of ε-Mn₂O₃ indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A₂O₃, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO₃-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

We synthesized single crystals of marokite (CaMn₂O₄)-type orthorhombic manganese (II,III) oxide, γ-Mn₃O₄, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn₃O₄ is a semiconductor with an indirect band gap E_g of 0.96 eV and two antiferromagnetic transitions (T_N) at ∼200 and ∼55 K. The phase stability of the γ-Mn₃O₄ crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn₃O₄ was determined to be B₀ = 235.3(2) GPa with B' = 2.6(6). The γ-Mn₃O₄ phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO₆ polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn³⁺ ions in the octahedra and the Mn²⁺ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Isothermal equation of state of crystalline and glassy materials from optical measurements in diamond anvil cells

Here, we present a method to study the equation of state of opaque amorphous and crystalline materials in diamond anvil cells. The approach is based on measurements of sample dimensions using high-resolution optical microscopy. Data on the volumetric strain as a function of pressure allow deriving the isothermal equation of state of the studied material. The analysis of optical images is fully automatized and allows measuring the sample dimensions with the precision of about 60 nm. The methodology was validated by studying isothermal compression of ω-Ti up to 30 GPa in a Ne pressure transmitting medium. Within the accuracy of the measurements, the bulk modulus of ω-Ti determined using optical microscopy was similar to that obtained from x-ray diffraction. For glassy carbon compressed to ∼30 GPa, the previously unknown bulk modulus was found to be equal to K₀ = 28 (2) GPa [K' = 5.5(5)].

Nuclear spin coupling crossover in dense molecular hydrogen

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 H2 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.

High-Pressure Synthesis of Metal-Inorganic Frameworks Hf4 N20 ⋅N2 , WN8 ⋅N2 , and Os5 N28 ⋅3 N2 with Polymeric Nitrogen Linkers

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf₄N₂₀⋅N₂, WN₈⋅N₂, and Os₅N₂₈⋅3 N₂ via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf₄N₂₀, WN₈, and Os₅N₂₈) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N₂ also leads to a non‐centrosymmetric polynitride Hf₂N₁₁ that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]_∞.

Face-Centered Cubic Refractory Alloys Prepared from Single-Source Precursors

Three binary fcc-structured alloys (fcc–Ir_0.50Pt_0.50, fcc–Rh_0.66Pt_0.33 and fcc–Rh_0.50Pd_0.50) were prepared from [Ir(NH₃)₅Cl][PtCl₆], [Ir(NH₃)₅Cl][PtBr₆], [Rh(NH₃)₅Cl]₂[PtCl₆]Cl₂ and [Rh(NH₃)₅Cl][PdCl₄]·H₂O, respectively, as single-source precursors. All alloys were prepared by thermal decomposition in gaseous hydrogen flow below 800 °C. Fcc–Ir_0.50Pt_0.50 and fcc–Rh_0.50Pd_0.50 correspond to miscibility gaps on binary metallic phase diagrams and can be considered as metastable alloys. Detailed comparison of [Ir(NH₃)₅Cl][PtCl₆] and [Ir(NH₃)₅Cl][PtBr₆] crystal structures suggests that two isoformular salts are not isostructural. In [Ir(NH₃)₅Cl][PtBr₆], specific Br…Br interactions are responsible for a crystal structure arrangement. Room temperature compressibility of fcc–Ir_0.50Pt_0.50, fcc–Rh_0.66Pt_0.33 and fcc–Rh_0.50Pd_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_0.50Pt_0.50, V₀/Z = 14.597(6) ų·atom⁻¹, B₀ = 321(6) GPa and B₀’ = 6(1); for fcc–Rh_0.66Pt_0.33, V₀/Z = 14.211(3) ų·atom⁻¹, B₀ =259(1) GPa and B₀’ = 6.66(9) and for fcc–Rh_0.50Pd_0.50, V₀/Z = 14.18(2) ų·atom⁻¹, B₀ =223(4) GPa and B₀’ = 5.0(3).

Table-top nuclear magnetic resonance system for high-pressure studies with in situ laser heating

High pressure Nuclear Magnetic Resonance (NMR) is known to reveal the behavior of matter under extreme conditions. However, until now, 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 a diameter of 25 cm and height of 4 cm. 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 setups. Feasibility of high-pressure high-temperature NMR was demonstrated by collecting ¹H-NMR spectra of H₂O at 25 GPa and 1063(50) K. The change in the signal intensity in a laser-heated NMR diamond anvil cell has been found to yield a convenient way for temperature measurements.

Synthesis of FeN4 at 180 GPa and its crystal structure from a submicron-sized grain

Iron tetra-nitride, 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] 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 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.

Observation of nuclear quantum effects and hydrogen bond symmetrisation in high pressure ice

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 1H-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.

NMR at pressures up to 90 GPa

The past 15 years have seen an astonishing increase in Nuclear Magnetic Resonance (NMR) sensitivity and accessible pressure range in high-pressure NMR experiments, owing to a series of new developments of NMR spectroscopy applied to the diamond anvil cell (DAC). Recently, with the application of electro-magnetic lenses, so-called Lenz lenses, in toroidal diamond indenter cells, pressures of up to 72 GPa with NMR spin sensitivities of about 10¹² spin/Hz^1/2 has been achieved. Here, we describe the implementation of a refined NMR resonator structure using a pair of double stage Lenz lenses driven by a Helmholtz coil within a standard DAC, allowing to measure sample volumes as small as 100 pl prior to compression. With this set-up, pressures close to 100 GPa could be realised repeatedly, with enhanced spin sensitivities of about 5 × 10¹¹ spin/Hz^1/2. The manufacturing and handling of these new NMR-DACs is relatively easy and straightforward, which will allow for further applications in physics, chemistry, or biochemistry.