Over the last 60 years, the diamond anvil cell (DAC) has emerged as the tool of choice in high pressure science because materials can be studied at megabar pressures using X-ray and spectroscopic ...probes. In contrast, the pressure range for neutron diffraction has been limited due to low neutron flux even at the strongest sources and the resulting large sample sizes. Here, we introduce a neutron DAC that enables break-out of the previously limited pressure range. Key elements are ball-bearing guides for improved mechanical stability, gem-quality synthetic diamonds with novel anvil support and improved in-seat collimation. We demonstrate a pressure record of 1.15 Mbar and crystallographic analysis at 1 Mbar on the example of nickel. Additionally, insights into the phase behavior of graphite to 0.5 Mbar are described. These technical and analytical developments will further allow structural studies on low-Z materials that are difficult to characterize by X-rays.
Carbon nanothreads are a new one-dimensional sp3 carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of ...benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp3 product: C5NH5 carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.
Synthesis of well-ordered reduced dimensional carbon solids with extended bonding remains a challenge. For example, few single-crystal organic monomers react under topochemical control to produce ...single-crystal extended solids. We report a mechanochemical synthesis in which slow compression at room temperature under uniaxial stress can convert polycrystalline or single-crystal benzene monomer into single-crystalline packings of carbon nanothreads, a one-dimensional sp3 carbon nanomaterial. The long-range order over hundreds of microns of these crystals allows them to readily exfoliate into fibers. The mechanochemical reaction produces macroscopic single crystals despite large dimensional changes caused by the formation of multiple strong, covalent C–C bonds to each monomer and a lack of reactant single-crystal order. Therefore, it appears not to follow a topochemical pathway, but rather one guided by uniaxial stress, to which the nanothreads consistently align. Slow-compression room-temperature synthesis may allow diverse molecular monomers to form single-crystalline packings of polymers, threads, and higher dimensional carbon networks.
The motif of distinct H ₂O molecules in H-bonded networks is believed to persist up to the densest molecular phase of ice. At even higher pressures, where the molecule dissociates, it is generally ...assumed that the proton remains localized within these same networks. We report neutron-diffraction measurements on D ₂O that reveal the location of the D atoms directly up to 52 GPa, a pressure regime not previously accessible to this technique. The data show the onset of a structural change at ∼13 GPa and cannot be described by the conventional network structure of ice VII above ∼26 GPa. Our measurements are consistent with substantial deuteron density in the octahedral, interstitial voids of the oxygen lattice. The observation of this “interstitial” ice VII form provides a framework for understanding the evolution of hydrogen bonding in ice that contrasts with the conventional picture. It may also be a precursor for the superionic phase reported at even higher pressure with important consequences for our understanding of dense matter and planetary interiors.
Acetonitrile (CH3CN) is the simplest and one of the most stable nitriles. Reactions usually occur on the C≡N triple bond, while the C−H bond is very inert and can only be activated by a very strong ...base or a metal catalyst. It is demonstrated that C−H bonds can be activated by the cyano group under high pressure, but at room temperature. The hydrogen atom transfers from the CH3 to CN along the CH⋅⋅⋅N hydrogen bond, which produces an amino group and initiates polymerization to form a dimer, 1D chain, and 2D nanoribbon with mixed sp2 and sp3 bonded carbon. Finally, it transforms into a graphitic polymer by eliminating ammonia. This study shows that applying pressure can induce a distinctive reaction which is guided by the structure of the molecular crystal. It highlights the fact that very inert C−H can be activated by high pressure, even at room temperature and without a catalyst.
Under high pressure, the C≡N bond of acetonitrile is expected to polymerize into conjugated C=N bonds. It is however shown that when compressing CH3CN at 25 GPa, a hydrogen atom transfers along the −H2C−H⋅⋅⋅NC− hydrogen bond, which triggers the polymerization to form a dimer, 1D chain, and 2D nanoribbon. Finally, it converts into a graphitic polymer accompanied by the release of ammonia.
A robust and comprehensive method for determining the orientation matrix of a single‐crystal sample using the neutron Laue time‐of‐flight (TOF) technique is described. The new method enables the ...measurement of the unit‐cell parameters with an uncertainty in the range 0.015–0.06%, depending upon the crystal symmetry and the number of reflections measured. The improved technique also facilitates the location and integration of weak reflections, which are often more difficult to discern amongst the increased background at higher energies. The technique uses a mathematical model of the relative positions of all the detector pixels of the instrument, together with a methodology that establishes a reproducible reference frame and a method for determining the parameters of the instrument detector model. Since all neutron TOF instruments require precise detector calibration for their effective use, it is possible that the method described here may be of use on other instruments where the detector calibration cannot be determined by other means.
Techniques have been developed that allow the measurement of accurate single‐crystal neutron‐diffraction data at pressures up to 10 GPa, using angle‐dispersive methods. High‐quality data have been ...collected up to 10 GPa, to a resolution of sinθ/λ≃ 1.5 Å−1, from samples of size 3–4 mm. This article presents the methods developed to mount and centre the sample accurately on the instrument; to reduce the background and hence increase the precision of the measured reflection intensities; and to increase further the accessible region of reciprocal space with a single sample loading. Developments are also highlighted, with a view to increasing the range of both science and pressures that can be achieved at the Institut Laue–Langevin reactor source using single‐crystal techniques.
Recent theoretical and experimental studies have produced several unusual and interesting results on dense lithium, the first metal in the periodic table. These include the deviation from simple ...metal behaviour, superconductivity at 17 K, and a metal to semiconductor transition. Despite these efforts, at present there is no agreement on the location of the high-pressure solid phases and melting curve of Li, and there is no clear picture of its phase diagram above 50 GPa (refs 4, 5, 6, 7). Using powder and single-crystal high-pressure diffraction techniques, we have mapped out the lithium phase diagram up to 130 GPa over a wide temperature range between 77 and 300 K. Whereas the melting temperatures of materials usually rise under pressure, and even the lightest condensed gases, hydrogen and helium, melt at temperatures of the order of 103 K at 50 GPa (refs 8, 9), we find that at these pressures lithium remains a liquid at temperatures as low as 190 K, by far the lowest melting temperature observed for any material at such pressure. We also find that in its solid state above 60 GPa, lithium adopts three novel and complex crystal structures not previously observed in any element. Estimates of the zero-point energy suggest that quantum effects play a significant role in shaping the lithium phase diagram.
Low-dimensional carbon nanomaterials such as fullerenes, nanotubes, graphene and diamondoids have extraordinary physical and chemical properties. Compression-induced polymerization of aromatic ...molecules could provide a viable synthetic route to ordered carbon nanomaterials, but despite almost a century of study this approach has produced only amorphous products. Here we report recovery to ambient pressure of macroscopic quantities of a crystalline one- dimensional sp(3) carbon nanomaterial formed by high-pressure solid-state reaction of benzene. X-ray and neutron diffraction, Raman spectroscopy, solid-state NMR, transmission electron microscopy and first-principles calculations reveal close- packed bundles of subnanometre-diameter sp(3)-bonded carbon threads capped with hydrogen, crystalline in two dimensions and short-range ordered in the third. These nanothreads promise extraordinary properties such as strength and stiffness higher than that of sp(2) carbon nanotubes or conventional high-strength polymers. They may be the first member of a new class of ordered sp(3) nanomaterials synthesized by kinetic control of high-pressure solid-state reactions.
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