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 sp 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.

At ambient pressure, the hydrogen bond in materials such as ice, hydrates, and hydrous minerals that compose the Earth and icy planets generally takes an asymmetric O-H···O configuration. Pressure significantly affects this configuration, and it is predicted to become symmetric, such that the hydrogen is centered between the two oxygen atoms at high pressure. Changes of physical properties of minerals relevant to this symmetrization have been found; however, the atomic configuration around this symmetrization has remained elusive so far. Here we observed the pressure response of the hydrogen bonds in the aluminous hydrous minerals δ-AlOOH and δ-AlOOD by means of a neutron diffraction experiment. We find that the transition from P21nm to Pnnm at 9.0 GPa, accompanied by a change in the axial ratios of δ-AlOOH, corresponds to the disorder of hydrogen bond between two equivalent sites across the center of the O···O line. Symmetrization of the hydrogen bond is observed at 18.1 GPa, which is considerably higher than the disorder pressure. Moreover, there is a significant isotope effect on hydrogen bond geometry and transition pressure. This study indicates that disorder of the hydrogen bond as a precursor of symmetrization may also play an important role in determining the physical properties of minerals such as bulk modulus and seismic wave velocities in the Earth’s mantle.

We provide the first experimental evidence for a giant, conventional barocaloric effect (BCE) associated with a pressure-driven spin crossover transition near room temperature. We use magnetometry, neutron scattering and calorimetry to explore the pressure dependence of the SCO phase transition in polycrystalline samples of protonated and partially deuterated [FeL2][BF4]2 [L = 2,6-di(pyrazol-1-yl)pyridine] in pressures of up to 120 MPa (1200 bar). Our data indicate that, in a pressure change of only 0-300 bar (0-30 MPa), an adiabatic temperature change of 3 K is observed at 262 K or 257 K in the protonated and deuterated materials, respectively. This BCE is equivalent to the magnetocaloric effect (MCE) observed in gadolinium in a magnetic field change of 0-1 Tesla. Our work confirms recent predictions that giant, conventional BCEs will be found in a wide range of SCO compounds. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

The motif of distinct H 2 O 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 2 O 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.crystallography | high pressure | water A ll of the some 16 phases of crystalline ice documented to date exhibit a tetrahedrally coordinated structure in which each molecule is H-bonded to four neighbors in an extended network. Above ∼2 GPa, the phase diagram contains just two, closely related, molecular phases: orientationally disordered ice VII and its ordered low-temperature analog ice VIII (1). Both phases have body-centered, close-packed oxygen sublattices with cubic and tetragonal symmetry, respectively. Infrared spectroscopy has provided evidence for bond symmetrization in ice VII-meaning covalent and H-bonds become geometrically equivalent, and ice becomes a simple oxide-starting around 60 GPa (2-4). Meanwhile, X-ray diffraction indicates the persistence of a closely body-centered cubic (bcc) oxygen sublattice up to at least 210 GPa (5-9). At the highest pressures, this system has been studied extensively by numerous experimental techniques [including infrared (3-4, 10) and Raman spectroscopy (2, 11), Brillouin scattering (12), and other optical techniques (13) as well as computational theory (e.g., refs. 14-18 and references therein)]. These efforts, spanning almost 50 years, have led to a consensus view that ice VII moves gradually toward a symmetric H-bonded phase (ice X) across a broad pressure range with protons essentially localized between neighboring O-atoms on network sites. However, the positions of the hydrogen nuclei have not been determined directly at pressures sufficient to significantly change the molecular geometry from that found in common ice Ih (19). In addition, the nature of the proposed intermediate states between ices VII and X remains uncertain (11,20), and changes in ices VII and VIII themselves have been suggested by anomalies in X-ray (5,7,8,21,22) and spectroscopic (12,22,23) data beginning as low as 13-14 GPa.In contrast to other techniques, neutron diffraction can resolve the location of deuterons (D) directly within the l...

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