Phase transitions and enhanced conductivity of CaC₂ under high pressure

Calcium carbide is widely used in the industry for the production of acetylene and other purposes. Its phase transitions under ambient pressure have been studied since 1930s [1]. In recent years, with the development of high pressure science, its phase transitions under high pressure attracted more attentions [2], and its physical properties such as conductivity and superconductivity were focused [3]. Up to now, most of the researches on CaC2 under high pressure are theoretical, and experimental investigations are expected to figure out the structural transitions. In this work, we investigated the structural transitions of CaC2 (phase I, tetragonal, I4/mmm) up to ~30 GPa by powder XRD, neutron diffraction, and neutron PDF analysis on the recovered samples, and measured the conductivity of CaC2 up to ~20 GPa. XRD data are employed to refine the unit cell parameters, based on which the equation of state is fitted. As identified by series of fittings, the tetragonal phase stabilizes up to 10 GPa, above which it has a minor phase transition. The crystal structures were refined by the structural model of phase I with in-situ neutron diffraction data. Both of the bond length of C-C triple bond and the nearest intergroup C...C distance show a turning point at around 10-12 GPa. The critical pressure is in consistent with the predicted phase transition from phase I to phase VI (monoclinic, I2/m), though the phase VI can't be identified and refined with the data under the current resolution. The resistivity of CaC2 decreases from 1000 Ω·m at 2 GPa to 0.0001 Ω·m at 22 GPa, which can be attributed to the compression of intergroup C...C distance from 0.335nm to 0.315nm. The resistivity-pressure curve also shows a turning point at ~10GPa, corresponding to the phase transition. Above 18 GPa, CaC2 starts to amorphize, which is reversible but sluggish. The C22- may get connected to each other, as observed in the neutron PDF data of the recovered sample.

Phase Transition and Polymerization of Acetonitrile under High Pressure

Successful application of high pressure on synthesis of organic polymer, including the conducting polymer and super hard materials depends on the knowledge of reaction mechanism. The evolution of crystal structure under high pressure especially the structure close to transition pressure is crucial to conclude the reaction mechanism. Nitriles represent a large class of interstellar molecules and are the potential source of amino acids. Understanding its behavior at extreme conditions has gained increasing attention recently. Acetonitrile (CH3CN), the simplest organic compound with C≡N triple bond, can act as a model system for studying the pressure induced polymerization. The phase transition of acetonitrile under high pressure has been studied extensively.[1-3] However, it is still controversial and there is no any detailed discussion about its polymerization mechanism under high pressure. Here, we report the in-situ high pressure Raman spectra and powder neutron diffraction results on CD3CN, which indicates a minor phase transition at 5 GPa. The neutron diffraction shows that CD3CN keeps the orthorhombic phase from 1.66 GPa to 20.58 GPa which is very close to the reaction pressure. The week hydrogen bonding CD...N arranges the molecule into 3-dimensional framework which can be treated as two sets of diamond like structures interpenetrating with each other. Interestingly, the observed N...D distance is 1.984 Å at 20.58 GPa, shorter than the van der Waals distance of N...H (2.75 Å) by 28%. The van der Waals separation is often taken as a reference distance for the molecular instability. Thus, a hydrogen transfer process during the polymerization can be concluded. This deduction is also supported by the solid state NMR and FTIR results of the recovered polymerized CH3CN (p-CH3CN) from high pressure. In addition, the atomic pair distribution function and Raman spectra indicate the p-CD3CN or p-CH3CN has a random packed layer structure with nano-graphene lattice.

Neutron diffraction in diamond anvil cells

Neutron diffraction provides many unique advantages for structural studies of materials under extremes of pressure. In addition to the famous sensitivity to light atom positions, neutrons are sensitive to long-range magnetic order and have an extremely high spatial resolution. However, a major downside of neutron techniques, that is keenly felt in high pressure studies, is the comparative weakness of available sources. Some of these limitations have been recently overcome at the Spallation Neutron Source, ORNL, using a newly developed supported diamond-anvil device. For the first time, this new capability allows the possibility of conducting neutron diffraction measurements at pressures approaching 100 GPa. These new developments will be discussed with a look towards the prospects for advances in neutron scattering at high pressure in the near future.