title: Pressure-induced phase transition and polymerization of tetracyanoethylene (TCNE).
abstract: We have studied the pressure-induced physical and chemical transformations of tetracyanoethylene (TCNE or C6N4) in diamond anvil cells using micro-Raman spectroscopy, laser-heating, emission spectroscopy, and synchrotron x-ray diffraction. The results indicate that TCNE in a quasi-hydrostatic condition undergoes a shear-induced phase transition at 10 GPa and then a chemical change to two-dimensional (2D) C=N polymers above 14 GPa. These phase and chemical transformations depend strongly on the state of stress in the sample and occur sluggishly in non-hydrostatic conditions over a large pressure range between 7 and 14 GPa. The x-ray diffraction data indicate that the phase transition occurs isostructurally within the monoclinic structure (P21∕c) without any apparent volume discontinuity and the C=N polymer is highly disordered but remains stable to 60 GPa-the maximum pressure studied. On the other hand, laser-heating of the C=N polymer above 25 GPa further converts to a theoretically predicted 3D C-N network structure, evident from an emergence of new Raman νs(C-N) at 1404 cm(-1) at 25 GPa and the visual appearance of translucent solid. The C-N product is, however, unstable upon pressure unloading below 10 GPa, resulting in a grayish powder that can be considered as nano-diamonds with high-nitrogen content at ambient pressure. The C-N product shows a strong emission line centered at 640 nm at 30 GPa, which linearly shifts toward shorter wavelength at the rate of -1.38 nm∕GPa. We conjecture that the observed red shift upon unloading pressure is due to increase of defects in the C-N product and thereby weakening of C-N bonds.
authors.affiliation: Department of Chemistry and Institute for Shock Physics, Washington State University, Pullman, Washington 99164, USA.
authors.forename: Dane
authors.initials: D
authors.lastname: Tomasino
authors.forename: Jing-Yin
authors.initials: JY
authors.lastname: Chen
authors.forename: Minesob
authors.initials: M
authors.lastname: Kim
authors.forename: Choong-Shik
authors.initials: CS
authors.lastname: Yoo
mesh_terms: D005030:Ethylenes / Q000737:chemistry*; D007834:Lasers; D049449:Luminescence; D009570:Nitriles / Q000737:chemistry*; D044367:Phase Transition; D058105:Polymerization; D011312:Pressure; D013059:Spectrum Analysis, Raman
chemical_list: D005030:Ethylenes; D009570:Nitriles; C003393:tetracyanoethylene