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.

Effect of pressure on heterocyclic compounds: Pyrimidine and s-triazine

“…[12][13][14] Hydrogen cyanide, cyanogen, and tetracyanoethylene polymerize progressively through saturating C≡N triple bonds upon compression above 1.3, 3.5, and 14 GPa, respectively. [15][16][17] The polymer of perdeuterated acrylic acid is produced upon decompression from 7.21 GPa, different from the conventional ones. 18 Additionally, topological reactions of diiodobutadiyne have been achieved through combination with supramolecular chemistry and high-pressure techniques.…”

High pressure study of a highly energetic nitrogen-rich carbon nitride, cyanuric triazide

“…7 clearly shows that both the intermolecular and intramolecular Raman lines broaden and decrease in intensity as a function of pressure. Two additional broad bands, centered 9,[30][31][32] As illustrated by Raman spectra at selected pressures, shown in Fig. 9, the loss in Raman signal associated with crystalline CTA is correlated to the laser light exposure time of the sample, used to record the Raman scattering.…”

“…This has been observed in other carbon-nitride molecular crystals. 5,[8][9][10]26 Due to amorphization and sample thinning at increasing pressures, many of the x-ray diffraction lines could not be differentiated from the background. Nonetheless, at least nine x-ray diffraction lines were used to determine the lattice parameters and the volume of the unit cell of CTA low pressure points.…”

“…Pathways to a stable single-bonded polymeric network at room conditions need to be studied. In that context, nitrogenrich molecular precursors, particularly ionic azides [5][6][7][8] and carbon nitrides, 3,[9][10][11][12][13] have been extensively investigated under extreme conditions. Such compounds could form a metastable network comprising single-bonded nitrogen atoms which would have mainly N 2 as a decomposition product.…”

“…Sodium azide (NaN 3 ) and tetracyanoethylene (TCNE, C 6 N 4 ) have both yielded a three dimensional single-bonded polymeric network, metastable down to <0.5 GPa and 10 GPa, respectively. 5,9 Other nitrogen-rich molecular compounds show a potential for storing chemical energy. Among them, we have examined the case of another carbon nitride, namely, cyanuric triazide (CTA, C 3 N 12 ).…”