Physical and chemical transformations of sodium cyanide at high pressures

Abstract

Pressure-induced physical and chemical transformations of sodium cyanide (NaCN) have been studied up to 50 GPa in diamond-anvil cells, using micro-Raman spectroscopy and angle-resolved synchrotron x-ray diffraction. We observe three phase transitions in this pressure range: NaCN-IIA (orthorhombic, Immm), to NaCN-IIB (orthorhombic, Pmmn) at 4 GPa, to NaCN-III (monoclinic, Cm) at 8 GPa, and to NaCN-IV (tetragonal, P4mm) at 15 GPa, which is stable to 25 GPa. At higher pressures, NaCN-IV undergoes an irreversible chemical change, which occurs over a large pressure range between 25 and 34 GPa. The new material exhibits a broad yet strong Raman band at around 1550 cm(-1), indicating the formation of C=N bonds in a similar configuration of carbon graphite. The absence of sharp diffraction lines in this material suggests an amorphous nature of CN polymer products.

Pressure-induced phase transition and polymerization of tetracyanoethylene (TCNE)

“…Upon the occurrence of chemical reactions, all Raman modes but one broad feature at ∼1585 cm −1 are lost and the sample becomes opaque. The peak at ∼1585 cm −1 corresponding to the C=N stretching mode in highly conjugated 2D C=N layers [7][8][9][10][11] gains the intensity, as the C=C stretching mode in TCNE is lost and the sample polymerizes. Interestingly, the polymerization occurs rather continuously or sluggishly over a large pressure region between 8 GPa and 14 GPa in non-hydrostatic conditions, whereas it occurs abruptly in quasi-hydrostatic conditions at pressures between 14 and 16 GPa depending on the sample.…”

“…A similar pressure-induced polymerization has been observed previously in other cyanide compounds such as NaCN, HCN, and C 2 N 2 . [7][8][9][10][11] These transitions are also accompanied with the color change to near opaque and the loss of sharp internal and external vibrational modes. The products typically develop a broad Raman band at ∼1600 cm −1 , indicating the formation of C=N bonds.…”

“…Therefore, these transformations have been interpreted in terms of the pressure-induced polymerization via breaking C≡N bonds and forming interlinked C=N bonds in a graphite-like two-dimensional (2D) layer structure. 2,7,11 Since the previous theoretical predictions of stable 3D network structures made of C-N single bonds that may be harder than diamond, 12,13 there have been a large number of experimental efforts to synthesize superhard carbonnitrogen compounds. Yet, there has not been an unambiguous demonstration of the presence of such a compound in bulk at any pressure conditions.…”

“…Yet, there has not been an unambiguous demonstration of the presence of such a compound in bulk at any pressure conditions. Of those efforts, various cyanide compounds including TCNE have been subjected to 0021-9606/2013/138(9)/094506/10/$30.00 © 2013 American Institute of Physics 138, 094506-1 high pressures, [7][8][9][10] but the results have typically been the formation of a black amorphous C=N solid, not a transparent singly bonded C-N 3D network. Laser-heating experiments of TCNE thin films or amorphous carbon aerogel at high pressures, on the other hand, have produced some transparent products that can be considered as sp 3 -bonded CN materials or nitrogen containing nano-diamonds.…”

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.…”