No abstract

The high-pressure behavior of nitrogen in NaN(3) was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N(3-) are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.

Nanodiamond in a 2–5-nm size interval (which is typical for an appearance of quantum confinement effect) show Raman spectra composed of 3 bands at 1325, 1600, and 1500 cm−1 (at the 458-nm laser excitation) which shifts to 1630 cm−1 at the 257-nm laser excitation. Contrary to sp2-bonded carbon, relative intensities of the bands do not depend on the 458- and 257-nm excitation wavelengths, and a halfwidth and the intensity of the 1600 cm−1 band does not change visibly under pressure at least up to 50 GPa. Bulk modulus of the 2–5-nm nanodiamond determined from the high-pressure study is around 560 GPa. Studied 2–5-nm nanodiamond was purified from contamination layers and dispersed in Si or NaCl.