High-Pressure-Induced Structural and Chemical Transformations in NaN3

Polynitrogen compounds have attracted great interest due to their potential application in the field of high-energy-density materials (HEDMs). Alkali azides are powerful candidates for the high-pressure preparation of HEDMs. In this work, we report a study of the crystal structure evolution and chemical transformation of NaN3 under high pressure by in situ Raman spectroscopy and synchrotron X-ray diffraction methods up to 57.9 GPa. The initial rhombohedral NaN3 transforms into the monoclinic C2/m-NaN3 at 0.6 GPa, which is in agreement with previous studies. The monoclinic NaN3 transforms into the tetragonal I4/mcm-NaN3 at 15.5 GPa. With further compression, in the pressure range of 19.6-21.7 GPa, both the Raman spectrum and X-ray powder diffraction (XRD) diffraction pattern indicate the chemical transition from NaN3 to new kinds of Na-N compounds with the appearance of experimental signals, which cannot be attributed to NaN3. It is very likely that a partial chemical transformation from NaN3 to NaN5 occurs at 19.6 GPa. NaN5 is not stable at ambient conditions upon decompression. Our study indicates that NaN5 can be formed by cold compression of NaN3, providing a potential route for the synthesis of binary alkali pentazolate compounds.