Searching for polynitrogen compounds has attracted great attention due to their potential applications as high energy density materials. Here, we report a facile approach to the synthesis of a solid lithium pentazolate (LiN5) compound by compressing lithium azide and molecular nitrogen under high pressure with laser heating. The formation of LiN5 with the crystal structure P21/m-LiN5 was identified according to the appearance of the vibrational modes of N5 – rings in Raman spectra and synchrotron X-ray diffraction measurements under high pressure. The LiN5 remains stable down to 18.5 GPa upon decompression. The bond lengths in N5 – are between single- and double-bond lengths. This study indicates that the precursor can effectively tune the high-pressure phase of pentazolate, providing us an alternative route to synthesize a polynitrogen compound with a novel structure.

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 NaN 3 under high pressure by in situ Raman spectroscopy and synchrotron X-ray diffraction methods up to 57.9 GPa. The initial rhombohedral NaN 3 transforms into the monoclinic C2/m-NaN 3 at 0.6 GPa, which is in agreement with previous studies. The monoclinic NaN 3 transforms into the tetragonal I4/mcm-NaN 3 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 NaN 3 to new kinds of Na−N compounds with the appearance of experimental signals, which cannot be attributed to NaN 3 . It is very likely that a partial chemical transformation from NaN 3 to NaN 5 occurs at 19.6 GPa. NaN 5 is not stable at ambient conditions upon decompression. Our study indicates that NaN 5 can be formed by cold compression of NaN 3 , providing a potential route for the synthesis of binary alkali pentazolate compounds.

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