Stabilization of Pentazolate Anions in High-Pressure Compounds Na2N5, NaN5 and in Sodium Pentazolate Framework NaN5·N2

Synthesis and characterization of nitrogen-rich materials is important for the design of novel high energy density materials due to extremely energetic low-order nitrogen-nitrogen bonds. The balance between energy output and stability may be achieved if polynitrogen units are stabilized by resonance as in cyclo-N₅^– pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na₂N₅, NaN₅ and NaN₅·N₂ from sodium azide NaN₃ and molecular nitrogen N₂ at ~50 GPa. NaN₅·N₂ is a metal-pentazolate framework (MPF) obtained via a self-templated synthesis with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common for a variety of ionic pentazolate compounds. The formation of Na₂N₅ demonstrates that cyclo-N₅ group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.

M. Bykov, E. E. Bykova, S. S. Chariton, V. B. Prakapenka, I. G. Batyrev, M. F. M. Mahmood and A. F. A. Goncharov, Dalton Trans., 2021, DOI: 10.1039/D1DT00722J. abstract

(a) Raman spectra collected at different positions of the NaN3 sample (sample #1) compressed up to ~50 GPa. Colour codes correspond to Fig. 1b. (b) Microscopic image of the sample chamber at ~50 GPa. Red circle shows an area, where the sample was laser-heated, and X-ray exposed before the collection of the Raman spectrum. Green area – was laser-heated only, orange area was subject to X-ray exposure during 20 min at 37 keV. (c) Le-Bail fit of the diffraction pattern collected during laser-heating of NaN3 at 1800 K (P6/m, a = 4.8457(6), c = 2.6289(8) Å). (d) Le-Bail fit of the diffraction pattern collected after laser-heating of NaN3 (Pm, a = 4.781 (10) Å, b = 2.5873(7) Å, c = 4.934(12) Å, β = 119.6(3)°) (e) Comparison of the diffraction patterns of hot and temperature-quenched samples.

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