Stabilization of pentazolate anions in the high-pressure compounds Na2N5 and NaN5 and in the sodium pentazolate framework NaN5·N2

Abstract

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 the 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 method with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common in a variety of ionic pentazolate compounds. The formation of Na₂N₅ demonstrates that the cyclo-N₅ group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.