Stabilization of pentazolate anions in the high-pressure compounds Na2N5 and NaN5 and in the sodium pentazolate framework NaN5 center dot N-2

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-N-5(-) pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na2N5, NaN5 and NaN5 center dot N-2 from sodium azide NaN3 and molecular nitrogen N-2 at similar to 50 GPa. NaN5 center dot N-2 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-N-5 group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.