Packing high-energy together: Binding the power of pentazolate and high-valence metals with strong bonds

Stabilizing pentazolate anion in solid compounds is a challenging and important approach toward making environmentally friendly high energy density materials. Using a constrained crystal structure search method and rigorous bond analysis, we predicted the metastable structures of Al (N5)3 and Mg(N5)2. In contrast to common wisdom, our work demonstrates that metals, especially the multivalent metals such as Al and Mg, can stabilize pentazolate anions by forming strong metal-N bonds. Both compounds are metastable at ambient pressure and temperatures as high as 600 K. The multivalent metals not only stabilize the pentazolate anions but also greatly enhance the energy density. The decomposition of Al(N5)3 or Mg(N5)2 can release an energy of 4.61 kJ/g or 3.10 kJ/g, respectively, making them promising candidates as high energy density materials.