How Stable and Powerful Can Metal cyclo-Pentazolate Salts Be? An Answer through Theoretical Crystal Design

The cyclo-pentazolate species have long been considered as promising candidates of ultrahigh energy density materials due to the potential ability of cyclo-N₅^– to store large amounts of energy compared to the azide ion. However, because of the presence of nonenergetic components in the reported metal cyclo-pentazolate hydrates, such as H₂O, NH₄⁺, and H₃O⁺, their practical power is far from expected. Herewith, we report the design the crystal structures of metal cyclo-pentazolate salts of NaN₅, Mg­(N₅)₂, Co­(N₅)₂, and Mn­(N₅)₂, devoid of nonenergetic components, using a self-developed crystal design methodology. The stabilization level and detonation performance of the newly designed crystals are thoroughly investigated by systematic static and dynamical quantum calculations. We find that the designed metal cyclo-pentazolate salts have slightly lower stability but are significantly more powerful than the corresponding hydrates. Among the four newly designed crystals, Mn­(N₅)₂ presents both outstanding crystal stability (approximately double the lattice energy of NaCl) and better performance than hexanitrohexaazaisowurtzitane (CL-20). To synthesize advanced pentazolate crystals with high stability and explosive power, a large cyclo-N₅^– coordination number and an atomic crystal type are theoretically recommended.