First-principles study on phase transition-induced initial decomposition of nitrogen-rich energetic azidotetrazolate oxide anion salts under external electric fields: A case study of ammonium azidotetrazolate 2-oxide and guanidinium 5-azidotetrazolate 1N-oxide

A first-principles study was carried out to investigate the influence mechanism of external electric field on crystal structures, electronic structures, Hirshfeld surfaces, hydrogen bonds, vibrational properties and initial decomposition mechanisms of nitrogen-rich CN7- energetic compounds ammonium azidotetrazolate 2-oxide (AAO) and guanidinium 5-azidotetrazolate 1 N-oxide (GAO). The results reveal that AAO and GAO undergo phase transitions into more sensitive crystalline phases at 0.006 a.u. and 0.007 a.u. respectively, thereby facilitating their initial decompositions, and the phase transitions are reflected in their all structural properties. The gradual decline in band gaps of AAO and GAO suggests that external electric field favors the electron transition ability from the occupied orbitals to empty ones. The discussion on DOS shows that N -N and N = O bonds breaking could be external electric field-induced initial decomposition pathways for AAO and GAO. The analysis on Hirshfeld surfaces and hydrogen bonds shows that external electric fields could adjust the hydrogen bonds in crystals, thereby changing their physicochemical properties. The vibrational analysis denotes that all main IR characteristic peaks in AAO gradually move towards lower frequency regions while those in GAO vary significantly. External electric field-induced initial decomposition of AAO and the most possible one of GAO are breaking of N -N single bond in tetrazole, and breaking of N -N single bond connected by N on carbonyl and N on tetrazole ring, respectively. Our findings offer a new insight into the effect of external electric field on energetic materials that phase transition induces initial decomposition under external electric fields.