Predicting the crystal structure of $$\hbox {N}_5\hbox {AsF}_6$$ high energy density material using ab initio evolutionary algorithms

Abstract

Abstract$$\hbox {N}_5\hbox {AsF}_6$$ N 5 AsF 6 is the first successfully synthesized salt that has a polymeric nitrogen moeity ($$\hbox {N}_5^+$$ N 5 + ). Although 12 other $$\hbox {N}_5^+$$ N 5 + salts followed, with $$\hbox {N}_5\hbox {SbF}_6$$ N 5 SbF 6 and $$\hbox {N}_5\hbox {Sb}_2\hbox {F}_{11}$$ N 5 Sb 2 F 11 being the most stable, the crystal structure of $$\hbox {N}_5\hbox {AsF}_6$$ N 5 AsF 6 remains unknown. Currently, it is impossible to experimentally determine the structures of $$\hbox {N}_5\hbox {AsF}_6$$ N 5 AsF 6 due to its marginal stability and explosive nature. Here, following an ab initio evolutionary prediction and using only the stoichiometry of $$\hbox {N}_5\hbox {AsF}_6$$ N 5 AsF 6 as a starting point, we were able to reveal the crystal structure of this high energy density material (HEDM). The $$\hbox {C}_{2V}$$ C 2 V symmetry of the $$\hbox {N}_5^+$$ N 5 + cation, as suggested from earlier investigations, is confirmed to be the symmetry adopted by this polymeric nitrogen within the crystal. This result gave full confidence in the validity of this crystal prediction approach. While stability of the $$\hbox {N}_5^+$$ N 5 + within the crystal is found to be driven by electronic considerations, the marginal stability of this HEDM is found to be related to a partial softening of its phonon modes.

A polymeric nitrogen N$$_6$$–N$$_2$$ system with enhanced stability at low pressure

“…PBEsol 56 GGA 55 functionals were utilized for DFT 47 , 48 calculations regarding all relaxation steps and phonon calculations. This follows our findings dealing with N AsF crystal structure prediction 51 where only GGA PBEsol was able to polymerize nitrogen as C N within the crystal. The C (2/m) phase found at 5 GPa and C found at 0 GPa using PBEsol and was reoptimized by PBE 55 and LDA 48 to assess the impact of the level of theory on the lattice parameters (See Supplementary Table 1 ).…”

“…The USPEX 32 – 34 algorithm and the DFT 47 , 48 Quantum Espresso 49 , 50 package for crystal structure relaxations were used throughout this investigation. The N –N discovery follows the same methodology we implemented to unravel the crystal structure of N AsF 51 . The N AsF HEDM, which marked the history of polymeric nitrogen as the first synthesized polynitrogen after the discovery of N back in the 1800's 52 – 54 , was highly explosive and impossible to characterize by XRD and spectroscopy techniques during data acquisition.…”

Understanding the Structural Properties and the Stability of the Solid Electrolyte Material Li4PS4I with First-Principles Simulations

“…Formalism for structural search.-The Universal Structure Predictor: Evolutionary Xtallography (USPEX) [32][33][34] version 10.4, known for its high success rate in predicting materials structures, [35][36][37][38][39][40][41] was employed in this study. An ab initio evolutionary search was performed using USPEX to investigate the system of interest.…”