  Silver pentazolate, a high energy density compound containing cyclo-N5-
anion, has recently been synthesized at ambient conditions. However, due to
high sensitivity to irradiation, its crystal structure has not been determined.
In this work, silver-nitrogen crystalline compounds at ambient conditions and
high pressures, up to 100 GPa, are predicted and characterized by performing
first-principles evolutionary crystal structure searching with variable
stoichiometry. It is found that newly discovered AgN5 and AgN6 are the only
thermodynamically stable silver-nitrogen compounds at pressures between 42 and
80 GPa. In contrast to AgN5, pentazolate AgN6 compound contains N2 diatomic
molecules in addition to cyclo-N5-. These AgN5 and AgN6 crystals are metastable
at ambient conditions with positive formation enthalpies of 54.95 kJ/mol and
46.24 kJ/mol, respectively. The underlying cause of the stability of cyclo-N5-
silver pentazolates is the enhanced aromaticity enabled by the charge transfer
from silver atoms to nitrogen rings. To aid in experimental identification of
these materials, calculated Raman spectra are reported at ambient pressure: the
frequencies of N5- vibrational modes of AgN5 are in good agreement with those
measured in experiment.
