Pressure induced polymorphism in ammonium azide (NH4N3)

Graphical abstract: Polymorph phase transition is observed in NH4N3 at ∼3 GPa by pressure dependent Raman studies. The strength of hydrogen bond appears to be modified at the phase transition as illustrated by dependence of N-H stretching frequency on pressure shown on figure. Highlights: → Ammonium azide (NH4N3) studied at high pressures by Raman spectroscopy. → Phase transition is observed at pressure ∼3 GPa. → Strength of hydrogen bond appears to be modified at the phase transition. → NH4N3 remain in molecular form up to pressures above 50 GPa. - Abstract: Pressure-dependent Raman spectroscopy studies reveal polymorph phase transition in simple molecular ionic crystal NH4N3 at pressure ∼3 GPa unobserved by recent abinitio evolutionary structure searches. Hydrogen bonding is spectroscopically evident in both low- and high-pressure phases. The strength of hydrogen bond appears to be modified at the phase transition: in the low-pressure phase NH4N3 behaves as system with very strong hydrogen bonding whereas changes of spectra with pressure in the high-pressure phase are indicative of weak or medium-strength hydrogen bonds. The high pressure phase is most likely thermodynamically stable at least up to pressure ∼55 GPa contradicting the abinitio studies predicting transformation of NH4N3 to nonmolecular hydronitrogen solid at 36 GPa.