Pressure induced polymorphism in ammonium azide (NH4N3)

Polymorph phase transition is observed in NH 4 N 3 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. ► Ammonium azide (NH 4 N 3 ) 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. ► NH 4 N 3 remain in molecular form up to pressures above 50 GPa. Pressure-dependent Raman spectroscopy studies reveal polymorph phase transition in simple molecular ionic crystal NH 4 N 3 at pressure ≈3 GPa unobserved by recent ab initio 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 NH 4 N 3 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 ab initio studies predicting transformation of NH 4 N 3 to nonmolecular hydronitrogen solid at 36 GPa.