Pressure induced polymorphism in ammonium azide (NH4N3)

Pressure-dependent Raman spectroscopy studies reveal polymorph phase transition in simple molecular ionic crystal NH4N3 at pressure approximate to 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 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 approximate to 55 GPa contradicting the ab initio studies predicting transformation of NH4N3 to nonmolecular hydronitrogen solid at 36 GPa. (C) 2011 Elsevier B.V. All rights reserved.