Pressure induced isostructural metastable phase transition of ammonium nitrate

The role of H-bond in the high-pressure chemistry of model molecules

Pressure is an extraordinary tool to modify direction and strength of intermolecular interactions with important consequences on the chemical stability of molecular materials. The decrease of the distance among nearest neighbour molecules can give rise to reactive configurations reflecting the crystal arrangement and leading to association processes. In this context, the role of the H-bonds is very peculiar because their usual strengthening with rising pressure does not necessarily configure a decrease of the reaction activation energy but, on the contrary, can give rise to an anomalous stability of the system. In spite of this central role, the mechanisms by which a chemical reaction is favoured or prevented by H-bonding under high pressure conditions is a poorly explored field. Here we review a few studies where the chemical behaviour of simple molecular systems under static compression was related to the H-bonding evolution with pressure. These results are able to clarify a wealth of changes of the chemical and physical properties caused by the strengthening with pressure of the H-bonding network and provide additional tools to understand the mechanisms of high-pressure reactivity, a mandatory step to make these synthetic methods of potential interest for applicative purposes.

Does increasing pressure always accelerate the condensed material decay initiated through bimolecular reactions? A case of the thermal decomposition of TKX-50 at high pressures

Performances and behaviors under high temperature-high pressure conditions are fundamentals for many materials. We study in the present work the pressure effect on the thermal decomposition of a new energetic ionic salt (EIS), TKX-50, by confining samples in a diamond anvil cell, using Raman spectroscopy measurements and ab initio simulations. As a result, we find a quadratic increase in decomposition temperature (T_d) of TKX-50 with increasing pressure (P) (T_d = 6.28P² + 12.94P + 493.33, T_d and P in K and GPa, respectively, and R² = 0.995) and the decomposition under various pressures initiated by an intermolecular H-transfer reaction (a bimolecular reaction). Surprisingly, this finding is contrary to a general observation about the pressure effect on the decomposition of common energetic materials (EMs) composed of neutral molecules: increasing pressure will impede the decomposition if it starts from a bimolecular reaction. Our results also demonstrate that increasing pressure impedes the H-transfer via the enhanced long-range electrostatic repulsion of H^+δH^+δ of neighboring NH₃OH⁺, with blue shifts of the intermolecular H-bonds. And the subsequent decomposition of the H-transferred intermediates is also suppressed, because the decomposition proceeds from a bimolecular reaction to a unimolecular one, which is generally prevented by compression. These two factors are the basic root for which the decomposition retarded with increasing pressure of TKX-50. Therefore, our finding breaks through the previously proposed concept that, for the condensed materials, increasing pressure will accelerate the thermal decomposition initiated by bimolecular reactions, and reveals a distinct mechanism of the pressure effect on thermal decomposition. That is to say, increasing pressure does not always promote the condensed material decay initiated through bimolecular reactions. Moreover, such a mechanism may be feasible to other EISs due to the similar intermolecular interactions.

New phase of ammonium nitrate: A monoclinic distortion of AN-IV

A new phase of ammonium nitrate (AN) is found using first principles evolutionary crystal structure search. It is this polymorph that is associated with the phase transition to previously unidentified phase, which was detected in experiment at 17 GPa upon appearance of the two extra peaks in Raman spectrum. The new phase has a monoclinic unit cell in the P21/m space group symmetry (AN-P21/m) and is similar to the known phase IV of AN (AN-IV) except the ammonium molecules are oriented differently relative to the nitrate molecules. The calculated free energy of AN-P21/m is found to be lower than AN-IV at pressures above 10.83 GPa. The equation of state of both AN-P21/m and AN-IV phases (volume vs hydrostatic pressure at room temperature) has been obtained within the quasi-harmonic approximation. The calculated Raman spectrum of both AN-P21/m and AN-IV as a function of pressure is in a good agreement with experiment. The energetic competitiveness of AN-IV and AN-P21/m at ambient conditions suggests a possibility of the phase transition in a small pressure-temperature range near ambient pressure and temperature.

Ab initio molecular dynamic study of solid-state transitions of ammonium nitrate

High-pressure polymorphism and phase transitions have wide ranging consequences on the basic properties of ammonium nitrate. However, the phase diagram of ammonium nitrate at high pressure and high temperature is still under debate. This study systematically investigates the phase transitions and structural properties of ammonium nitrate at a pressure range of 5-60 GPa and temperature range of 250-400 K by ab initio molecular dynamics simulations. Two new phases are identified: one corresponds to the experimentally observed phase IV' and the other is named AN-X. Simultaneously, the lattice strains play a significant role in the formation and stabilization of phase IV', providing a reasonable explanation for experimental observation of phase IV-IV' transition which only appears under nonhydrostatic pressure. In addition, 12 O atoms neighboring the NH (N atom in ammonium cation) atom are selected as reference system to clearly display the tanglesome rotation of ammonium cation.

Development of a ReaxFF reactive force field for ammonium nitrate and application to shock compression and thermal decomposition

We have developed a new ReaxFF reactive force field parametrization for ammonium nitrate. Starting with an existing nitramine/TATB ReaxFF parametrization, we optimized it to reproduce electronic structure calculations for dissociation barriers, heats of formation, and crystal structure properties of ammonium nitrate phases. We have used it to predict the isothermal pressure-volume curve and the unreacted principal Hugoniot states. The predicted isothermal pressure-volume curve for phase IV solid ammonium nitrate agreed with electronic structure calculations and experimental data within 10% error for the considered range of compression. The predicted unreacted principal Hugoniot states were approximately 17% stiffer than experimental measurements. We then simulated thermal decomposition during heating to 2500 K. Thermal decomposition pathways agreed with experimental findings.