Polymorphism, Phase Transition, and Lattice Dynamics of Energetic Oxidizer Ammonium Perchlorate under High Pressure

Abstract

Ammonium perchlorate (AP) is an efficient energetic oxidizer with high density (1.95 g/cm3) and positive oxygen balance (34%), and it has been used as a potential rocket propellant with proper mixture of metal powders/polymeric binders for a long time. In this work, we have systematically investigated the polymorphic phase stability, structural transition, and lattice dynamics of AP under high pressure. From our total energy calculations, it is vivid that AP attains global minimum energy structure in Pnma symmetry over Pna21; however, the difference is very small, ∼1 meV per formula unit. Moreover, the calculated phonon dispersion curves reveal that AP is dynamically stable in both Pnma and Pna21 crystal symmetries at ambient pressure which unambiguously shows the existence of polymorphism in AP. The Pnma phase of AP is found to be mechanically unstable above 4 GPa from the computed mechanical stability criteria. The calculated pressure-dependent phonon dispersion curves of the Pnma phase disclose its dynamical instability at 10 GPa. Mechanical and dynamical instability of the Pnma phase clearly demonstrates that AP undergoes a first-order structural phase transition above 4 GPa. The high-pressure phase crystallizes in orthorhombic crystal symmetry (P212121) at 6.9 GPa which is consistent with the recent experimental results. The anticooperative nature of hydrogen bonding and electrostatic interactions is the driving force for this structural phase transition in AP under high pressure.