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.

Density Functional Theory Analysis Identifying the Mechanism for Ignition Sensitivity of Ammonium Periodate Compared with Ammonium Perchlorate

“…The mechanical stability criteria for the orthorhombic crystal requires the elastic stiffness constants to obey the conditions below: .25ex2ex C 11 > 0 ; C 11 C 22 > C 12 2 C 11 C 22 C 33 + 2 C 12 C 13 C 23 − C 11 C 23 2 − C 22 C 13 2 − C 33 C 12 2 > 0 C 44 > 0 ; C 55 > 0 ; C 66 > 0 It is evident from Table that the calculated elastic constants of this work satisfy the above requirements. Note that the results for APC from this work are in a very good agreement with DFT results by Yedukondalu and Vaitheeswaran, and Liu et al and are larger in average by about 35% than the experimental data , and by other DFT results , by 18%.…”

“… a Ref , PBE-TS method b Ref , using finite pressure temperature elasticity (FPTE) method. c Ref , PBE-TS method …”

“… a Ref , PBE-TS method b Ref , using finite pressure temperature elasticity (FPTE) method. c Ref , PBE-TS method d Ref , Brillouin scattering measurement. e Ref , Ultrasonic resonance frequency measurement. …”

Computational Study of Structural and Energetic Properties of Ammonium Perchlorate at Interfaces

“…Very recently, the structural and energetic properties of the polymorphs were investigated using the DFT method, and it was found that the Pnma polymorph is slightly more stable than the Pna21 one at ambient conditions. 21 Using the DFT calculation method described in the Supporting Information, we also have found that the two polymorphs exhibit total energies very similar to that of the Pnma form, slightly lower by 0.02 kcal/mol, which is within the accuracy of our method. These results indicate that the polymorphs may coexist at ambient conditions.…”

“…The unit cell in both polymorphs of the AP crystal includes four ClO 4 – and four NH 4 + ions that are connected by a three-dimensional N–H···O network of hydrogen bonds, as shown in Figure e for the Pnma polymorph. Very recently, the structural and energetic properties of the polymorphs were investigated using the DFT method, and it was found that the Pnma polymorph is slightly more stable than the Pna 21 one at ambient conditions . Using the DFT calculation method described in the Supporting Information, we also have found that the two polymorphs exhibit total energies very similar to that of the Pnma form, slightly lower by 0.02 kcal/mol, which is within the accuracy of our method.…”