First principles hydrostatic compression study of phase I ammonium perchlorate and α-, γ-, and ε-RDX

摘要

We have performed density functional theory (DFT) calculations to study the hydrostatic compression of phase I of ammonium perchlorate (AP) and the α-,γ-, and ε- polymorphs of cyclotrimethylenetrinitramine (RDX). A comparison of the performance of different DFT dispersion- correction schemes in calculating crystal geometries and vibrational frequencies at high pressure has been performed for AP. Using 'on-the-fly' (OTF) pseudopo-tentials, the experimental compression behaviour of phase I AP has been reproduced for all DFT dispersion correction schemes studied. DFT-D calculations on the α-form of RDX reproduced the lattice energy in exceptional agreement with experiment, thus the lattice energy of the metastable β-form was confidently predicted. The calculated lattice parameters and unit cell volumes for α-, γ-, and ε-RDX polymorphs were in excellent agreement with experiment both at ambient and at elevated hydrostatic pressures. Comprehensive vibrational mode calculations (including symmetry) as a function of pressure have been performed with the frequency of all predicted internal modes, again in good agreement to experiment.