Phase transition, thermodynamics properties and IR spectrum of α- and γ-RDX: First principles and MD studies

Abstract

In present letter, based on density functional theory plus dispersion (DFT-D) and a self-consistent charge density-functional tight-binding (SCC-DFTB) method, the structural and electronic properties are reported, and the phase transition are investigated by analyzing its thermodynamics properties and IR spectrum of RDX. The anisotropy of α- and γ-RDX were discussed at 0⿿10GPa. By fitting the third-order Birch⿿Murnaghan equation of states, the bulk modulus and its pressure derivative of RDX were determined. The α-RDX phase is found stable at ambient condition, however, under pressures, both the values of lattice constants a, b, c and the οEvdw at around 4GPa show abrupt changes which indicate a structural transition occurred. By analyzing the linear compressibility of a, b, c axes at 0⿿8GPa, one clearly see that the molecules in α-RDX phase underwent rotations and translational motion to their position in the γ-RDX phase at about 4GPa, which validates the α⿿γ phase transition. The IR spectra of α-form and γ-form RDX was calculated by analyzing the trajectory of molecules motion, which also show the phase transition from the spectra changes. Employing the quasi-harmonic Debye model, the enthalpy and specific heat were investigated at various pressures of both phases. The condition of equal enthalpies in both phases also indicates the phase transition of α-form to γ-form at around 4GPa. The variation of specific heat with temperature approaches to the classical Dulong⿿Petit's law at high temperature, while at low-temperature it obeys the Debye's T3 law.