First-Principles Study of High-Pressure Behavior of Solid β-HMX

Abstract

A first-principles plane-wave method with an ultrasoft pseudopotential scheme in the framework of the generalized gradient approximation (GGA) was used to calculate the lattice parameters, bulk modulus and its pressure derivative, energy band structures, density of states, phonon density of states, thermodynamic properties, and absorption spectra of solid beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX). The current study is focused on the thermodynamics and electronic properties that were not reported previously. The bulk modulus and its pressure derivative are also consistent with experimental data and other theoretical results. From the results for the band gaps and density of states, it was found that beta-HMX has the tendency to become a semiconductor with increasing pressure. As the temperature increases, the heat capacity, enthalpy, product of temperature and entropy, and Debye temperature all increase, whereas the free energy decreases. The optical absorption coefficients shift to higher frequencies/energies with increasing pressure. The present study leads to a better understanding of how energetic materials respond to compression.

Dynamic Responses and Initial Decomposition under Shock Loading: A DFTB Calculation Combined with MSST Method for β-HMX with Molecular Vacancy

“…The change of the N−O bond was ignored because of its nonsensitivity for pressure. 16 From the MD trajectories, the first reaction had already started within 0.4 ps for all of the simulations; thus, the following discussion has been primarily focused on the period around 0.4 ps to uncover the physical response process and chemical reaction behavior of HMX at an early compression stage. Figure 2a shows the changes of bond length for C−N, N−N, and C−H at the shock speeds of 9 and 10 km·s −1 , both in the conditions of perfect and defective HMX crystal.…”

“…10,22 However, the N−N bond was significantly compressed at high pressure. 13,16 It was still contradicted which one was the dominant pathway among these reaction models.…”

Vibrational and thermodynamic properties of β-HMX: A first-principles investigation

“…This crystal has been investigated using DFT with various exchange-correlation functional. 9,20,52,58,72 The calculated lattice parameters are listed in Table I along with experimental values. GGA calculation overestimates the equilibrium volume by 17%, while LDA underestimates it by 11%.…”

“…26,27 HMX crystals have been extensively investigated by a variety of experimental methods and first-principles calculations. 8,9,19,20,22,[52][53][54][55][56][57][58][59][60][61][62][63] To our surprise, although LDA and GGA calculations have predicted a totally different equilibrium volume of β-HMX, 9,61 with difference of nearly 30%, both LDA and GGA describe the thermal expansion coefficient and heat capacity quite well. Furthermore, LDA also predicts the bulk modulus well.…”

A far- and mid-infrared study of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) under high pressure

“…Thus, high pressure studies of HMX -particularly those that can discern conformational molecular alterations and the role of these alterations in supporting molecular breakdown leading to detonation -would be of great importance toward better-understanding chemical reactions under extreme conditions of pressure and temperature. Despite this, few static high pressure experimental [4,5,[8][9][10][11] and theoretical [12][13][14] studies of HMX exist in the literature. At pressures above ambient, there has been a report of two more phases (e and u) found above 12 and 27 GPa respectively [4] about which little is known.…”