**Molecular dynamics study of diffusionless phase transformations in HMX:
β-HMX twinning and β-ε phase transition**

A Pereverzev, JOURNAL OF APPLIED PHYSICS, 134, 125105 (2023).

DOI: 10.1063/5.0171571

We use molecular dynamics to study the mechanism of deformation twinning
of beta-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (beta-HMX) in the P2(1)/n
space group setting for the twin system specified by K-1 = (101), eta(1)
= **10 (1) over bar**, K-2 = (101), and eta(2) = **101** at T = 1 and 300 K.
Twinning of a single perfect crystal was induced by imposing increasing
stress. The following three forms of stress were considered: uniaxial
compression along **001**, shear stress in the K1 plane along the eta(1)
direction, and shear stress in the K-2 plane along the eta(2) direction.
In all cases, the crystal transforms to its twin by the same mechanism:
as the stress increases, the a and c lattice parameters become,
respectively, longer and shorter; soon after the magnitude of a exceeds
that of c the system undergoes a quick phase-transition-like
transformation. This transformation can be approximately separated into
two stages: glide of the essentially intact *101* crystal planes along <
10 (1) over bar > crystal directions followed by rotations of all HMX
molecules accompanied by N-NO2 and CH2 group rearrangements. The overall
process corresponds to a military transformation. If uniaxial
compression along **001** is applied to a beta-HMX crystal which is
already subject to a hydrostatic pressure greater than or similar to 10
GPa, the transformation described above proceeds through the crystal-
plane gliding stage but only minor molecular rearrangements occurs. This
results in a high-pressure phase of HMX which belongs to the P2(1)/n
space group. The coexistence curve for this high-pressure phase and
beta-HMX is constructed using the harmonic approximation for the crystal
Hamiltonians.

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