First-principles study of the structural transformation, electronic structure, and optical properties of crystalline 2,6-diamino-3,5-dinitropyrazine-1-oxide under high pressure

Pressure and temperature effects on the Raman spectra of LLM-105

Currently, only one crystal structure of LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) (P21/n) has been discovered, and there are still debates on its phase transition point and phase diagram. Based on previous work, we performed crystal structure, Raman spectra, and vibrational properties calculations on LLM-105 crystal. Our results indicate that the crystal structure of LLM-105 remains stable until compressed to 49 GPa, beyond which it may undergo two phase transitions at pressure intervals of 49.0-49.1 GPa and 51.4-51.5 GPa, respectively. Analysis of Raman shift results suggests that these two phase transitions may be reversible, with an intermediate phase possibly acting as a transition phase. Additionally, based on the quasi-harmonic approximation, we fitted the experimental data of LLM-105 lattice expansion state, obtaining the volume at zero pressure and using it for Raman spectra calculations. The results demonstrated the accuracy of this quasi-harmonic approximation method in describing the redshift of Raman peaks during the heating process and the excitation ratio of Raman peaks in different wavenumber ranges.

Ab Initio Crystal Structure Prediction of the Energetic Materials LLM-105, RDX, and HMX

Crystal structure prediction (CSP) is performed for the energetic materials (EMs) LLM-105 and α-RDX, as well as the α and β conformational polymorphs of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), using the genetic algorithm (GA) code, GAtor, and its associated random structure generator, Genarris. Genarris and GAtor successfully generate the experimental structures of all targets. GAtor's symmetric crossover scheme, where the space group symmetries of parent structures are treated as genes inherited by offspring, is found to be particularly effective. However, conducting several GA runs with different settings is still important for achieving diverse samplings of the potential energy surface. For LLM-105 and α-RDX, the experimental structure is ranked as the most stable, with all of the dispersion-inclusive density functional theory (DFT) methods used here. For HMX, the α form was persistently ranked as more stable than the β form, in contrast to experimental observations, even when correcting for vibrational contributions and thermal expansion. This may be attributed to insufficient accuracy of dispersion-inclusive DFT methods or to kinetic effects not considered here. In general, the ranking of some putative structures is found to be sensitive to the choice of the DFT functional and the dispersion method. For LLM-105, GAtor generates a putative structure with a layered packing motif, which is desirable thanks to its correlation with low sensitivity. Our results demonstrate that CSP is a useful tool for studying the ubiquitous polymorphism of EMs and shows promise of becoming an integral part of the EM development pipeline.

Pressure-Modulated Dissolution Behavior of LLM-105 Crystals in High-Temperature Water

The exploration of the microstructural evolution and reaction kinetics of energetic materials with high-temperature and high-pressure water contributes to the understanding of their microscopic physicochemical origin, which can provide critical experimental data for the use of energetic materials. As a promising high-energy and insensitive energetic material, LLM-105 has been investigated under extreme conditions such as high pressure and high temperature. However, little information is available about the effect of water on LLM-105 under high pressure and high temperature. In this work, the interaction between LLM-105 and water under HP-HT was investigated in detail. As a result, the dissolving behavior of LLM-105 in water under high pressure and high temperature is related to the initial pressure. When the initial pressure is less than 1 GPa, LLM-105 crystals are dissolved in high-temperature water; when the initial pressure is above 1 GPa, LLM-105 particles are only decomposed in high-temperature water. When the solution is saturated at a high temperature, recrystallization of the LLM-105 sample appears in the solution. High pressure hindered the dissolution process of the sample in HP-HT water because the interaction between the solute and the solvent was weakened by high pressure. The initial pressure is one of the significant parameters that determines whether LLM-105 crystals can be dissolved in high-temperature water. More importantly, water under high pressure and high temperature can not only act as a solvent when dissolving the samples but also act as a catalyst to accelerate the decomposition process. In addition, the HP-HT water reduced the decomposition temperature of the LLM-105 crystal to a large extent. The research in this paper not only provides insights into the interaction between LLM-105 and water but also contributes to the performance of energetic materials under extreme conditions and their practical applications in complex conditions.

Compression Behavior and Vibrational Properties of New Energetic Material LLM-105 Analyzed Using the Dispersion-Corrected Density Functional Theory

The 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a newly energetic material with an excellent performance and low sensitivity and has attracted considerable attention. On the basis of the dispersion-corrected density functional theory (DFT-D), the high-pressure responses of vibrational properties, in conjunction with structural properties, are used to understand its intermolecular interactions and anisotropic properties under hydrostatic and uniaxial compressions. At ambient and pressure conditions, the DFT-D scheme could reasonably describe the structural parameters of LLM-105. The hydrogen bond network, resembling a parallelogram shape, links two adjacent molecules and contributes to the structure stability under hydrostatic compression. The anisotropy of LLM-105 is pronounced, especially for Raman spectra under uniaxial compression. Specifically, the red-shifts of modes are obtained for [100] and [010] compressions, which are caused by the pressure-induced enhance of the strength of the hydrogen bonds. Importantly, coupling modes and discontinuous Raman shifts are observed along [010] and [001] compressions, which are related to the intramolecular vibrational redistribution and possible structural transformations under uniaxial compressions. Overall, the detailed knowledge of the high-pressure responses of LLM-105 is established from the atomistic level. Uniaxial compression responses provide useful insights for realistic shock conditions.

First-Principles-Based Force Field for 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a highly promising energetic material (EM) with high safety. Understanding its microscopic response mechanisms within the external stimulus is meaningful for the design of EMs. In order to comprehend the complicated phenomena, it is necessary to employ molecular simulation methods to investigate the response mechanisms with the force field (FF) at an atomic level. In this work, we developed a tailored FF for LLM-105 based on first-principles calculations. The validity of the FF was evaluated by molecular dynamics simulations. The structural parameters of LLM-105 predicted by FF are in good agreement with the experimental values, such as lattice constant, bond length, bond angle, dihedral angle and center of mass, and so forth. Moreover, the FF possesses good performance to describe the structural response on pressure accurately. In general, our work not only builds a balanced FF in gas and condensed phases, but also provides a useful tool to study the properties about LLM-105 at a large scale, which is helpful to improve the understanding about the balance between energy and safety in EMs.

Structural, mechanical properties, and vibrational spectra of LLM-105 under high pressures from a first-principles study

In this work, we report the structure, mechanical properties, and vibrational spectra of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), an energetic molecular crystal, with a first-principles method based on density functional theory (DFT) using the recentely developped HASEM package. The elastic constants, acoustic velocity, and parameters of equations of state were calculated, and the predicted ordering of stiffness constants is C ₃₃ (38.5 GPa) > C ₁₁ (24.0 GPa) > C ₂₂ (17.7 GPa). We also investigated the structure and equation of state of LLM-105 under hydrostatic pressure up to 100 GPa. The predicted structures are in good agreement with experimental results available from ambient pressure to 20 GPa. Under compressions, the LLM-105 crystal exhibits anisotropic compressibility, with a highly incompressible response along the a-axis and c-axis. It is worth noting that there is a sudden change in the lattice parameters and change rate of volume at ~30 GPa. Based on the intermolecular interaction analysis and vibrational spectra, a phase transition at the hydrostatic pressure of ~30 GPa is predicted.