Solid-solid polymorphic transition of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) at high
Chaoyu Wang1,2, Yuchuan Shi2, Chaoyang Zhang2
1College of Smart Materials and Future Energy, Fudan University, Shanghai 200433, China.
Physical Chemistry Chemical Physics : PCCP
|July 14, 2025
Summary
Investigating 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) polymorphic transitions reveals a stability order of ε-, γ-, and β-CL-20. This study elucidates the mechanism behind solid-state transformations in flexible molecular crystals.
Area of Science:
- Materials Science
- Crystallography
- Computational Chemistry
Background:
- Solid-solid polymorphic transitions in flexible molecular crystals are complex due to conformational changes and packing rearrangements.
- Understanding these transitions is crucial for materials design and predicting crystal behavior.
Purpose of the Study:
- To investigate the polymorphic behavior of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) at high temperatures.
- To elucidate the mechanism of solid-solid polymorphic transitions in CL-20.
- To determine the relative stability and transition pathways of CL-20 polymorphs.
Main Methods:
- Development and application of a refined force field (OPLS-CL-20) for accurate CL-20 molecular and lattice simulations.
- Utilizing the well-tempered metadynamics method to simulate polymorphic transitions.
- Analysis of free energy landscapes to determine relative polymorph stability.
Main Results:
- Successful simulation of solid-solid polymorphic transitions among ε-, β-, and γ-CL-20 forms.
- Verification of transition state structures and reproduction of all ambient stable forms.
- Established stability order: ε-CL-20 > γ-CL-20 > β-CL-20.
- Observed cracking in heated ε-CL-20 due to highest energy deposition and stress.
Conclusions:
- The study provides a deep understanding of the solid-solid polymorphic transition mechanism in CL-20.
- Facile transitions are attributed to -NO2 group rotation and high lattice similarity among polymorphs.
- Findings contribute to the broader understanding of polymorphic transitions in molecular crystalline materials.
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