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Crystals 2018, 8(7), 265; https://doi.org/10.3390/cryst8070265

Evolution of Interatomic and Intermolecular Interactions and Polymorphism of Melamine at High Pressure

1
Department of Geology & Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Mānoa, 1680 East West Road, POST Bldg., Honolulu, HI 96822, USA
2
Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Mānoa, 1680 East West Road, POST Bldg., Honolulu, HI 96822, USA
3
Center for Advanced Radiation Sources, University of Chicago, Argonne National Laboratory, 9700 S. Cass Ave., Bldg. 434, Argonne, IL 60439, USA
*
Author to whom correspondence should be addressed.
Received: 1 June 2018 / Revised: 20 June 2018 / Accepted: 21 June 2018 / Published: 27 June 2018
(This article belongs to the Special Issue High-Pressure Studies of Crystalline Materials)
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Abstract

Melamine (C3H6N6; 1,3,5-triazine-2,4,6-triamine) is an aromatic substituted s-triazine, with carbon and nitrogen atoms forming the ring body, and amino groups bonded to each carbon. Melamine is widely used to produce laminate products, adhesives, and flame retardants, but is also similar chemically and structurally to many energetic materials, including TATB (2,4,6-triamino-1,3,5- trinitrobenzene) and RDX (1,3,5-trinitroperhydro-1,3,5-triazine). Additionally, melamine may be a precursor in the synthesis of superhard carbon-nitrides, such as β-C3N4. In the crystalline state melamine forms corrugated sheets of individual molecules, which are stacked on top of one another, and linked by intra- and inter-plane N-H hydrogen bonds. Several previous high-pressure X-ray diffraction and Raman spectroscopy studies have claimed that melamine undergoes two or more phase transformations below 25 GPa. Our results show no indication of previously reported low pressure polymorphism up to approximately 30 GPa. High-pressure crystal structure refinements demonstrate that the individual molecular units of melamine are remarkably rigid, and their geometry changes very little despite volume decrease by almost a factor of two at 30 GPa and major re-arrangements of the intermolecular interactions, as seen through the Hirshfeld surface analysis. A symmetry change from monoclinic to triclinic, indicated by both dramatic changes in diffraction pattern, as well as discontinuities in the vibration mode behavior, was observed above approximately 36 GPa in helium and 30 GPa in neon pressure media. Examination of the hydrogen bonding behavior in melamine’s structure will allow its improved utilization as a chemical feedstock and analog for related energetic compounds. View Full-Text
Keywords: melamine; s-triazine; hydrogen bond melamine; s-triazine; hydrogen bond
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Shelton, H.; Dera, P.; Tkachev, S. Evolution of Interatomic and Intermolecular Interactions and Polymorphism of Melamine at High Pressure. Crystals 2018, 8, 265.

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