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Crystal Structure and Non‐Hydrostatic Stress‐Induced Phase Transition of Urotropine Under High Pressure
Author(s) -
Guńka Piotr A.,
Olejniczak Anna,
Fanetti Samuele,
Bini Roberto,
Collings Ines E.,
Svitlyk Volodymyr,
Dziubek Kamil F.
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202003928
Subject(s) - hexamethylenetetramine , tetragonal crystal system , phase transition , diffraction , hydrostatic pressure , phase (matter) , materials science , crystallography , fourier transform infrared spectroscopy , atom (system on chip) , chemistry , crystal (programming language) , crystal structure , thermodynamics , optics , organic chemistry , physics , embedded system , programming language , computer science
High‐pressure behavior of hexamethylenetetramine (urotropine) was studied in situ using angle‐dispersive single‐crystal synchrotron X‐ray diffraction (XRD) and Fourier‐transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure‐transmitting media to study the effect of deviatoric stress on phase transformations. Up to 4 GPa significant damping of molecular librations and atomic thermal motion was observed. A first‐order phase transition to a tetragonal structure was observed with an onset at approximately 12.5 GPa and characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non‐hydrostatic conditions, induced by deviatoric or uniaxial stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage‐like molecules and may be considered a common feature of such systems. DFT computations were performed to model urotropine equation of state and pressure dependence of vibrational modes. The first successful Hirshfeld atom refinements carried out for high‐pressure diffraction data are reported. The refinements yielded more realistic C−H bond lengths than the independent atom model even though the high‐pressure diffraction data are incomplete.