Crystal structure of urotropine under high pressure and non-hydrostatic stress-induced phase transitions in cage compounds.

Crystal structure of urotropine under high pressure and non-hydrostatic stress-induced phase transitions in cage compounds.
Chemistry. 2020 Oct 23;:
Authors: Guńka PA, Olejniczak A, Fanetti S, Bini R, Collings IE, Svitlyk V, Dziubek KF
Abstract
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 ~12.5 GPa which is 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 ato...

Source: Chemistry - October 23, 2020 Category: Chemistry Authors: Guńka PA, Olejniczak A, Fanetti S, Bini R, Collings IE, Svitlyk V, Dziubek KF Tags: Chemistry Source Type: research