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Abstract
The polymorphism of molecular crystals is a well-known phenomenon, resulting in modifications of physicochemical properties of solid phases. Low temperatures and high pressures are widely used to find phase transitions and quench new solid forms. In this study, L-Leucinium hydrogen maleate (LLHM), the first molecular crystal that preserves its anomalous plasticity at cryogenic temperatures, is studied at extreme conditions using Raman spectroscopy and optical microscopy. LLHM was cooled down to 11 K without any phase transition, while high pressure impact leads to perceptible changes in crystal structure in the interval of 0.0–1.35 GPa using pentane-isopentane media. Surprisingly, pressure transmitting media (PTM) play a significant role in the behavior of the LLHM system at extreme conditions—we did not find any phase change up to 3.05 GPa using paraffin as PTM. A phase transition of LLHM to amorphous form or solid–solid phase transition(s) that results in crystal fracture is reported at high pressures. LLHM stability at low temperatures suggests an alluring idea to prove LLHM preserves plasticity below 77 K.
Highlights
Studying molecular crystals and their phase transitions is of great importance for many scientific fields such as crystallography [1,2], thermodynamics [3,4], computational [5,6,7] and solid state chemistry [8,9,10], etc
Scrupulous analysis of crystal structure allows authors to provide a simple model for bending L-Leucinium hydrogen maleate (LLHM) crystals—layers of L-Leucinium cations and maleic acid anions connected via H-bonds were stacked over a direction
One crystal was used for the low temperature experiment, and another two were used in high pressure experiments in pentane–isopentane mixture (PIP) and paraffin pressure transmitting media (PTM)
Summary
Studying molecular crystals and their phase transitions is of great importance for many scientific fields such as crystallography [1,2], thermodynamics [3,4], computational [5,6,7] and solid state chemistry [8,9,10], etc. L-Leucinium hydrogen maleate (LLHM) is a unique example of organic crystals that preserves plasticity at a cryogenic temperature [34]. Scrupulous analysis of crystal structure allows authors to provide a simple model for bending LLHM crystals—layers of L-Leucinium cations and maleic acid anions connected via H-bonds (forming bc planes) were stacked over a direction (interacting with weak VdW interactions). This results in the possible slipping of layers along b direction (Figure 1). No experiments below 100 K and high pressures were provided before, leaving an opportunity for a combined study of possible phase transitions at extreme conditions using powerful methods of XRD and Raman spectroscopy
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