Low Temperature and High-Pressure Study of Bending L-Leucinium Hydrogen Maleate Crystals

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.

Crystal design by CH...N and N...N interactions: high-pressure structures of high-nitrogen-content azido-triazolopyridazines compounds

High-nitrogen-content compounds 6-azido-1,2,4-triazolo[4,3-b]pyridazine (C₅H₃N₇) and its 3-methyl derivative (C₆H₅N₇) have been in situ crystallized in a diamond-anvil cell and their structures determined by single-crystal X-ray diffraction. Under ambient and high-pressure conditions the crystallizations yield the same phases: the C₅H₃N₇ anhydrate and C₆H₅N₇ hydrated clathrate. In both the structures there are clearly distinguished regions of short CH...N and N...N intermolecular contacts, the latter involving exclusively the azide groups. High pressure initially increases the contents of water in the channel pores of the clathrate.