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.
AbstractIn this work, we have determined the mean square relative displacement, elastic constant, anharmonic effective potential, correlated function, local force constant, and other thermodynamic parameters of diamond-type structured crystals under high-pressure up to 14 GPa. The parameters are calculated through theoretical interatomic Morse potential parameters, by using the sublimation energy, the compressibility, and the lattice constant in the expanded X-ray absorption fine structure spectrum. Numerical results agree well with the experimental values and other theories.
Theoretical High Pressure Study of Phonon Density of State and Debye Temperature of Solid C60: Grüneisen Approximation Approach
