Phase Transition in the Jumping Crystal l-Pyroglutamic Acid: Insights from Dynamic Quantum Crystallography and Spectroscopy

Crystals of l-pyroglutamic acid exhibit a thermosalient phenomenon. During nondestructive, reversible phase transitions, the crystals can jump vertically by several centimeters. Such phase transitions have been described as martensitic; displacive, diffusionless transitions. The molecular reorganizations that impart the thermosalient effect have previously been characterized in detail; however, less attention has been given to the dynamics that precede the phase transition. In this study, we analyze the thermal motion and structural organization of the crystals at temperatures close to the phase transition using X-ray diffraction and low-frequency Raman spectroscopy. These analyses are supported by periodic density functional theory (DFT) calculations. The free energies derived from the lattice dynamics models refined against X-ray data provide a qualitative picture of the relative free energies of the involved crystal phases. The low-frequency phonons are analyzed to find possible molecular motion that can drive the phase transitions. The Raman measurements interpreted in light of spectra derived the periodic DFT calculations, as well as the observed diffuse scattering and correlated disorder, imply that a simplistic picture of a clean phase transition from one periodic crystal lattice to another must be abandoned.