Pressure-Induced Phase Transitions in Ammonium Squarate: A Supramolecular Structure Based on Hydrogen-Bonding and π-Stacking Interactions

摘要

We report the results of high-pressure Raman and X-ray diffraction measurements performed on ammonium squarate ((NH4)2C4O4, AS), a representative supramolecular architecture based on hydrogen bonding and π-stacking interactions, at various pressures up to 19 GPa. Two phase transitions at ～2.7 GPa and in the pressure range of 11.1 —13.6 GPa were observed. Both Raman and XRD results provide convincing evidence for these two phase transitions. The first phase transition is attributed to the rearrangements of hydrogen-bonding networks, resulting in the symmetry transformation from F2_1/c to P1. The second one, which is identified as an order—disorder phase transition, arises from significant modifications of squarate rings and random orientations of NH_4~+ cations. The cooperative effects between hydrogen-bonding and π-stacking interactions, as well as mechanisms for the phase transitions, are discussed by virtue of the local structure of AS,