title: Pressure-induced phase transitions in ammonium squarate: a supramolecular structure based on hydrogen-bonding and π-stacking interactions.
abstract: We report the results of high-pressure Raman and X-ray diffraction measurements performed on ammonium squarate ((NH(4))(2)C(4)O(4), 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 P2(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.
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.
authors.forename: Shourui
authors.initials: S
authors.lastname: Li
authors.forename: Kai
authors.initials: K
authors.lastname: Wang
authors.forename: Mi
authors.initials: M
authors.lastname: Zhou
authors.forename: Qian
authors.initials: Q
authors.lastname: Li
authors.forename: Bingbing
authors.initials: B
authors.lastname: Liu
authors.forename: Guangtian
authors.initials: G
authors.lastname: Zou
authors.forename: Bo
authors.initials: B
authors.lastname: Zou