title: Pressure-driven variations of hydrogen bonding energy in ammonium azide (NH4N3): IR absorption and Raman scattering studies.
abstract: In this study, high pressure infrared (IR) absorption and Raman scattering studies for ammonium azide (NH4N3) were carried out at room temperature up to 20 GPa and 22 GPa, respectively. For comparison and further assignment, the vibrational spectra at ambient conditions were calculated using CASTEP code, particularly for the far- and mid-IR modes. The recorded vibrational data consistently indicated a pressure-induced phase transition at 2.9 GPa. All observed vibrational modes maintained their identities at the high pressure phase, indicating that NH4N3 was still presented in the form of ammonium cations and azide anions linked by the hydrogen bond (N-H⋯N). Above 2.9 GPa, the relative magnitude of the torsional mode weakened and the N-H symmetric stretch displayed a redshift, indicating strengthened hydrogen bonding energy. The opposite effects were observed above 12 GPa, where the relative magnitude of the torsional mode strengthened and the N-H symmetric stretch reverted to a blueshift, indicating weakened hydrogen bonding energy. It can be concluded that the hydrogen bonding energy exhibited a weakening (0-2.9 GPa), strengthening (2.9-12 GPa), and then again weakening (12-22 GPa) phenomena with the increasing of compression. The hydrogen bonding energy changing with the increase of pressure can be ascribed to a phase transition at 2.9 GPa and a rotational or bending behavior of azide ions at 12 GPa.
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Xiaoxin
authors.initials: X
authors.lastname: Wu
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Fengxian
authors.initials: F
authors.lastname: Ma
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Chunli
authors.initials: C
authors.lastname: Ma
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Hang
authors.initials: H
authors.lastname: Cui
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015, USA.
authors.forename: Zhenxian
authors.initials: Z
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Hongyang
authors.initials: H
authors.lastname: Zhu
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China.
authors.forename: Qiliang
authors.initials: Q
authors.lastname: Cui