title: Experimental verification of the high pressure crystal structures in NH3BH3.
abstract: A detailed high-pressure study on NH3BH3 has been carried out using in situ synchrotron X-ray diffraction (XRD) and Raman scattering with a diamond anvil cell up to 20 and 33 GPa, respectively. The Rietveld refinement based on the XRD pattern and analysis of Raman data indicate two first-order phase transitions from the ambient pressure I4 mm structure (α-NH3BH3) to a high pressure Cmc21 phase (β-NH3BH3) at 2.14 GPa, and further into a monoclinic P21 (Z = 2) phase (γ-NH3BH3) at 9.67 GPa. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (with fixed B0 (') = 4) for the β-NH3BH3 below and above 5 GPa, respectively. Still, with the splitting of the NBH rock mode in Raman experiment, it is concluded that a second-order isostructural phase transition occurs at 5 GPa. By analyzing the dihydrogen bonding framework, the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increasing from 12 to 14 at 5 GPa.
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Yanping
authors.initials: Y
authors.lastname: Huang
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Huang
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Zhonglong
authors.initials: Z
authors.lastname: Zhao
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Wenbo
authors.initials: W
authors.lastname: Li
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Shuqing
authors.initials: S
authors.lastname: Jiang
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Defang
authors.initials: D
authors.lastname: Duan
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Kuo
authors.initials: K
authors.lastname: Bao
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhou
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Bingbing
authors.initials: B
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
authors.forename: Tian
authors.initials: T
authors.lastname: Cui