Phase Study of NH3 to 100GPa by Infrared Absorption.

Abstract

Infrared absorption spectra for solid ammonia were measured with diamond anvil cell at room temperature and pressures up to 120 GPa. Two phase transitions were observed at about 40 and 70 GPa, transition from solid V to VI and to solid VII. Four branch peaks were observed for the NH stretching vibrations. One branch showed a significant frequency decrease from 3200 cm-1 at 10 GPa to 3000 cm-1 at 120 GPa, while the other two branches markedly showed the peak shifts to high frequency. No evidence was obtained for the hydrogen-bond symmetrization. [phase transition, hydrogen-bond symmetrization, solid ammonia, infrared spectroscopy, high pressure]

Ammonia as a case study for the spontaneous ionization of a simple hydrogen-bonded compound

“…We note that the measured spectrum of the known molecular phase, P2 1 2 1 2 1 , as well as its pressure evolution, remained in good agreement with the previously reported experimental 24 and theoretical data 19 for both low-and high-frequency spectral regions (Fig. 2b,d and Supplementary Fig.…”

Raman spectrum of ammonia IV

“…As a matter of fact, the phase diagram and bonding scheme of this molecular solid at high pressure appear, from recent experimental [1][2][3] and theoretical studies, 4,5 to be more complex than thought 20 years ago. Moreover, the contradictions found between different experimental reports 1,2,6-8 have led to some confusion.…”

Elastic properties study of single crystal NH3 up to 26 GPa

“…As an archetype of hydrogen‐bonded systems, it has a weaker hydrogen bond with respect to water, because the single electronic lone pair of the nitrogen atom bonds to three H atoms of neighboring molecules. There is a fundamental interest in compressed ammonia, yet the polymorphism and phase diagram of ammonia under high pressure seems to be more complex; some contradictions and confusions still exist according to different experimental reports …”

“…On the other hand, Sakashita et al . pointed out two phase changes at 40 and 70 GPa using infrared absorption spectroscopy . More recently, X‐ray diffraction of ammonia phase IV indicated that the orthorhombic symmetry may remain up to at least 123 GPa in NH 3 , while a subtle structural change has been noticed as evidence for an isostructural transition at 12 GPa in NH 3 and 18 GPa in ND 3 .…”

“…There is a fundamental interest in compressed ammonia, yet the polymorphism and phase diagram of ammonia under high pressure seems to be more complex; some contradictions and confusions still exist according to different experimental reports. [9][10][11][12][13][14][15][16] At room temperature and above 1 GPa, ammonia is known to crystallize into an orientationally disordered face-centered cubic phase (phase III, space group Fm3m), a so-called plastic phase in which the ammonia molecule rotates at each lattice point. [5] Increasing pressure up to 4 GPa leads to a first-order transition to phase IV, which was supposed to be a hexagonal close-packed structure initially, [5] but later neutron diffraction study on ND 3 revealed it is orthorhombic structure with rotationally ordered molecules in space group P212121.…”