Studies of Hydrogen Bond Vibrations of Hydrogen-Disordered Ice Ic

Xu-Hao Yu,Yan-Ju Sun,Zi-Xun Xu,Xu-Liang Zhu,Xiao-Ling Qin,Peng Zhang,Xiao-Tong Dong,Hao-Cheng Wang,Jing-Wen Cao

doi:10.3390/cryst11060668

Abstract

The hydrogen-disordered structure of ice, Ic, makes it difficult to analyze the vibrational normal modes in the far-infrared region (i.e., the molecular translation band). To clarify the origin of the energy-splitting of hydrogen bond vibrations in this area, a 64-molecule supercell was constructed and calculated using first-principles density functional theory. The results were in good agreement with inelastic neutron scattering experiments and our previous study of a hydrogen-ordered ice Ic model. Assisted by analytic equations, we concluded that the origin of the two hydrogen bond peaks in real ice Ic is consistent with that of hydrogen-ordered ice Ic: the peaks originate from two kinds of normal mode vibration. We categorize the four peaks in the far-infrared region recorded from inelastic neutron scattering experiments as the acoustic peak, the superposition peak, the two-hydrogen bond peak and the four-hydrogen bond peak. We conclude that the existence of two intrinsic hydrogen bond vibration modes represents a general rule among the ice family, except ice X.

Highlights

Water/ice is among the Earth’s most abundant materials
We demonstrate how to determine the intrinsic vibrational types of the hydrogen bonds (H-bonds) of ice Ic using a comparative method with a hydrogen-ordered ice Ic structure
We focus on the translation band, which comprises the optical and acoustic normal modes

Summary

Introduction

In 2019, a new ice phase, ‘ice XVIII’, was found to exist at pressures greater than 100 GPa and temperatures above 2000 K [11]. Most ice phases exist in pairs, such as ice Ih and ice XI, which have the same oxygen positions but differ in terms of hydrogen order or disorder. The vibrational spectrum of ice may be detected by infrared (IR) absorption, Raman scattering and inelastic neutron scattering (INS). These spectra can be simulated using the first-principles density functional theory (DFT) to reveal the lattice structure and the vibrational energies of atoms and molecules. We demonstrate how to determine the intrinsic vibrational types of the hydrogen bonds (H-bonds) of ice Ic using a comparative method with a hydrogen-ordered ice Ic structure

Methods

Results

Conclusion