Proton Ordering of Cubic Ice Ic: Spectroscopy and Computer Simulations

Area of Science:

• Solid-state physics
• Materials science
• Crystallography

Background:

• Proton disorder is common in crystalline ice structures.
• Proton ordering typically occurs at low temperatures in hexagonal ice (ice Ih) to form ice XI.
• Cubic ice (ice Ic) has not previously shown evidence of proton ordering.

Purpose of the Study:

• To investigate the possibility of proton ordering in cubic ice (ice Ic).
• To characterize the spectral changes associated with potential proton ordering in ice Ic.
• To explore the resulting proton-ordered structures and their properties.

Main Methods:

• Subjecting ice Ic to a low-temperature preparation protocol.
• Utilizing in situ Fourier Transform Infrared (FT-IR) spectroscopy to monitor spectral changes.
• Performing vibrational spectra computations using classical molecular dynamics and ab initio simulations for various ice Ic structures.

Main Results:

• Observed a narrowing and structural change in the librational band of the FT-IR spectrum of ice Ic.
• The observed spectral features differ from both proton-disordered ice Ic and ordered hexagonal ice (ice XI).
• Computed spectra indicate partial proton ordering in ice Ic, with two low-energy ferroelectric and weakly ferroelectric structures identified.

Conclusions:

• The study provides the first experimental evidence for proton ordering in cubic ice (ice Ic).
• The observed spectral modifications are attributed to partial proton ordering within the ice Ic lattice.
• While two potential ordered structures were identified, a unique assignment remains inconclusive due to similar spectral fits.