Structures of high and low density amorphous ice by neutron diffraction.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Amorphous ice exists in high-density (HDA) and low-density (LDA) forms, with their structural differences and relationship to crystalline ice and liquid water not fully understood.
• Understanding these structures is crucial for the ongoing debate about the nature of metastable water phases.

Purpose of the Study:

• To determine the detailed structures of high-density amorphous ice (HDA) and low-density amorphous ice (LDA).
• To elucidate the relationship between amorphous ice structures, crystalline ice (ice Ih), and liquid water.
• To investigate the structural factors governing the transition between HDA and LDA.

Main Methods:

• Neutron diffraction utilizing isotope substitution.
• Analysis of radial distribution functions to probe local and extended order.

Main Results:

• Both HDA and LDA are characterized by fully hydrogen-bonded, tetrahedral networks.
• LDA exhibits local order similar to hexagonal ice (ice Ih), while HDA shows similarities to liquid water.
• An increase in second shell radial order and a decrease in spatial order are observed when transitioning from HDA to LDA and then to ice Ih.
• A specific "interstitial" molecule, acting as a "lynch pin," was identified as crucial for maintaining the HDA structure.

Conclusions:

• The identified interstitial molecule plays a key role in the structural integrity of HDA.
• The findings provide critical insights into the mechanism of the HDA-LDA transition.
• This research contributes to the understanding of metastable states of water.