Molecular Rotations, Multiscale Order, Hyperuniformity, and Signatures of Metastability during the...

Area of Science:

• Condensed Matter Physics
• Materials Science
• Physical Chemistry

Background:

• Amorphous ices exist in low-density (LDA) and high-density (HDA) forms.
• Both LDA and HDA exhibit near hyperuniformity and possess a dynamical hexagonal boron nitride (HBN) network.
• Amorphous ices are dynamic materials, challenging traditional static solid descriptions.

Purpose of the Study:

• To model the compression and decompression of amorphous ices using molecular dynamics.
• To investigate the role of molecular rotations in phase transitions between LDA and HDA.
• To understand how these transitions affect the hyperuniformity and network topology of amorphous ices.

Main Methods:

• Large-scale molecular dynamics simulations were employed.
• Isothermal compression of LDA to HDA and decompression to negative pressures were simulated.
• Structural properties and network topology (HBN) were analyzed across various length scales.

Main Results:

• Both LDA and HDA phases are nearly hyperuniform and feature a dynamical HBN.
• Phase transitions (LDA-HDA and HDA-LDA) involve the activation of rotational degrees of freedom.
• This activation triggers a cascade effect, drastically altering connectivity and HBN topology, and disrupting hyperuniformity.
• Metastability signatures are observed across all length scales, supporting the liquid-liquid critical point scenario.
• LDA and LDA$_{}$ exhibit distinct HBN and structural properties, indicating they are different low-density glasses.

Conclusions:

• Molecular rotations play a crucial role in amorphous ice phase transitions.
• The unfreezing of rotational degrees of freedom induces a multi-scale cascade effect impacting network structure.
• Findings enhance understanding of water, amorphous ices, and network-forming materials.