Molecular rotations, multi-scale order, hyperuniformity and signatures of metastability during the compression/decompression cycles of amorphous ices.

Abstract

We model, via large-scale molecular dynamics simulations, the isothermal compression of low density amorphous ice (LDA) to generate high-density amorphous ice (HDA) and the corresponding decompression extending to negative pressures to recover the low-density amorphous phase $(LDA_{HDA})$. Both LDA and HDA are nearly hyperuniform and are characterized by a dynamical HBN, showing that amorphous ices are non-static materials, and implying that nearly hyperuniformity can be accommodated in dynamical networks. In correspondence with both the LDA-to-HDA and the HDA-to-$LDA_{HDA}$ phase transitions, the (partial) activation of rotational degrees of freedom activates a cascade effect that induces a drastic change in the connectivity and a pervasive reorganization of the HBN topology which, ultimately, break the samples hyperuniform character. Key to this effect is the rapid rate at which changes occur, and not their magnitude. The inspection of structural properties from the short- to the long-range shows that signatures of metastability are present at all length-scales, hence providing further solid evidence in support of the liquid-liquid critical point scenario. LDA and $LDA_{HDA}$ differ in terms of HBN and structural properties, implying that they are distinct low-density glasses. Our work unveils the role of molecular rotations in the phase transitions between amorphous ices and shows how the unfreezing of rotational degrees of freedom generates a cascade effect that propagates over multiple length-scales. Our findings greatly improve our basic understanding of water and amorphous ices and can potentially impact the field of molecular network-forming materials at large.