A Continuum of Amorphous Ices between Low- and High-Density Amorphous Ice

Amorphous ices at approximately P<1 GPa are usually classified as low-density or high-density amorphous ice (LDA and HDA) with densities ρ~0.94 g/cc and ρ~1.15 g/cc, respectively. However, a recent experiment crushing hexagonal ice (ball-milling) produced a medium-density amorphous ice (MDA, ρ~1.06 g/cc) adding complexity to our understanding of amorphous ice and supercooled water. Motivated by the discovery of MDA, we perform computer simulations where amorphous ices are produced by isobaric cooling and isothermal compression/decompression. We show that, depending on the pressure employed, isobaric cooling can generate a continuum of amorphous ices with densities that expand in between those of LDA and HDA (briefly, intermediate amorphous ices, IA). In particular, the IA generated at P ≈ 125 MPa has a remarkably similar density and average structure as MDA. Our results imply that MDA is not unique, and that it is not impossible that MDA may be related to liquid water. By using the potential energy landscape (PEL) formalism, we provide an intuitive understanding of the nature of LDA, HDA, and the IA generated at different pressures. In this view, LDA and HDA occupy specific and well-separated regions of the PEL; the IA prepared at P = 125 MPa is located in the intermediate region of the PEL that separates LDA and HDA.