A continuum of amorphous ices between low-density and high-density amorphous ice

Amorphous ices are usually classified as belonging to low-density or high-density amorphous ice (LDA and HDA) with densities rho LDA approximate to 0.94 g/cm3 and rho HDA approximate to 1.15-1.17 g/cm3. However, a recent experiment crushing hexagonal ice (ball-milling) produced a medium-density amorphous ice (MDA, rho MDA approximate to 1.06 g/cm3) adding complexity to our understanding of amorphous ice and the phase diagram of supercooled water. Motivated by the discovery of MDA, we perform computer simulations where amorphous ices are produced by isobaric cooling and isothermal compression/decompression. Our results 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 approximate to 125 MPa has a remarkably similar density and average structure as MDA, implying that MDA is not unique. Using the potential energy landscape formalism, we provide an intuitive qualitative 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. Recently, a medium-density amorphous ice (MDA) was synthesized via ball-milling of hexagonal ice, adding complexity to our understanding of the phase diagram of supercooled water and amorphous ice. Here, the authors use molecular dynamics simulations to show that, depending on the employed pressure, isobaric cooling can generate a continuum of amorphous ices with densities that expand in between those of low- and high-density amorphous ices, with some of these amorphous ices being remarkably similar to MDA.