The molecular description of amorphous ices and the mechanism governing their pressure induced interconversion

We employ classical molecular dynamics simulations to investigate the molecular-level structure of water during the isothermal compression of hexagonal ice (Ih) and low-density amorphous (LDA) ice at low temperatures to high-density amorphous ice (HDA) [1], and the isothermal decompression of HDA by employing a sensitive local order metric [2]. Our results confirm that LDA and HDA are indeed amorphous, i.e., they lack of polydispersed ice domains. Interestingly, HDA contains a small number of domains that are reminiscent of the unit cell of ice IV, although the hydrogen-bond network (HBN) of these domains differ from the HBN of ice IV. Both nonequilibrium LDA-to-HDA and Ih-to-HDA transformations are two-steps processes where a small distortion of the HBN first occurs at low pressures and then, a sudden, extensive re-arrangement of hydrogen bonds at the corresponding transformation pressure follows.

[1] F. Martelli, N. Giovambattista, S. Torquato and R. Car, Phys. Rev. Materials, 2, 075601 (2018)
[2] F. Martelli, H.-Y. Ko, E. C. Oguz and R. Car, Phys. Rev. B, 97, 064105 (2018)