Pressure-induced structural change from low-density to high-density amorphous ice by molecular-dynamics simulations

Abstract

We have investigated the pressure dependence of the structure of amorphous ices by molecular-dynamics simulations to clarify the characteristic features of the structural change from the low-density amorphous (LDA) ice to the high-density amorphous (HDA) ice. We have found that, with increasing pressure at the temperature of 77 K, (1) the mean-square-displacement and its fluctuation as a function of time decrease, (2) the second peak of the oxygen–oxygen (O O) correlation function g ( r ) shifts to shorter distances and changes its shape drastically, e.g. it splits into two peaks, though the first peak remains almost same, (3) the average coordination number estimated from the first peak of g ( r ) increases from 4 to 5, (4) the broad peak of the O O O bond-angle distribution around 110° decreases and shifts towards 60°. From these results we conclude that the four-fold coordinated tetrahedral local structure in LDA ice changes to the five-fold coordinated structure in HDA ice, where the ‘fifth’ neighboring water molecule corresponds to the ‘interstitial’ water molecule.