Prediction of a new ice clathrate with record low density: A potential candidate as ice XIX in guest-free form

Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a new cubic crystalline phase of ice clathrate, named as s-IV, which is composed of eight large icosihexahedral cavities (12(4)6(4)4(18)), eight intermediate dodecahedral cavities (6(6)4(6)), and sixteen small octahedral cavities (6(2)4(6)) per unit cell. Based on DFT calculations, we find that the s-IV ice clathrate with an extremely low mass density of 0.506 g/cm3. In the P-T phase diagram of water described by the TIP4P/2005 water model, the s-IV ice clathrate becomes a more stable ice polymorph in the negative-pressure region, e.g., below-3830 bar at 0 K, below-4882 bar at 115 K, and below-7292 bar at 200 K. (c) 2017 Elsevier B.V. All rights reserved.