New Phase Diagram of Water under Negative Pressure: Rise of The Lowest-Density Ice Clathrates

Using Monte Carlo packing algorithm and dispersion-corrected density functional theory, we predict a crystalline clathrate of cubic structure III (s-III) with mass density of 0.593 g/cm³ composed of two large icosihexahedral cavities (8⁶6⁸4¹²) and six small decahedral cavities (8²4⁸) per unit cell [Sci. Adv. 2016, 2, e1501010]. A new phase diagram of water ice with TIP4P/2005 model potential is constructed, where the s-III clathrate emerges as the most stable ice polymorph in the pressure region below −5834 bar at 0 K and below −3411 bar at 300 K. Inspired by this discovery, another cubic crystalline phase of s-IV with even lower density of 0.506 g/cm³ is found a stable phase in the phase diagram. Given that s-III and s-IV can be related to the silica analog of ROH and FAU, respectively, we explore all the possible clathrates from database of zeolite structures and find that a FAU ice clathrate with ultralow density (0.5 g/cm³) in the phase diagram. Finally, the new phase diagram under negative pressure is updated. With pressure decreasing, ice Ih emerges firstly, then s-II ice clathrate dominates. Below s-II, s-IV arises at low temperature and FAU dominates at high temperature.