title: A new phase diagram of water under negative pressure: The rise of the lowest-density clathrate s-III.
abstract: Ice and ice clathrate are not only omnipresent across polar regions of Earth or under terrestrial oceans but also ubiquitous in the solar system such as on comets, asteroids, or icy moons of the giant planets. Depending on the surrounding environment (temperature and pressure), ice alone exhibits an exceptionally rich and complicated phase diagram with 17 known crystalline polymorphs. Water molecules also form clathrate compounds with inclusion of guest molecules, such as cubic structure I (s-I), cubic structure II (s-II), hexagonal structure H (s-H), tetragonal structure T (s-T), and tetragonal structure K (s-K). Recently, guest-free clathrate structure II (s-II), also known as ice XVI located in the negative-pressure region of the phase diagram of water, is synthesized in the laboratory and motivates scientists to reexamine other ice clathrates with low density. Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a crystalline clathrate of cubic structure III (s-III) composed of two large icosihexahedral cavities (8(6)6(8)4(12)) and six small decahedral cavities (8(2)4(8)) per unit cell, which is dynamically stable by itself and can be fully stabilized by encapsulating an appropriate guest molecule in the large cavity. A new phase diagram of water ice with TIP4P/2005 (four-point transferable intermolecular potential/2005) model potential is constructed by considering a variety of candidate phases. The guest-free s-III clathrate with ultralow density overtakes s-II and s-H phases and emerges as the most stable ice polymorph in the pressure region below -5834 bar at 0 K and below -3411 bar at 300 K.
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China.; Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA.
authors.forename: Yingying
authors.initials: Y
authors.lastname: Huang
authors.affiliation: Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA.; Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Chongqin
authors.initials: C
authors.lastname: Zhu
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Collaborative Innovation Center of Chemistry for Energy Materials, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China.
authors.forename: Lu
authors.initials: L
authors.lastname: Wang
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China.
authors.forename: Xiaoxiao
authors.initials: X
authors.lastname: Cao
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China.
authors.forename: Yan
authors.initials: Y
authors.lastname: Su
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China.
authors.forename: Xue
authors.initials: X
authors.lastname: Jiang
authors.affiliation: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Sheng
authors.initials: S
authors.lastname: Meng
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China.
authors.forename: Jijun
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA.; Hefei National Laboratory for Physical Sciences at the Microscale and Collaborative Innovation Center of Chemistry for Energy Materials, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China.
authors.forename: Xiao Cheng
authors.initials: XC
authors.lastname: Zeng
keywords: density-functional theory; ice clathrate s-III; icosihexahedral cavity; lowest density ice; molecular dynamics; negative pressure; phase diagram; water