A New Phase Diagram of Water Under Negative Pressure

RESEARCH ARTICLE

WATER CHEMISTRY 2016 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed A new phase diagram of water under negative under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). pressure: The rise of the lowest-density 10.1126/sciadv.1501010 clathrate s-III

Yingying Huang,1,2* Chongqin Zhu,2,3* Lu Wang,4 Xiaoxiao Cao,1 Yan Su,1 Xue Jiang,1 Sheng Meng,3 Jijun Zhao,1† Xiao Cheng Zeng2,4†

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 Downloaded from 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 (8668412) and six small decahedral cavities (8248) per unit cell,

which is dynamically stable by itself and can be fully stabilized by encapsulating an appropriate guest molecule in the http://advances.sciencemag.org/ 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.

INTRODUCTION thegashydrateoriceclathrate(15–18). In the ice clathrates, water Water molecules are the third most abundant molecular species in molecules form loosely hydrogen-bonded framework connected by in- the universe (1). Ice, the form of water in condensed state, is the most terlinked cages whose inner cavities are either completely or partially common molecular solid on Earth and can be also detected in the occupied by guest molecules. So far, at least five types of ice clathrate giant planetary interior. The existence of water and ice has implication structures have been experimentally identified (11, 15, 19)ortheoret- to the diversity of nature and possible presence of life. Because of the ically proposed (20), namely, cubic structure I (s-I) with 2 512 cages flexible hydrogen bonds, water ice exhibits an exceptionally rich and and 6 51262 cages per unit cell; cubic structure II (s-II) with 16 512 complicated phase diagram (2–10). Under different conditions of cages and 8 51264 cages; hexagonal structure H (s-H) with 3 512 cages, on February 15, 2016 pressure (P)andtemperature(T), there are 17 experimentally estab- 2435663 cages, and 1 51268 cage; tetragonal structure T (s-T) with 2 lished crystalline phases of ice so far (2, 3, 11). Among them, ice XI 425864 cages; and tetragonal structure K (s-K) with 6 512 cages, 4 51263 has the lowest mass density of 0.930 g/cm3 and forms a proton- cages, and 4 51262 cages per unit cell, respectively. Usually, the clathrate ordered phase at zero temperature and ambient pressure (12). At hydrates are considered as hypothetical phases of water (21, 22)and elevated temperature (72 K), ice XI transforms into a proton-disordered have to be stabilized by favorable van der Waals (vdW) interactions phase, that is, the well-known ice Ih (12). In addition, many hypo- between guest molecules and host water cages (16–18, 23). However, thetical ice phases are also predicted and wait experimental confir- Falenty and co-workers (11) recently synthesized guest-free s-II mation, for example, virtual ices i and i′ with low density (13), ice 0 clathrate (also named ice XVI) by leaching Ne atoms from the s-II as a precursor to ice nucleation (5), silica-like ice polymorphs (14), Ne clathrate. The guest-free s-II clathrate is mechanically stable at and partially ionic phase of water ice under extremely high pressure atmospheric pressure up to a temperature of 145 K and exhibits neg- (5 to 50 Mbar) (6–10). ative thermal expansion with good mechanical stability and larger Apart from pure water ices, water and various gas molecules (such lattice constants than the filled clathrate. The experimental realiza- as Ne, Ar, H2,CO2,CH4,C2H6, adamantane, and methylcyclohexane) tion of empty s-II lattice not only confirms that water molecules can form a class of nonstoichiometric inclusion compounds, namely, themselves can form loose crystalline phase with density lower than ice XI but also motivates us to further explore other possible low- 1Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian density polymorphs of water ice that are stable at low temperatures University of Technology, Ministry of Education, Dalian 116024, China. 2Department of and negative pressures. Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, In contrast to the comprehensive knowledge of phase diagram of Lincoln, NE 68588, USA. 3Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 4Hefei water under positive pressures, the negative-pressure region of the National Laboratory for Physical Sciences at the Microscale and Collaborative diagram is much less explored (21, 22, 24, 25). Some efforts have Innovation Center of Chemistry for Energy Materials, Department of Materials Science been devoted to determining the limiting mechanical tension that and Engineering, University of Science and Technology of China, Hefei 230026, China. *These authors contribute equally to this work. the stretched liquid water can sustain before the event of nucleation †Corresponding author. E-mail: [email protected] (J.Z.); [email protected] (X.C.Z.) occurs, namely, the cavitation pressure (24–28). Classical nucleation

Huang et al. Sci. Adv. 2016; 2 :e1501010 12 February 2016 1of6 ARTICLE

https://doi.org/10.1038/s41467-019-09950-z OPEN Room temperature electrofreezing of water yields a missing dense ice phase in the phase diagram

Weiduo Zhu1,2,6, Yingying Huang2,3,4,6, Chongqin Zhu2,6, Hong-Hui Wu 2, Lu Wang1, Jaeil Bai2, Jinlong Yang 1, Joseph S. Francisco2, Jijun Zhao3, Lan-Feng Yuan1 & Xiao Cheng Zeng 1,2,5

Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of

1234567890():,; a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric ﬁeld. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T–P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T–P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric ﬁelds.

1 Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 2 Department of Chemistry, University of Nebraska, Lincoln, NE 68588, USA. 3 Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024, China. 4 Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China. 5 Department of Chemical & Biomolecular Engineering and Department of Mechanical and Materials Engineering, University of Nebraska, Lincoln, NE 68588, USA. 6These authors contributed equally: Weiduo Zhu, Yingying Huang, Chongqin Zhu. Correspondence and requests for materials should be addressed to J.Z. (email: [email protected]) or to L.-F.Y. (email: [email protected]) or to X.C.Z. (email: [email protected])

NATURE COMMUNICATIONS | (2019) 10:1925 | https://doi.org/10.1038/s41467-019-09950-z | www.nature.com/naturecommunications 1 An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram

Yuan Liua,b,1, Yingying Huangc,d,1, Chongqin Zhub,e,f, Hui Lia, Jijun Zhaod, Lu Wangg,2, Lars Ojamäeh, Joseph S. Franciscob,e,f,2, and Xiao Cheng Zenga,b,2 aBeijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, 100029 Beijing, China; bDepartment of Chemistry, University of Nebraska–Lincoln, Lincoln, NE 68588; cShanghai Advanced Research Institute, Chinese Academy of Sciences, 201210 Shanghai, China; dKey Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, 116024 Dalian, China; eDepartment of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316; fDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6316; gCollaborative Innovation Center of Chemistry for Energy Materials, Department of Materials Science and Engineering, University of Science and Technology of China, 230026 Hefei, China; and hDepartment of Physics, Chemistry, and Biology, Linköping University, SE-58 183 Linköping, Sweden

Contributed by Joseph S. Francisco, April 25, 2019 (sent for review January 15, 2019; reviewed by Ben Slater and Amadeu K. Sum) The recent back-to-back findings of low-density porous ice XVI and dictions. This porous ice phase is named ice XVI. Later, ice XVII, XVII have rekindled the century-old field of the solid-state physics a metastable porous ice phase, was also produced by emptying the and chemistry of water. Experimentally, both ice XVI and XVII crystals clathrate hydrogen hydrate (3). The mass density of both ice XVI can be produced by extracting guest atoms or molecules enclosed in and XVII is in the range of 0.81–0.85 g/cm3, lower than that the cavities of preformed ice clathrate hydrates. Herein, we examine (0.93 g/cm3) of normal ice Ih. Note that among the known more than 200 hypothetical low-density porous ices whose struc- clathrate hydrogen hydrates, the highest capacity for hydrogen tures were generated according to a database of zeolite structures. storage is 5.3 wt %, achieved by the sII type (15, 16). This record Hitherto unreported porous EMT ice, named according to zeolite hydrogen storage among ice clathrate hydrates can be broken if a nomenclature, is identified to have an extremely low density of 3 stable porous ice with a lower mass density than the sII type can be 0.5 g/cm and the largest internal cavity (7.88 Å in average radius). produced in the laboratory. Several guest-free porous ices with The EMT ice can be viewed as dumbbell-shaped motifs in a hexago- ultralow density were predicted recently via computer simulations nal close-packed structure. Our first-principles computations and mo- (9, 17, 18). One ice phase, named guest-free sIV, was predicted to lecular dynamics simulations confirm that the EMT ice is stable under be a stable phase in the temperature–pressure (T-P) phase dia- negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen gram of water ice at deeply negative pressures (9). molecules, the EMT ice hydrate can easily outperform the record Zeolite-like ices belong to the hypothetical low-density porous hydrogen storage capacity of 5.3 wt % achieved with sII hydro- ices because of their large cavities that can potentially be exploited gen hydrate. Most importantly, in the reconstructed temperature– for gas storage. In fact, the three most common types of ice pressure (T-P) phase diagram of water, the EMT ice is located at clathrate hydrates, sI, sII, and sH, are isostructural with various deeply negative pressure regions below ice XVI and at higher tem- silica clathrate minerals, i.e., MEP with sI, MTN with sII, and perature regions next to FAU. Last, the phonon spectra of empty-sII, DOH with sH, based on the nomenclature of zeolites (19). In FAU, EMT, and other zeolite-like ice structures are computed by particular, the tetrahedrally coordinated frameworks of oxygen in using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm3), both the bending and stretching vibra- Significance tional modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds. Among 18 known ice structures, ice XVI and XVII were produced by emptying the guest atoms/molecules encapsulated in cavities porous ice | ultralow density | EMT ice | reconstructed temperature– of porous ices. We demonstrate simulation evidence that the pressure phase diagram | record hydrogen storage capacity ultralow-density porous EMT ice (named according to zeolite nomenclature) is thermodynamically stable under negative ater is a unique form of matter with many intriguing pressures. Such a low-density solid (∼60% of the mass density of Wproperties. One such physical property is its wide variety ice XVI) can be exploited for hydrogen storage with H2 mass of stable and metastable crystal structures. To date, 18 different density of 12.9 wt %, which is more than twice that (5.3 wt %) crystalline ice phases have been established experimentally (1– achieved by sII clathrate hydrate. With EMT ice, the temperature– 3). Many more ice phases ranging from 1D to 3D have been pressure phase diagram of water under negative pressures is predicted from computer simulations (4–10). Another known ice reconstructed. Like ice XVII, EMT ice could be produced by form is ice clathrate hydrates with large internal cavities that can pumping off guest molecules in EMT hydrates preformed at high host guest molecules. Clathrate natural gas hydrates have re- pressure. ceived considerable attention because they are an enormous energy source on Earth. Indeed, the amount of carbon in natural Author contributions: Y.L., L.O., J.S.F., and X.C.Z. designed research; Y.L., Y.H., L.W., and X.C.Z. performed research; C.Z., H.L., J.Z., L.O., J.S.F., and X.C.Z. contributed new reagents/ gas hydrates is estimated to be at least twice that in all other analytic tools; Y.L., Y.H., C.Z., H.L., J.Z., L.W., L.O., J.S.F., and X.C.Z. analyzed data; and Y.L., fossil energies combined (11). Clathrate hydrogen hydrates have Y.H., L.W., J.S.F., and X.C.Z. wrote the paper. also received growing attention as they are a renewable and Reviewers: B.S., University College London; and A.K.S., Colorado School of Mines. carbon-free energy source (12). The authors declare no conflict of interest. In previous computer simulation studies, guest-free clathrate Published under the PNAS license. hydrates of type sII were independently predicted to be a stable 1Y.L. and Y.H. contributed equally to this work. phase at negative pressures by Jacobson et al. (13), Conde et al. 2To whom correspondence may be addressed. Email: [email protected], frjoseph@sas. (14), and Huang et al. (8). Remarkably, the guest-free clathrate upenn.edu, or [email protected]. hydrate of type sII was recently produced in the laboratory by This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Falenty et al. (2) by pumping off guest Ne atoms from the cav- 1073/pnas.1900739116/-/DCSupplemental. ities of sII clathrate hydrate, confirming earlier theoretical pre- Published online June 10, 2019.

12684–12691 | PNAS | June 25, 2019 | vol. 116 | no. 26 www.pnas.org/cgi/doi/10.1073/pnas.1900739116 Article

Cite This: J. Phys. Chem. A 2018, 122, 6007−6013 pubs.acs.org/JPCA

Phase Diagram of Methane Hydrates and Discovery of MH-VI Hydrate † ‡ † † † § † Yingying Huang, , Keyao Li, Xue Jiang, Yan Su, Xiaoxiao Cao,*, and Jijun Zhao*, † Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China ‡ Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588, United States § College of Physics and Electronic Engineering, Jiangsu Second Normal University, Nanjing 210013, China

*S Supporting Information

ABSTRACT: Methane hydrate is not only the predominant natural deposits of permafrost and continental margins of Earth but also the dominant methane- containing phase in the nebula and major moons of gas giants. Depending on the surrounding environment (mainly pressure), seven methane hydrate phases have been discovered by experiment or predicted by computer simulation, such as clathrate methane hydrates I, II, H, and K, and filled-ice methane hydrates III, IV, and V. Using extensive Monte Carlo packing algorithm and density functional theory optimization, here we predict a partial clathrate methane hydrate VI built by basic units of 4262 water bowl encapsulating a methane molecule, which is dynamically stable from the computed phonon dispersion. Its density and structural characteristics are comparable to that of filled-ice methane hydrate III. By calculating the formation enthalpies of a variety of candidate phases at different pressures, a phase diagram of methane hydrates is constructed. As pressure rises, phase transitions occur among the methane hydrates along with decreasing water/methane molecular ratios. The newly predicted methane hydrate VI emerges as the most stable phase in the region between clathrate phase II and filled-ice phase III, suggesting that methane hydrate VI might be synthesized in a laboratory under accessible conditions.

■ INTRODUCTION structure I methane hydrate (MH-I), with two 512 (i.e., 12 pentagons) cages and six 51262 cages per unit cell; cubic Gas hydrates are crystalline inclusion compounds consisting of 12 a hydrogen-bonded network of polyhedral water cavities, structure II methane hydrate (MH-II), with 16 5 cages and eight 51264 cages per unit cell; hexagonal structure H methane which encage small gas molecules (such as Ne, Ar, H2,H2S, 12 3 6 3 1−3 hydrate (MH-H), with three 5 cages, two 4 5 6 cages, and CO2,CH4,C2H6,andC3H8). Particularly, when the 12 8 methane molecules contact water at low temperature (close one 5 6 cage per unit cell; and tetragonal structure K ° methane hydrate (MH-K), with six 512 cages, four 51263 cages, to 0 C) and under moderate pressure (a few MPa and up), 12 2 fi methane hydrates (MHs) would form. On the earth, methane and four 5 6 cages per unit cell, respectively. The three lled- hydrate (as the major component of natural gas hydrates) is ice phases of methane hydrate are orthorhombic structure III See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Downloaded via SHANGHAI INST OF APPLIED PHYSICS on February 21, 2019 at 07:47:56 (UTC). the predominant natural deposit of permafrost and continental methane hydrate (MH-III), orthorhombic structure IV margins. By estimation, the amount of energy stored in the methane hydrate (MH-IV), and monoclinic structure V form of MHs is twice that of all conventional fossil deposits methane hydrate (MH-V). For all the clathrate hydrates, together.4,5 On the other hand, methane hydrate is thought to methane molecules are encapsulated in the water cages to stabilize the hydrogen-bonded framework via van der Waals be the dominant methane-containing phase in the nebula of 12 3 6 3 Saturn, Uranus, Neptune, Titan, and their major moons.6,7 interaction. Small cages such as 5 and 4 5 6 can only accommodate one methane molecule; medium cages such as Therefore, studies on the formation and phase transformation 12 2 12 3 12 4 behavior of methane hydrates under high pressure are of 5 6 ,5 6 , and 5 6 are able to encapsulate two or three 12 8 fi importance to solve the urgent problems of future energy methane molecules; large cages such as 5 6 can be lled with up to seven methane molecules.16 For the filled-ice phases, resources, gas storage and transportation, and to understand fi the formation process of outer solar system. methane molecules are lled in the channels of water MH, also known as flammable ice, is a special class of ice that hydrogen-bonded networks. The topological water networks contains methane molecules in water cages or networks of of MH-III, MH-IV, and MH-V are closely related to that of hydrogen-bonded water molecules. To date, at least four clathrate methane hydrates and three filled-ice methane Received: March 17, 2018 − hydrates have been experimentally identified7 13 or theoret- Revised: June 23, 2018 ically proposed.14,15 The four clathrate hydrates are cubic Published: July 2, 2018

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Chemical Physics Letters

journal homepage: www.elsevier.com/locate/cplett

Frontiers article Prediction of a new ice clathrate with record low density: A potential candidate as ice XIX in guest-free form ⇑ ⇑ Yingying Huang a,b, Chongqin Zhu b, Lu Wang c, Jijun Zhao a, , Xiao Cheng Zeng b,c, a Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China b Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA c Collaborative Innovation Center of Chemistry for Energy Materials, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China article info abstract

Article history: Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory opti- Received 19 December 2016 mization, we predict a new cubic crystalline phase of ice clathrate, named as s-IV, which is composed of In ﬁnal form 16 January 2017 eight large icosihexahedral cavities (12464418), eight intermediate dodecahedral cavities (6646), and six- Available online 18 January 2017 teen small octahedral cavities (6246) 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 Keywords: by the TIP4P/2005 water model, the s-IV ice clathrate becomes a more stable ice polymorph in the Ice clathrate negative-pressure region, e.g., below –3830 bar at 0 K, below –4882 bar at 115 K, and below –7292 bar Negative pressure at 200 K. Phase diagram Ó Low density 2017 Elsevier B.V. All rights reserved.

1. Introduction H2S, CH4,C2H6,C3H8, adamantane, tetrahydrofuran, dimethylpen- tane, sulfur hexaﬂuoride) are trapped inside hydrogen-bonded Water is the source of life on Earth and it covers 71% of Earth’s water cavities to form a class of nonstoichiometric inclusion com- surface. Ice Ih, as a solid state of water, not only is omnipresent on pounds, namely, the clathrate hydrate [14–19]. Once the guest Earth but also ubiquitous in the solar system such as on comets, molecules or atoms are removed from the cavities of clathrate asteroids, and icy moons of the giant planets. Hence, exploration hydrate, a special ice phase, the guest-free ice clathrate can be of different forms of ice in different environment has important obtained. A well-known example for the co-crystal ice is the struc- implication to both chemical science and planetary science. The ture II (s-II) ice clathrate, whose guest-free structure is known as moderate strength and ﬂexible directionality of the hydrogen the ice XVI. The latter was recently produced in the laboratory by bonds, combined with the bent molecular geometry of water, give leaching Ne atoms from the s-II Ne clathrate hydrate [14]. The suc- water ice an exceptionally rich and complex phase diagram, from cess in making ice XVI (or guest-free s-II) attests the feasibility of ultrahigh pressure region to deeply negative pressure region converting an ice clathrate into a distinct ice polymorph.

[1–12]. Recently, using Monte Carlo packing algorithm with (CH4) To date, at least seventeen crystalline ice phases (ice Ih, Ic to ice (H2O)2 stoichiometry and dispersion-corrected density functional XVI) are confirmed in the laboratory, and their stabilities depend theory (DFT) optimization, we predicted a new clathrate hydrate on the condition of external pressure (P) and temperature (T) phase (i.e., s-III ice clathrate) that can stabilize in the deeply nega- [13,14]. In a way, these experimentally established ice phases tive pressure region of water, suggesting an alternative co-crystal can be classified into two forms of crystals: (1) The self-crystal ice polymorph [12]. Thus far, at least six forms of ice clathrate ice, in which water molecules can spontaneously crystalize into hydrates have been experimentally confirmed or theoretically pro- different phases, such as ice Ih, Ic, ice II-XV, depending on the posed, including cubic structure I (s-I) with two 512 cages and six external P-T conditions. (2) The co-crystal ice, in which hydropho- 51262 cages per unit cell; cubic structure II (s-II) with sixteen 512 12 4 bic guest atoms or molecules (e.g., He, Ne, Ar, Kr, Xe, H2,N2,CO2, cages and eight 5 6 cages; hexagonal structure H (s-H) with three 512 cages, two 435663 cages and one 51268 cage; cubic struc- 6 8 12 2 8 ⇑ ture III (s-III) with two 8 6 4 cages and six 8 4 cages; tetragonal Corresponding authors at: Department of Chemistry and Nebraska Center for 2 8 4 Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA structure T (s-T) with two 4 5 6 cages; and tetragonal structure K 12 12 3 12 2 (X.C. Zeng). (s-K) with six 5 cages, four 5 6 cages and four 5 6 cages E-mail addresses: [email protected] (J. Zhao), [email protected] (X.C. Zeng). [12,17,20,21]. Compared to the self-crystal ice phases, the http://dx.doi.org/10.1016/j.cplett.2017.01.035 0009-2614/Ó 2017 Elsevier B.V. All rights reserved. 第 33 卷第 1 期高压物理学报 Vol. 33, No. 1 2019 年 2 月 CHINESE JOURNAL OF HIGH PRESSURE PHYSICS Feb. , 2019

DOI: 10.11858/gywlxb.20180643

超低密度笼形冰相及其负压相图

黄盈盈1,2，苏艳1，赵纪军1 （1. 大连理工大学，辽宁大连 116024； 2. 中国科学院上海应用物理研究所，上海 201800）

摘要：水不仅在地球上无处不在，而且在太阳系中（如彗星、小行星及巨行星的卫星上）也普遍存在。因此，探索存在于不同环境条件下不同形态的水冰晶体对物理学、化学、生物学、地球科学以及行星科学都有着重要意义。根据周围的环境条件（压强和温度），冰呈现出极其丰富和复杂的相图。目前，实验上已合成了18个晶体冰相，分别是ice Ic、ice Ih、ice II 直至 ice XVII。此外，还有一些来自于笼形包合物的假想超低密度冰相，分别是I型、II型、H型、K型和T型笼形冰。近期，在实验室中合成的II型笼形冰（即ice XVI）出现在了水的负压相图中，极大地激发了人们去探索其他低密度笼形冰。结合带有色散修正的密度泛函理论计算和经典的蒙特卡罗、分子动力学模拟，我们预测了两个具有超低密度的立方笼形冰相，将其依次命名为 s-III笼形冰（ρ=0.593 g/cm3）和s-IV笼形冰（ρ=0.506 g/cm3）。s-III笼形冰的元胞由2个二十六面体的大笼子（8668412）和6个十面体的小笼子（8248）组成。s-IV笼形冰的元胞中含有 8个二十六面体的大笼子（12464418）、8个十二面体的中等尺寸笼子（6646）和6个八面体的小笼子（6246）。对于这两种笼形冰，超大尺寸的二十六面体水笼子以及不同笼子之间的独特堆积方式使它们的密度极低。把所有低密度冰相（其密度小于或者等于ice XI）考虑在内，我们基于TIP4P/2005模型势函数构建了水在负压下的p-T（压强-温度）相图。在s-II笼形冰下方的极低负压区域内，s-III和s-IV笼形冰取代了之前认为的s-H笼形冰，分别占据了高温和低温部分，因此在相图中产生了一个三相点（T=115 K，p=–488.2 MPa）。密度泛函理论计算表

明，通过在二十六面体大笼子中添加合适尺寸的客体分子，比如十二面烷（C20H20）和富勒烯

（C60），能够分别充分地稳定s-III和s-IV笼形冰晶格。基于实验室中已经制备出的无客体分子填充的s-II笼形冰，且被认定为ice XVI相，那么s-III和s-IV笼形冰很可能是ice XVIII或 ice XIX的候选结构。它们一旦在实验室中被合成，则可以作为一种储存气体的材料用来封装

气体分子（如H2、CH4、CO2等）。计算表明：s-III笼形冰在低温和室温下的储氢能力均为s-II 的两倍左右，达到了美国能源部在海陆运输上制订的储氢目标。关键词：笼形冰；相图；负压；超低密度中图分类号：O521.2; O642.4; O641.3 文献标识码：A

水是由氢和氧两种元素组成的无机物，在宇宙中是第三丰富的分子种类[1]。水是地球上最常见的物质之一，地球表面有71%的地方被水所覆盖。水也是生命之源，是包括人类在内所有生命生存的重要资源，是生物体最重要的组成部分，对于自然界内物种的多样性和生命的起源都有着重要影响。冰

* 收稿日期： 2018-09-25；修回日期：2018-10-29 基金项目：国家自然科学基金（11674046）作者简介：黄盈盈（1988－），女，博士，助理研究员，主要从事低密度冰和水合物研究. E-mail: [email protected] 苏艳（1983－），女，博士，副教授，主要从事天然气水合物和含能材料理论研究. E-mail: [email protected] 通信作者：赵纪军（1973－），男，博士，教授，主要从事低维凝聚态物理和计算材料学研究. E-mail: [email protected]

010001-1 2019/9/16 Structural and Electronic Properties of Binary Clusters SimGen (m + n = 6-13). - PubMed - NCBI

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J Nanosci Nanotechnol. 2019 Dec 1;19(12):7879-7885. doi: 10.1166/jnn.2019.15370.

Structural and Electronic Properties of Binary Clusters SimGen (m + n = 6-13). Huang Y1, Liang X1, Li Z1, Sai L2, Zhao J1, Zeng XC3. Author information 1 Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China. 2 Department of Mathematics and Physics, Hohai University, Changzhou 213022, China. 3 Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA.

Abstract Using genetic algorithm combined with density functional theory calculations, we performed an unbiased global search for the most-stable structures of binary clusters SimGen with size s = m + n from 6 to 13. Further, we studied the structural and electronic properties of SimGen clusters using the B3LYP and CCSD(T) methods coupled with 6-311G + (d) basis set. For s = 6-12, SimGes-m clusters exhibit similar geometries to Sis and Ges clusters, respectively. However, for s = 13, the geometries of SimGes-m clusters fall into five completely different patterns. The negative mixing energies of SimGes-m clusters indicate that they possess higher energetic stability than Sis and Ges clusters. Among all clusters investigated, Si₂Ge₄, Si₂Ge5, Si₂Ge6, Si6Ge₃, Si5Ge5, Si7Ge₄, Si₃Ge9, and Si8Ge5 clusters have the relatively lower mixing energies and thus the highest energetic stabilities. Moreover, the Si₂Ge₄, Si₂Ge6, Si5Ge5, Si7Ge₄, and Si8Ge5 clusters with higher HOMO-LOMO gaps should have higher chemical stabilities than the same-sized Sis and Ges clusters. The Si5Ge5 cluster has a higher ionization potential than Si10 and Ge10. At the size s = 13, the geometry with the highest symmetry has the highest energetic and chemical stabilities.

PMID: 31196303 DOI: 10.1166/jnn.2019.15370

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https://www.ncbi.nlm.nih.gov/pubmed/31196303 1/1 Molecular Simulation, 2015 Vol. 41, No. 13, 1086–1094, http://dx.doi.org/10.1080/08927022.2014.940522

Dissociation mechanism of gas hydrates (I, II, H) of alkane molecules: a comparative molecular dynamics simulation Yingying Huang, Yuan Liu, Yan Su and Jijun Zhao* Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams, College of Advanced Science and Technology, Dalian University of Technology, Ministry of Education, Dalian 116024, P.R. China (Received 9 September 2013; ﬁnal version received 23 June 2014)

Employing NPT molecular dynamics method with consistent valence force ﬁeld, the dissociation processes of sI, sII and sH gas hydrates are simulated at different temperatures and at a constant pressure of 100 MPa. The dissociation mechanisms of gas hydrates are revealed by analysing the structural snapshots, radial distribution functions and diffusion coefﬁcients at different temperatures. As temperature increases, the diffusion rates of water molecules and guest molecules increase; thus the clathrate skeleton formed by water molecules with hydrogen bonds distorts and breaks down; meanwhile the guest molecules encapsulated in the water cavities are released. The size of guest molecules affects the dissociation behaviour of gas hydrate. In addition, the dissociation behaviour also relies on the structural phase of gas hydrates. Keywords: gas hydrate; dissociation; molecular dynamics

1. Introduction the green house effects. On the other side, uncontrolled Gas hydrates are a class of non-stoichiometric ice-like decomposition of hydrate leads to a catastrophic release of inclusion clathrates formed by the hydrogen-bonded methane (as a severe greenhouse gas) into the atmosphere. network lattice of water molecules (host) and encapsulated [6] In the petroleum industry, formation of gas hydrates gas molecules (guest), such as methane, ethane, propane, during gas/oil transportation can plug the transportation hydrogen and carbon oxide, under appropriate conditions pipelines and has been a critical concern for many years. [7] In addition, gas hydrates also act as a storage material of pressure and temperature. The van der Waals (vdW) interactions between host lattice and guest molecules for natural gas [8–9] or hydrogen [10–12] with high stabilise the gas hydrates. Depending on the size of guest storage capacity. molecules and external temperature–pressure conditions, Since the first discovery of clathrate hydrate of there are three major structural types of gas hydrates, i.e. chloride in 1810 by Sir Humphrey Davy,[4] there have I, II and H.[1] As a cubic crystal, the unit cell of structure been tremendous efforts on the research of hydrate and I (sI) hydrate is made up of two 512 pentagonal great progresses have been made in the structures, dodecahedral cages and six 51262 tetrakaidecahedral thermodynamic, formation and dissociation mechanisms, cages, while structure II (sII) hydrate has sixteen 512 and physical properties of gas hydrates.[6,7,13–20] pentagonal dodecahedral cages and eight 51264 hexakai- Among those studies, understanding the dissociation decahedral cages. Meanwhile, the unit cell of hexagonal process of gas hydrates is of key importance for future lattice for structure H (sH) consists of three types of cages: exploitation of natural gas hydrates. three small 512 pentagonal dodecahedral cages, two Some laboratory-scale hydrate experiments have been medium-sized 435663 cages and one large 51268 cage. undertaken to accurately predict the dissociation beha- In nature, hydrates primarily exist in permafrost viour of clathrate hydrate by control of pressure and regions in the Arctic and sediments on the bottom and temperature. Liang et al. [21] developed a 2D axisym- beneath ocean floors.[2,3] It was estimated that the amount metric simulator to model methane hydrate dissociation in of organic carbon contents in these hydrated deposits is at porous media by depressurisation. Their results showed least twice as large as that of total fossil fuel resource that a fast hydrate dissociation rate can be related to available.[1,4,5] Therefore, natural gas hydrates are several factors, including high initial gas saturation, low considered as an important energy backup to solve the outlet pressure, high surrounding temperature and high imminent energy crisis. Meanwhile, gas hydrates may absolute permeability. Masuda et al. [22] developed a two- have profound effects on climate change and environ- phase, gas–water numerical finite-difference simulator to mental protection. On the one side, gas hydrates can serve model the depressurisation experimental results. The as a great reserve to store carbon dioxide and to mitigate Kim–Bishnoi equation was used to determine the

*Corresponding author. Email: [email protected] q 2014 Taylor & Francis PCCP

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Phase diagrams for clathrate hydrates of methane, ethane, and propane from first-principles Cite this: Phys. Chem. Chem. Phys., 2016, 18,3272 thermodynamics

Xiaoxiao Cao,a Yingying Huang,a Wenbo Li,b Zhaoyang Zheng,a Xue Jiang,a Yan Su,*a Jijun Zhaoac and Changling Liude

Natural gas hydrates are inclusion compounds composed of major light hydrocarbon gaseous molecules

(CH4,C2H6,andC3H8) and a water clathrate framework. Understanding the phase stability and formation conditions of natural gas hydrates is crucial for their future exploitation and applications and requires an accurate description of intermolecular interactions. Previous ab initio calculationsongashydratesweremainly limited by the cluster models, whereas the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations. For the first time,weconstructthechemicalpotentialphasediagramsoftypeIIclathratehydratesencapsulatedwith methane/ethane/propane guest molecules using first-principles thermodynamics. We find that the partially

occupied structures (136H2O1CH4, 136H2O16CH4,136H2O20CH4,136H2O1C2H6,and136H2O1C3H8)

and fully occupied structures (136H2O24CH4, 136H2O8C2H6, and 136H2O8C3H8) are thermodynamically Received 28th October 2015, favorable under given pressure–temperature (p–T) conditions. The theoretically predicted equilibrium Accepted 21st December 2015 pressures for pure CH4,C2H6 and C3H8 hydrates at the phase transition point are consistent with the DOI: 10.1039/c5cp06570d experimental data. These results provide valuable guidance for establishing the relationship between the accurate description of intermolecular noncovalent interactions and the p–T equilibrium conditions of www.rsc.org/pccp clathrate hydrates and other molecular crystals.

I. Introduction Most notably, natural gas hydrates are considered as an important energy resource in near future due to the tremendous carbon 4

Published on 22 December 2015. Downloaded by Dalian University of Technology 11/03/2016 00:16:11. Gas clathrate hydrates are a class of nonstoichiometric crystal- content in the natural gas hydrate deposits on the Earth. On the line inclusion compounds. The ‘‘host’’ lattice is made up of other hand, one has to be aware of the negative influences of gas water molecules by hydrogen bonding, whereas small gaseous hydrates, for instance, the dissociationofalargeamountofgas molecules (as ‘‘guest’’) are encapsulated in various water cavities hydrate deposits would induce geological disasters and release of of the host framework.1 The van der Waals (vdW) interactions methane gas from hydrates into the atmosphere would aggravate between the host lattices and the guest molecules maintain the the greenhouse eﬀect. Clearly, understanding the phase stability stability of clathrate crystals under appropriate temperature and and formation conditions of gas hydrates is crucial for their pressure conditions.2 future exploitation and applications.

Natural gas hydrates exist in permafrost regions or beneath deep Small hydrocarbon molecules, such as methane (CH4), ethane

oceans and play an important role in the fields of energy and (C2H6), and propane (C3H8), are the major components of natural environment.3 The pipelines of oil and natural gas might be blocked gas.5 In the presence of small hydrocarbon guest molecules (with by chunks of gas hydrates formed under certain conditions. less than five carbon atoms), there are commonly two kinds of clathrate lattices: structure I (sI) and structure II (sII).6 Generally

a Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, speaking, small molecules such as methane or ethane prefer to Dalian University of Technology, Dalian 116024, China. form sI hydrates, while larger molecules like propane tend to 7 E-mail: [email protected] form sII hydrates. However, the specific type of clathrate hydrate b School of Electronic Science and Technology, Faculty of Electronic Information and depends on not only the size of gas molecules included in Electrical Engineering, Dalian University of Technology, Dalian 116024, China c the hydrate, but also the external pressure–temperature ( p–T ) Beijing Computational Science Research Center, Beijing 100089, China 8 d Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, conditions. As temperature varies, structural transformation Ministry of Land and Resources, Qingdao 266071, China from sI to sII or sII to sI as well as the possible coexistence of sI 9–12 e Qingdao Institute of Marine Geology, Qingdao 266071, China and sII phases may occur in gas clathrates.

3272 | Phys. Chem. Chem. Phys., 2016, 18, 3272--3279 This journal is © the Owner Societies 2016 PCCP

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Phase diagram of water–methane by first-principles thermodynamics: discovery Cite this: Phys. Chem. Chem. Phys., 2017, 19, 15996 of MH-IV and MH-V hydrates†

Xiaoxiao Cao,a Yingying Huang,b Xue Jiang, *b Yan Sub and Jijun Zhao b

Searching novel gas hydrates is an enduring topic of scientific investigations, owing to its outstanding implications on planetology, the origin of life and the exploitation of energy resources. Taking the methane–water system as a representative, we disclose two new dense methane hydrate phases (MH-IV and MH-V) using the Monte-Carlo packing algorithm and density-functional theory (DFT) optimization.

Both of these methane clathrates with (CH4)(H2O)4 stoichiometry can be regarded as filled ices, since their hydrogen bond networks are closely related to that of ice i and ice XI, respectively. In particular, the former ice i network is observed for the first time in all gas hydrates. A new chemical composition phase diagram of methane hydrate is constructed. Our newly identified methane hydrate IV emerges in Received 21st February 2017, the transition zone for a water–methane ratio between 2 : 1 and 5.75 : 1. It suggests that our MH-IV Accepted 13th May 2017 phase can be stabilized without external pressure, which is superior to previous reported filled ices to DOI: 10.1039/c7cp01147d apply to energy storage. These findings attest to the importance of composition effects on the packing mechanism of gas hydrate, and provide new perspectives for understanding the physicochemical and rsc.li/pccp geophysical processes in the giant planets of the solar system.

I. Introduction Therefore, knowledge of the existence conditions and phase stability of gas hydrates is desirable from various standpoints Gas hydrates are ice-like crystals formed from a mixture of water for overcoming humankind’s urgent problems of energy crisis and common gases or hydrocarbon molecules.1 Gas hydrates of and global warming, as well as for answering the fundamental methane, hydrogen, oxygen, nitrogen, noble gases, and carbon questions about the internal structure and evolution of the icy 8 Published on 17 May 2017. Downloaded by Dalian University of Technology 23/06/2017 11:14:49. dioxide are widely found in nature and are important over a planetary bodies. broad range of fields, including energy deposit and storage, Gas hydrates have several unique crystalline structures, environmental protection, planetary science, and life science. accommodating a wide variety of guest species under high pressure. For instance, the behavior and properties of gas hydrates are One common class of gas hydrates is clathrate structure where water crucial for understanding both the origin of life and the molecules form a hydrogen-bonded network connected by inter- planetary processes. Methane (36%) and water (56%) (the unit linked cages and the water cavities are occupied by guest molecules. of this percent is weight) as the major constituents of the So far, six types of clathrate structures have been experimentally interiors of icy giant planets most likely form gas hydrates identified9,10 or theoretically proposed,11–14 namely, cubic structure I, under high-pressure conditions.2 The global reserve of natural cubic structure II, hexagonal structure H, cubic structure s-III and gas in the hydrate form is estimated to be significantly larger s-IV, tetragonal structure T, and tetragonal structure K. Apart from than that in the traditional fossil fuels.3 Gas hydrates are also the low-density clathrate structures, water and various gas molecules relevant to a number of scientific and industrial applications, can form another class of gas hydrates with higher compactness, such as hydrogen storage,4 carbon dioxide capture,5 greenhouse namely filled ice.15 The filled ice structures consist of a host ice eﬀect and climate change,6 and blockage in oil/gas pipelines.7 framework (i.e., a water sublattice), with guest molecules ‘‘filled’’ within the channels of the host framework. Interestingly, the hydrogen-bonded network topology of filled ices can be closely a College of Physics and Electronic Engineering, Jiangsu Second Normal University, related to that of known ice polymorphs. To date, gas hydrate Nanjing 210013, China 15,16 17,18 17 structures based on ice Ih, ice II and ice Ic have been b Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China. observed for H2, Ne, Kr, Ar and CH4 hydrates, respectively. E-mail: [email protected] Both the clathrate structures and filled ice structures significantly † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cp01147d expand our imagination on the phase diagrams of gas hydrates.

15996 | Phys. Chem. Chem. Phys., 2017, 19, 15996--16002 This journal is © the Owner Societies 2017 Computational and Theoretical Chemistry 1123 (2018) 79–86

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Computational and Theoretical Chemistry

journal homepage: www.elsevier.com/locate/comptc

Dissociation mechanism of propane hydrate with methanol additive: A molecular dynamics simulation ⇑ Keyao Li a, Ruili Shi a, Yingying Huang a, Lingli Tang b, Xiaoxiao Cao a, Yan Su a, , Jijun Zhao a a Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China b School of Science, Dalian Nationalities University, Dalian 116600, China article info abstract

Article history: Employing NPT molecular dynamics method with consistent valence force field, dissociation processes of Received 21 August 2017 propane hydrate with and without methanol additive are simulated at different temperatures and a con- Received in revised form 26 October 2017 stant pressure of 50 MPa. We analyze structural snapshots, radial distribution functions, density distribu- Accepted 8 November 2017 tions, angle distributions, change of energies, mean square displacements and diffusion coefficients of Available online 10 November 2017 two comparative models and find that encaging methanol molecules in the 512 cavities could enhance

the diffusion behaviors of H2O and C3H8 molecules and shorten the decomposition time of propane Keywords: hydrate. The hydroxyl group of methanol could form hydrogen bonds with the water cage skeleton, Propane hydrate and destroy the original hydrogen bond balance of the hydrate simultaneously. Our theoretical results Decomposition Methanol could provide a useful guideline to understand gas hydrate blocking wellbore in the production and Molecular dynamics transportation of oil and gas. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction of structure II (sII) hydrate contains sixteen small 512 cages and eight large quasi spherical hexakaidecahedral (51264) cages. SH Natural gas hydrate is a class of non-stoichiometric ice-like contains three types of cages: three small 512 cages, two midsized inclusion clathrates formed by the hydrogen bond lattice of water oblate spheroid dodecahedral (435663) cages, and one large ellip- molecules (host) and encapsulated by alkane gas molecules soid icosahedral (51268) cage. The hydrate structure depends not

(guest), such as CH4,C2H6,C3H8, under appropriate conditions of only on the size of the guest molecules, but also on the external pressure and temperature. Natural gas hydrate, mainly depositing temperature and pressure [1]. For instance, once the temperature in the continental permafrost and deep seas, is considered as a sig- and pressure change, the crystal structure of the hydrate would niﬁcant clean energy backup to solve the energy crisis [1–5]. How- change from sI to sII or from sII to sII [8], moreover, the coexistence ever, unreasonable exploitation and utilization of hydrate could of sI and sII may occur [9]. cause submarine geological disasters and other issues. At the same The exploitation and application of hydrate are closely related time, the release of alkane gases from hydrate into the atmosphere to the decomposition and formation of hydrate, especially decom- would aggravate the greenhouse effect [6]. Therefore, safe and eco- position. If the exploitation of hydrate is unreasonable, it would nomic exploitation of hydrate is necessary and challenging [7]. cause serious greenhouse effect and submarine landslide. Mean- Hydrogen bonds between water molecules forming periodic while, in the exploitation of offshore oil and gas ﬁelds and the cage-based network structure and the van der Waals interactions transportation of deep-sea oil and gas, the formation of hydrate between guest molecules and host molecules are the key to the for- could plug oil and gas pipeline. The addition of additives to hydrate mation and stabilization of the hydrate structure [1]. The most could affect the formation or decomposition process of hydrate. common crystal structures of clathrate hydrate are structure I Thus, it is essential and necessary to understand the effects of (sI), structure II (sII), and structure H (sH), in which sI and sII are additives on formation and decomposition process of hydrate the most abundant hydrate structure in nature. The unit cell of sI (especially the inhibition of formation and acceleration of decom- hydrate consists of six small pentagonal dodecahedral (512) cages position) for the development of gas hydrate in the future [4]. and two large tetrakaidecahedral (51262) cages, while the unit cell In the past decades, there were many works on the effects of additives on decomposition of hydrate [10–20]. Experimentally,

by injecting 10–30 wt% ethylene glycol to CH4 hydrate, Fan et al. ⇑ Corresponding author. concluded that the dissociation rate of CH4 hydrate depended on E-mail address: [email protected] (Y. Su). https://doi.org/10.1016/j.comptc.2017.11.011 2210-271X/Ó 2017 Elsevier B.V. All rights reserved. JOURNAL OF APPLIED PHYSICS 121, 085107 (2017)

A new family of multifunctional silicon clathrates: Optoelectronic and thermoelectric applications Yinqiao Liu, Xue Jiang,a) Yingying Huang, Si Zhou, and Jijun Zhao Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China (Received 30 December 2016; accepted 12 February 2017; published online 28 February 2017) To develop Si structures for multifunctional applications, here we proposed four new low-density silicon clathrates (Si-CL-A, Si-CL-B, Si-CL-C, and Si-CL-D) based on the same bonding topologies of clathrate hydrates. The electronic and thermal properties have been revealed by first-principles calculations. By computing their equation of states, phonon dispersion, and elastic constants, the thermodynamic, dynamic, and mechanical stabilities of Si-CL-A, Si-CL-B, Si-CL-C, and Si-CL-D allotropes are confirmed. In the low-density region of the phase diagram, Si-CL-B, Si-CL-D, and Si- CL-C would overtake diamond silicon and type II clathrate (Si-CL-II) and emerge as the most stable Si allotropes successively. Among them, the two direct semiconductors with bandgaps of 1.147 eV (Si-CL-A) and 1.086 eV (Si-CL-D) are found. The suitable bandgaps close to the optimal Shockley- Queisser limit result in better absorption efficiency in solar spectrum than conventional diamond silicon. Owing to the unique cage-based framework, the thermal conductivity of these Si allotropes at room temperature are very low (2.7–5.7 Wm1 K1), which are lower than that of diamond structured Si by two orders of magnitude. The suitable bandgaps, small effective masses, and low thermal conductivity of our new silicon allotropes are anticipated to find applications in photovoltaic and thermoelectric devices. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4977245]

INTRODUCTION Compared to the dense Si allotropes, the low-density Si allotropes open a new era to achieve direct, optically allowed Silicon is the second most abundant element on Earth’s bandgaps.11–19 These low-density Si allotropes usually pos- crust and has been widely used in the semiconductor indus- sess bandgaps close to the optimal Shockley-Queisser limit try. In the common integrated circuits, silicon serves as a (1.4 eV),20 rendering potential applications in optoelectron- mechanical support for the circuits because it can withstand ics and photovoltaics. It resolves the long-standing short- high temperatures and great electrical activity. Moreover, sil- comings of diamond structured silicon for solar cell, that is, icon is an intrinsic semiconductor, and silicon can readily be indirect bandgap, large optical bandgap, and low absorption doped with elements to reach p or n type. Hence, its conduc- efficiency. In the past few years, this topic has gained much tivity and electrical response can be controllable by the con- attention, and a few low-density silicon candidates with qua- centrations and charge of activated carriers, which is sidirect and direct bandgap of 0.81–1.5 eV have been discov- necessary for transistor and semiconductor detectors. ered by theoretical predictions.11–19 In experiment, a novel In contrast to the booming application in the semiconductor industry, applications of silicon in the other fields are rela- silicon allotrope oC24 with Cmcm space group has been suc- tively less. Exploring new silicon allotropes with crystal cessfully prepared by removing sodium from a Na4Si24 pre- structures and physical properties distinct from the diamond cursor, which displays an open channeled framework, a direct bandgap of 1.3 eV, and a stronger light absorption effi- structured silicon would lead to some new functional silicon 10 allotropes. With sp3 hybridized bond like carbon, silicon can cient than diamond silicon. form many metastable allotropes. Under extreme conditions, In addition to solar cell, low-density silicon phases, the ground-state diamond structured silicon (Si-diam) are especially the clathrate structures, have drawn attention as a promising thermoelectric material. As a typical example, known to transform into a sequence of dense phases, such as 9 BC8 structured Si-III,1 R8 structured Si-XII,2 b-tin structured Nolas reported the synthesis and thermal properties of poly- Si-II,3 and lonsdaleite phases.1 While using ultrafast laser crystalline Si-CL-II. This semiconducting elemental clath- 4 rate is essentially guest-free and possesses a very low induced confined microexplosion approach, some novel 1 1 dense phases of Si were observed after quenching under ultra- thermal conductivity of 2.5 Wm K . However, the mech- high depressurization and temperature.4,5 Meanwhile, low- anism of such low thermal conductivity for Si-CL-II is still controversial.21,22 Using molecular dynamics simulation, density Si allotropes can be synthesized by a metal atom 23 assisted two-step synthesis method.6 By using these methods, Schopf’s group investigated the lattice thermal conductiv- the cage-like silicon type-II clathrate (Si-CL-II)7–9 and open ity of type-I clathrate of silicon (Si-CL-I). They found that 1 1 10 the thermal conductivity of Si-CL-I is 43 Wm K for the framework Si24 (oC24) have been synthesized. empty-cage phase and it will further reduce when guest a)Author to whom correspondence should be addressed. Electronic mail: atoms are included in the cages. Using first-principles calcu- [email protected] lations within quasiharmonic approximation, Norouzzadeh

0021-8979/2017/121(8)/085107/9/$30.00 121, 085107-1 Published by AIP Publishing. PHYSICAL REVIEW B 95, 144104 (2017)

Ionic and superionic phases in ammonia dihydrate NH3 · 2H2O under high pressure

Xue Jiang,1 Xue Wu,1 Zhaoyang Zheng,2 Yingying Huang,1,3 and Jijun Zhao1,4,* 1Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China 2National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, Chinese Academy of Engineering Physics, Mianyang 621900, China 3Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA 4Beijing Computational Science Research Center, Beijing 100089, China (Received 27 May 2016; revised manuscript received 8 March 2017; published 6 April 2017)

Water and ammonia have long been seen as the main species of extraterrestrial space, especially on solar giants, moons, comets, and numerous extrasolar planets. The phases formed by their admixtures under temperature and pressure conditions of the giant planets are important for understanding many observable properties (gravitational moments, atmospheric composition, and magnetic ﬁeld). Here we employ a Monte Carlo packing algorithm combined with ﬁrst-principles calculations to search the low-energy crystal structures of ammonia dihydrate

(ADH). At high pressure above 11.81 GPa, we predict an unusual ionic phase (tetragonal, I41cd) consisting + − + of three alternating layers of H2O, NH4 ,andOH . The occurrence of ionic phase is attributed to the NH4 and OH− electrostatic interaction induced volume reduction, which lowers the energy barrier of molecular to ionic phase transition. Analysis of proton transfer under pressure further supports the transformation mechanism between molecular and ionic phase. According to the mobility of hydrogen atoms from ab initio molecular dynamics, this ionic crystal will transform into a superionic phase under high temperature and high pressure. The existence of ionic or superionic ADH may have important implications for understanding the interiors of Neptune, Uranus, and many extrasolar planets.

DOI: 10.1103/PhysRevB.95.144104

I. INTRODUCTION phases were further confirmed theoretically [8,12]. At ambient pressure, powder neutron diffraction showed that ADH-I is a The connection between the appearance of life and plane- cubic phase with space group P 2 3 at 150 K (Z = 4) [13]. tary environments is a longstanding unresolved issue in many 1 By compressing ADH-I to 470 MPa at 175 K, a mixture scientific fields. Water and ammonia play a decisive role in of ADH-II and ADH-III was obtained by Fortes et al. [11]. the origin and chemical evolution of matter, from simple Afterwards, Fortes et al. [14] and Griffiths et al. [12] carried out molecules to complicated types of organic. They are also density functional theory (DFT) calculations and established believed to be the major constituents of the interiors of icy that the most stable structures of ADH-III and ADH-II are or- solar giants (Neptune and Uranus) [1,2], large icy moons thorhombic (P 2 2 2 ) and monoclinic (P 2 /n), respectively. (Titan, Triton, and the dwarf planet Pluto) [3,4], and numerous 1 1 1 1 The transition pressures between these two phases on the extrasolar planets discovered recently [5,6]. PBE and PBE+G06 phase diagrams are 3.11 and 0.71 GPa, The predicted abundances of ammonia (8%) and water respectively [12]. However, the P 2 /n phase does not satisfy (56%) in the “hot ice” layers of Uranus and Neptune are likely 1 all peaks of the powder diffraction pattern; thus it is possible to form ammonia monohydrate (AMH), ammonia dihydrate to coexist with accessory monoclinic ADH-II structure [12]. (ADH), and ammonia hemihydrate (AHH), corresponding to Beyond that, other ADH phases (IV and bcc-ADH) still have the condensation conditions of pressure up to 600 GPa and not been fully identified from the diffraction patterns due to temperature up to 7000 K [7]. Among them, ADH might be phase mixture [11]. Most importantly, all previous studies a major condensed phase suggested by the cosmochemical focused on a relatively narrow range of pressures (0–5 GPa) models [2,8]. Hence, knowledge of the structure, stability, and temperatures (0–300 K), which are far below the pressures and physical properties of ADH under high-pressure and (20–600 GPa) and temperatures (2000–7000 K) of the ice high-temperature (HPHT) conditions is crucial for better layers in the giant planets [2,15]. understanding the interior structure and evolution of the ice In contrast to the limited knowledge about the NH -H O giants as well as the extrasolar planets. It can be related to many 3 2 mixtures, the extremely rich phase diagrams of pure water and observable phenomena, including the gravitational moment pure ammonia under HPHT conditions have been extensively and atmospheric composition, generation mechanism of the explored [15–25], leading to the discovery of a series of ionic multipolar magnetic field of Neptune and Uranus [9], global and superionic phases. The latter can be characterized as a expansion, and rifting on the Moon [10]. partially melted state and be compared with the ionic fluid or a The phase diagram of ADH under high pressure has been fully melted state in which neutral or ionized water molecules investigated by several groups [11]. To date, five crystalline diffuse freely. In those matters, protons diffuse almost freely phases have been found in experiments; among them three through the lattice formed by the heavy ions at extremely high temperatures and high pressures, showing an exceptionally high ionic conductivity [26]. For instance, DFT calculations * [email protected] by Pickard and Needs predicted that NH3 will transform into an

Communication: Interaction of BrO radical with the surface of water Chongqin Zhu,1 Yurui Gao,2 Jie Zhong,1 Yingying Huang,1,3 Joseph S. Francisco,1,a) and Xiao Cheng Zeng1,a) 1Department of Chemistry, University of Nebraska–Lincoln, Lincoln, Nebraska 68588, USA 2Department of Physics and Astronomy, California State University, Northridge, Northridge, California 91330-8268, USA 3Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China (Received 7 November 2016; accepted 13 December 2016; published online 23 December 2016)

Solvation of a BrO radical in a slab of water is investigated using adaptive buffered force quantum mechanics/molecular mechanics (QM/MM) dynamics simulations. The simulation results show that the BrO radical exhibits preference towards the water surface with respect to the interior region of the water slab, despite BrO’s high afﬁnity to water. Another important ﬁnding is the weaken- ing of (BrO)Br···O(water) interaction at the water surface due to competitive interactions between (BrO)Br···O(water) and (water)H···O(water). As such, the BrO-water slab interaction is dominated by (BrO)O···H(water) interaction, contrary to that in the gas phase, suggesting that the reactive site for the BrO radical at the air/water surface is more likely the Br site. The conclusion from this study can offer deeper insight into the reactivity of the BrO radical at the air/water interface, with regard to atmospheric implications. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4973242]

The halogen oxide species, such as ClO and BrO radi- radical at the surface of water droplets, the aim of this work is cal, play a central role in the depletion of the ozone layer in to study the interaction between the BrO radical and a water the atmosphere.1–9 In particular, bromine containing radical slab, which may bring new insights into the BrO chemistry species have been suggested to be more active than the chlorine in atmosphere. To this end, the adaptive buffered force quan- analogues in the destructive processes.10 One set of reactions tum mechanics/molecular mechanics (adbf-QM/MM) dynam- involving bromine4–6 that are crucial to ozone depletion within ics simulations are performed to investigate structural and the marine boundary layer are as follows: dynamic properties of the interaction of the BrO radical with a water slab. · · Br + O3 → BrO + O2, The adbf-QM/MM17 dynamics simulation was per- BrO· + hν → Br· + O·, (1) formed using the CP2K code.18 Speciﬁcally, the (quantum- · mechanical) QM part was described at the BLYP- O + O3 → 2O2. D/DZVP19–23 level of theory, while the water molecules The net reaction is 2O3 → 3O2, which is catalyzed by the in the (molecular-mechanical) MM part were described Br atom and BrO radical. On the other hand, it is widely using the TIP3P model.24 To assess the reliability of the accepted that hydrogen-bonded complexes with radicals and BLYP-D/DZVP level of theory used in this work,25,26 we water play key roles in atmospheric reactions, especially at use the water dimer as a benchmark reference. Calcula- the surface of the water droplet.5–14 The key species involved tions show that the binding energy of the water dimer 10,15,16 in the chemistry is the BrO·H2O complex. Galvez´ is 3.4 kcal/mol at the BLYP-D/DZVP level of theory, et al.15 have studied the equilibrium geometries, energies, and slightly higher than the experimental value from the litera- properties of the BrO hydrates by means of ab initio cal- ture: 3.16 ± 0.02 kcal/mol.27 The calculated binding energy culations. Upon analyzing electron density distribution, the is in good agreement with the theoretical calculation at intermolecular interactions for the formation of halogen oxide higher levels of theory (3.2 kcal/mol at the CCSD(T)/6- and water molecule complexes are characterized.16 In another 311+G(2d,2p) and CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc- 28 study, Hoehn et al. employed RCCSD(T)/aug-cc-pVQZ level pVTZ levels ). For the BrO···H2O complex, the computed electronic structure calculations to derive an analytic poten- binding energy is 3.7 kcal/mol at BLYP-D/DZVP level, tial energy function for the interaction between a BrO radical slightly higher than that computed at higher levels of theory and a water molecule.10 Most studies of the BrO radical have (3.48 kcal/mol at the MP2/aug-cc-PVTZ and 3.33 kcal/mol at been focused on the gas-phase interaction. Detailed informa- the B3LYP/6-311++G(d,p)levels15). Hence, we expect that the tion about the physico-chemical behavior of the BrO radical at BLYP-D functional would be reliable in predicting the quali- the surface of atmospheric aerosols and cloud droplets is still tative trend on the solvation of the BrO radical on the surface not fully understood. In view of the lack of data for the BrO of water. Prior to the adbf-QM/MM dynamics simulation, a water a) slab with initial thickness of about 25 Å and containing 495 Authors to whom correspondence should be addressed. Electronic 3 addresses: [email protected] and [email protected] water molecules was placed into a 24.6 × 24.6 × 60 Å

0021-9606/2016/145(24)/241102/4/$30.00 145, 241102-1 Published by AIP Publishing. IOP

Journal of Physics: Condensed Matter

J. Phys.: Condens. Matter J. Phys.: Condens. Matter 30 (2018) 335501 (11pp) https://doi.org/10.1088/1361-648X/aad2bf

30 Structural evolution and magnetic 2018 properties of anionic clusters Cr2Gen

JCOMEL density functional theory computation

335501 Xiaoqing Liang1,3, Xiangyu Kong2, Sheng-Jie Lu2, Yingying Huang1, Jijun Zhao1 , Hong-Guang Xu2, Weijun Zheng2 and Xiao Cheng Zeng3,4 X Liang et al 1 Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, People’s Republic of China 2 Beijing National Laboratory for Molecular Science, State Key Laboratory of Molecular Reaction Dy- namics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People s Republic of China Printed in the UK ’ 3 Department of Chemistry, University of Nebraska, Lincoln, NE 68588, United States of America 4 Department of Physics and Department of Mechanical & Materials Engineering, CM University of Nebraska, Lincoln, NE 68588, United States of America E-mail: [email protected], [email protected] and [email protected] 10.1088/1361-648X/aad2bf Received 15 May 2018, revised 2 July 2018 Accepted for publication 11 July 2018 Paper Published 26 July 2018 Abstract 1361-648X The structural, electronic and magnetic properties of dual Cr atoms doped germanium anionic clusters, Cr2Gen− (n = 3–14), have been investigated by using photoelectron spectroscopy in combination with density-functional theory calculations. The low-lying structures of Cr2Gen− are determined by DFT based genetic algorithm optimization. For Cr2Gen− with n ⩽ 8, the 33 structures are bipyramid-based geometries, while Cr2Ge9− cluster has an opening cage-like structure, and the half-encapsulated structure is gradually covered by the additional Ge atoms to form closed-cage configuration with one Cr atom interior for n = 10 to 14. Meanwhile, the two Cr atoms in Cr2Gen− clusters tend to form a Cr–Cr bond rather than be separated. Interestingly, the magnetic moment of all the anionic clusters considered is 1 μB. Almost all clusters exhibit antiferromagnetic Cr–Cr coupling, except for two clusters, Cr2Ge5− and Cr2Ge6−. To our knowledge, the Cr2Gen− cluster is the first kind of transition-metal doped semiconductor clusters that exhibit relatively stable antiferromagnetism within a wide size range. The experimental/theoretical results suggest high potential to modify the magnetic behavior of semiconductor clusters through introducing different transition-metal dopant atoms. Keywords: germanium cluster, chromium doping, photoelectron spectrum, antiferromagnetic, ferromagnetic

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