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H_2、CH_4、CO_2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。
Water is not only omnipresent on the Earth but also ubiquitous in the solar system such as on comets,asteroids,or icy moons of the giant planets.Hence,exploration of different forms of ice in different environment has significant implication to physical science,chemical science,bioscience,geoscience and planetary science.Depending on the surrounding conditions of pressure and temperature,water ice exhibits an exceptionally rich and complicated phase diagram.To date,at least eighteen crystalline ice phases(ice Ih,Ic,ice II to ice XVII)have been identified under laboratory conditions.In addition,there are many hypothetical ultralow-density ice phases from clathrate hydrates,such as structure I(s-I),structure II(s-II),structure H(s-H),structure K(s-K)and structure T(s-T)ices.Recently,the s-II clathrate ice(ice XVI)produced in the laboratory emerges in the negative pressure part of phase diagram,which stimulates greatly people to explore the other low-density clathrate ices.Using extensive Monte Carlo packing algorithm,classical molecular dynamins simulations,and dispersion-corrected density functional theory optimization,we predict two cubic clathrate ices with ultralow densities,and name them as s-III(ρ=0.593 g/cm^3)and s-IV(ρ=0.506 g/cm^3)clathrate ices.The unit cell of s-III clathrate ice is composed of two large icosihexahedral cavities(8^66^84^12)and six small decahedral cavities(8^24^8),while the unit cell of s-IV clathrate ice is constructed by eight large icosihexahedral cavities(12^46^44^18),eight intermediate dodecahedral cavities(6^64^6),and six small octahedral cavities(6^24^6).For these two clathrate ices,the large-sized icosihexahedral cavities and the unique packed patterns among different cavities result in their record low densities.Considering all the low-density(lower than ice XI or equal to ice XI)ices,we construct a new p-T(pressuretemperature)phase diagram of water with TIP4P/2005 model potential under negative pressures.Below the deeply negative-pressure region of s-II clathrate ice,s-III and s-IV clathrate ices replace s-H clathrate ice,arising as the most stable ice phases in the high-temperature part and the low-temperature part,respectively.As a result,a triple point(T=115 K,p=–488.2 MPa)appears in the phase diagram.The density functional theory calculations suggest that the s-III and s-IV clathrate ices can be fully stabilized by encapsulating an appropriate guest molecule such as dodecahedrane molecule(C20H20)and fullerene molecule(C60)in the large cavity,respectively.Considering that the guest-free s-II clathrate ice has been produced in the laboratory,which is also recognized as ice XVI,both the s-III and s-IV clathrate ices can be viewed as potential candidates of ice XVIII or ice XIX.Computations show that the hydrogen storage capacities of s-III ice clathrate amount to nearly twice of those for the s-II ice clathrate at low temperature and room temperature,which satisfies the DOE ultimate target for on-board hydrogen storage.
作者
黄盈盈
苏艳
赵纪军
HUANG Yingying;SU Yan;ZHAO Jijun(Dalian University of Technology,Dalian 116024,China;Shanghai Institute of Applied Physics,Chinese Academy of Science,Shanghai 201800,China)
出处
《高压物理学报》
EI
CAS
CSCD
北大核心
2019年第1期151-166,共16页
Chinese Journal of High Pressure Physics
基金
国家自然科学基金(11674046)
关键词
笼形冰
相图
负压
超低密度
clathrate ice
phase diagram
negative pressure
ultralow-density
