Helium is the most inert element in nature. It is difficult to react with other substances. However, its chemical activity will change significantly under high pressure.
Guide reading
Recently, Professor Sun Jian's research group from the school of physics of Nanjing University and Nanjing micro structure Collaborative Innovation Center worked with researchers from Cambridge University, Edinburgh University and Northridge University of California to predict that helium and ammonia can form a variety of stable compounds under high pressure by means of crystal structure search and first principles calculation, and found such kind of compounds Under the extreme conditions of high temperature and high pressure, there will be special state between solid and liquid - plastic crystal state and super ion state.
Their findings will provide important theoretical reference for further study of helium compounds, solid melting process and new state of matter, as well as the internal structure of planets. Relevant research results were published on Physical Review x on April 9, 2020 under the title of "plastic and super helium ammonia compounds under high pressure and high temperature". At the same time, three helium ammonia compounds found for ice planets is the title of the American Physical Society Physics magazine.
Research background
In recent years, the research on the evolution of the internal state of Uranus and Neptune is an important research direction for human beings to explore unknown celestial bodies.
In the current planetary model, the atmosphere of Uranus and Neptune is mainly composed of hydrogen and helium, while between the atmosphere and the core, most of the volume of the stars is filled with ice layer composed of water, ammonia and methane. On the other hand, although helium is the most inert element in nature, it is usually difficult to react with other substances, but recent studies show that under high pressure, the chemical properties of helium will change significantly, and it can form compounds with some substances.
Since the atmosphere of Uranus and Neptune is full of helium, whether it can react with planetary substances such as ammonia and have any impact on the evolution of the state of things in the ice layer is still a blank of human cognition.
innovation research
Stability and crystal structure of helium ammonia compounds
Based on this background, Professor Sun Jian's research group has systematically studied the compounds of helium and ammonia under high pressure, as well as their states under high temperature and pressure, and obtained a series of surprising theoretical results by using the self-developed crystal structure search method and the first principle calculation based on machine learning acceleration.
They predicted that helium and helium could react at high pressures of 0 to 500 GPa to form a variety of stable compounds (see Figure 1). The first is that he 2nh 3 and he NH 3 tend to form perovskite like structures, in which each nitrogen atom is connected by three covalent bonds and three hydrogen bonds to form a distorted octahedron, and helium atoms are distributed in the octahedron space. The second is that he 2nh 3 tends to form a host guest structure composed of four to eight membered rings, in which the nitrogen atom forms a four to eight membered ring and the helium atom is located in the center of the eight membered ring.
Figure 1: energy stability and crystal structure of helium ammonia compounds
Melting and kinetic behavior of helium ammonia compounds at high temperature and pressure
Then, they used first principles molecular dynamics to study the dynamics of helium ammonia compounds under high temperature and pressure in detail, and found very interesting results. They found that at lower temperatures all compounds maintain solid lattice (diffusion coefficients of all atoms are close to zero, slightly vibrating at lattice points), but at higher temperatures helium ammonia compounds produce novel material states.
As shown in Figure 2, at low pressure (about 10-40 GPA), when the temperature is 500-1000 K, the hydrogen atom in the ammonia molecule freely revolves around the nitrogen atom, which is the obvious plastic crystal state At high pressure (about 100-500gpa), when the temperature is about 1000-4000k, the hydrogen atom can freely travel through the solid sublattice structure composed of helium and nitrogen atoms, which is an obvious super ionic state.
Figure 2: the motion behavior of nitrogen (blue), helium (blue) and hydrogen (red to white) atoms in henh3o crystal. For the convenience of display, the tracks of hydrogen atoms in super ionic state and plastic crystal state are placed in the solid grid of nitrogen and helium atoms at the same time. The time scale is marked from red (initial) to white (end).
Pressure temperature phase diagram
Based on the states of different substances, they constructed a complete pressure temperature phase diagram of helium ammonia compound (as shown in Figure 3). At low pressure (Fig. 3b), there is a large plastic crystalline region (green) between the solid and liquid phases, while at high pressure, the hyperionic state occupies the main pressure temperature region (yellow).
Compared with the hyperionic region of pure ammonia (white dotted part), the introduction of helium expands the hyperionic region, especially over 200 GPa, and is very close to the pressure and temperature conditions of Uranus and Neptune. This shows that the helium insertion will greatly increase the possibility of the occurrence of hyperionic ammonia.
Figure 3: temperature and pressure phase diagram of helium ammonia compound predicted by theory. Different signs represent different states of matter, blue square represents solid, dark green prism represents plastic crystal, blue triangle represents Hyperion, and yellow circle represents liquid. The white dotted line represents the hyperionic region in pure ammonia, and the green and blue solid lines represent the isoenthalpy of Uranus and Neptune, respectively.
In this work, new helium ammonia compounds have been found, and a variety of peculiar motion states of helium ammonia compounds under high temperature and pressure have been found. Their discovery will provide important theoretical reference for people to understand the physical and chemical properties of helium under high pressure, the melting process of solid and new state of matter, as well as the structural evolution of Uranus and Neptune.
It is worth mentioning that this work is the second important progress of Professor Sun Jian's research team in the study of new state of matter under high temperature and pressure. Last year, they have found multiple hyperionic states in helium water compounds under high temperature and pressure. 【Nature Physics 15, 1065 (2019).】( DOI: 10.1038/s41567-019-0568-7 )
In addition, the machine learning assisted crystal structure search method [SCI. Bull. 63, 817 (2018)] (DOI: 10.1016 / j.scib. 2018.05.027) independently developed by Professor Sun Jian's research group provides a solid foundation for the development of this project. This method has been successfully applied to the search for high-pressure structures and design functional materials of multiple systems.
Summary
Liu Cong, Ph.D. student in the research group of Professor Sun Jian, School of physics, Nanjing University, is the first author of the article, Professor Sun Jian is the corresponding author, academician Xing Dingyu and Professor Wang Huitian of School of physics, Nanjing University, professor Chris Pickard and Richard needs of Cambridge University, Professor Andreas Hermann of University of Edinburgh, and Professor Maosheng of northern ridge University of California Professor Miao and others participated in the work.
This work is the latest research achievement of Nanjing micro structure collaborative innovation center. The work has been supported by the key R & D plan of the Ministry of science and technology, the National Natural Science Foundation, the basic business expenses of the Central University and other funds.
The computing work is mainly carried out on supercomputers in Nanjing micro structure Collaborative Innovation Center, Nanjing University High Performance Computing Center, Guangzhou Supercomputing Center "Tianhe II" and Cambridge University.
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Cong Liu, Hao Gao, Andreas Hermann, Yong Wang, Maosheng Miao, Chris J. Pickard, Richard J. Needs, Hui-Tian Wang, Dingyu Xing, and Jian Sun, Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature, Phys. Rev. X 10, 021007 (2020).
DOI: 10.1103/PhysRevX.10.021007
Highlights in the journal physics of the American Physical Society:
Synopsis: Three Helium-Ammonia Compounds Found for Icy Planets, Rachel Berkowitz
https://physics.aps.org/articles/v13/s48
Source: Nanjing University News Network
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