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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 6: 983 - 996

DOI: https://doi.org/10.7536/PC230221

Review

Gases under High Pressure and Their Associated Chemical Reactions

  • Peng Liu ,
  • Yong Zhou ,
  • Liangyu Liu ,
  • Yang Chen ,
  • Xiaoyang Liu , *
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  • State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University,Changchun 130031, China
*Corresponding authore-mail:

Received date: 2023-02-28

  Revised date: 2023-06-07

  Online published: 2023-06-10

Supported by

The National Natural Science Foundation of China(22171101)

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Abstract

The study of gases under high pressure is a very important research direction, which is of great significance to many disciplines. This paper introduces the special physical and chemical properties of gases and the chemical reactions they participate in under high pressure conditions. Gases behave very differently at high pressure than they do under ambient conditions. At extreme pressures, gases undergo structural transformations, change their electromagnetic properties, and exhibit interesting phase transitions. The chemical reactions of the gases also change and new reaction paths occur. Understanding the effect of high pressure on gas reactions is critical to improving our understanding of the synthesis of new compounds. In addition, the paper also introduces the practical significance of gas under high pressure. The unique properties of gas under high pressure make it widely used in other disciplines. This paper especially introduces the application of gas under high pressure in high-temperature superconductors, extremely high-energy materials and planetary science. In conclusion, the study of gases at high pressure provides valuable insights into the fundamental properties of matter, and understanding these phenomena is critical to advancing disciplines such as condensed matter physics, materials science, and chemistry. Finally, the prospect of further research on gases under high pressure is given.

Contents

1 Introduction

2 Simple gas under high pressure

2.1 Argon and hydrogen under high pressure

2.2 Metallization of xenon under high pressure

2.3 Unique structure of Xe-H2compounds under high pressure

2.4 Chemical reaction of xenon and fluorine under high pressure

3 Gases with superconductivity under high pressure

3.1 Overview of superconductivity

3.2 high-temperature superconductors predicted at High pressure

3.3 high temperature superconductivity of lanthanide polyhydrides under High pressure

3.4 Second group of lanthanide polyhydride superconductors under high pressure

4 Extreme energy materials

4.1 Nitrogen under high pressure

4.2 Hydrogen under high pressure

5 Applications of planetary science

5.1 Applications of helium in Planetary Science

5.2 Missing xenon paradox

6 Conclusion and outlook

Key words： gas; high-pressure; metallization; superconductivity; extreme energy materials; planetary science

Cite this article

Peng Liu , Yong Zhou , Liangyu Liu , Yang Chen , Xiaoyang Liu . Gases under High Pressure and Their Associated Chemical Reactions[J]. Progress in Chemistry, 2023 , 35(6) : 983 -996 . DOI: 10.7536/PC230221

1 Introduction

Pressure is an important thermodynamic parameter that affects the state of a substance and the chemical reactions in which it participates, because it can directly affect the crystal structure and electronic structure of a substance, effectively changing the distance and interaction between atoms or molecules, thereby changing the bonding mode and forming a high-pressure phase[1]. Under high pressure, the properties of many substances are often significantly different from those of substances at ambient pressure (i.e., one atmosphere). For example, at high pressure, the outer atomic orbitals of many substances overlap, making the stable valence state of an element different from that at ambient pressure. As a typical extreme physical condition, high pressure can effectively change the atomic distance and the state of the atomic shell of matter, so it is often used as an atomic distance modulation, information probe and other special application means, and almost permeates most of the frontier research topics[2].
High pressure is a relatively new science. Due to the limitations of many factors such as pressure vessels, it is challenging to reach pressures of hundreds of GPa (1 GPa = 10 000 atm) under experimental conditions, which makes the study of pressure lag behind other thermodynamic variables, such as temperature, for a long time. However, with the use of multi-anvil presses (MAPs), diamond anvil cells (DACs) and synchrotron technology in high-pressure experiments, pressures of hundreds of GPa can be easily achieved, which provides an opportunity for high-pressure research in a wide range of pressures. Pressure, as a thermodynamic parameter, has a huge impact on matter. Since the thermodynamic free energy G that determines a reaction or phase transition is a function of both pressure and temperature, high pressure broadens the pressure parameter space, thereby greatly increasing the likelihood of reactions[3]. At the same time, it has greatly expanded the scope of the "temperature-pressure-composition" phase space, making it possible to discover new materials and novel phenomena.
The study of gases at high pressures is a broad field involving many disciplines such as physics, chemistry, and energy. High-pressure science has been improved over the years. High-pressure equipment and techniques, new materials obtained at high pressure, and unique compounds discovered at high pressure have been summarized. This paper aims to summarize the special physical and chemical properties of gases at high pressure and briefly introduce their applications in related disciplines[4⇓⇓⇓⇓⇓~10]. Under the condition of high pressure, the physical and chemical properties and behavior of gas have changed significantly, so it has important theoretical and practical value to explore the behavior and properties of gas under the extreme condition of high pressure. Among them, the phase transition of gases under high pressure is an important research direction. The study of gas phase change process under high pressure can provide a new research perspective and theoretical support for the law of gas phase change. Under high pressure conditions, the phase change of gases will lead to the emergence of new States of matter, which provides the possibility for the discovery of new substances and their new physical and chemical properties. In addition, the study of gas phase transition process under high pressure is of great significance for the storage and utilization of nitrogen and other gases, as well as the preparation of extreme energy storage materials. At high pressure, the chemical reaction of the gas changes. Many gases are susceptible to chemical reactions at high pressure, and even some gases that are inert at ambient pressure can undergo chemical reactions at high pressure. The high pressure allows the gas to cross the barrier of chemical reaction and non-chemical reaction. In addition, new chemical reaction paths and compounds may appear under high pressure. With the increase of pressure, the global properties of matter change and unexpected physical phenomena appear. At ambient pressure superconductivity is found in only a few element and compounds, whereas at high pressure it is found in most element of that periodic table. To sum up, the study of gases under high pressure conditions not only deepens our understanding of the physical and chemical properties of gases, but also provides new ideas and methods for the application of gases under high pressure. Since most of the matter in the universe is actually under the conditions of high temperature and high pressure, the changes in these properties and physical phenomena of gases found under high pressure may be very important for in-depth understanding of geoscience and astrophysics[11]. In addition, from this point of view, high pressure should not be regarded as an extreme condition, but as an unexplored dimension, because the range of pressure in the whole universe is very wide after all, up to about 50 orders of magnitude. In contrast, most of the compounds and their related physical and chemical properties we know so far, as well as the chemical reaction laws we know so far, are obtained under the condition of one atmosphere or close to one atmosphere[12]. The environmental pressure in which we live is instead a special case. In a word, research under both environmental and high-pressure conditions has promoted the development of related disciplines such as physics, chemistry and astronomy. However, the research on high pressure is still extremely limited, and high pressure should be studied more widely.

2 Simple gas at high pressure

The properties of gas under high pressure are completely different from those under normal temperature and pressure. Under high pressure, gas can be compressed and then undergo phase transition, and metallization and strange phenomena such as superconductivity and magnetism that have not been found under environmental conditions may occur. For example, O2 is metallized at a pressure of 96 GPa, and superconductivity is found at about 100 GPa, with a superconducting critical transition temperature (Tc) of 0.6 K[13][14]. In addition, gases that are generally less reactive at ambient conditions may also undergo chemical reactions induced by pressure, such as xenon[15,16]. In contrast, high pressure can also render inert gases that are highly reactive at ambient conditions, for example, H2 and O2 are mutually inert and coexist in the same crystal[17]. In conclusion, the study of these simple gases at high pressure helps to understand these novel phenomena.

2.1 Argon and hydrogen at high pressure

Pressure-induced hydrogen metallization has been a hot issue in high-pressure science, but it is very challenging to achieve it experimentally[18,19]. It is theoretically proposed that the "chemical pre-compression" method in the form of hydrogen-rich materials obtained by introducing impurity atoms or molecules into hydrogen can promote the metallization of hydrogen under pressure[20⇓~22]. As one of the hydrogen-rich van der Waals compounds found at high pressure, Ar(H2)2 can be used to explore the effect of "doping" Ar on the intermolecular interaction of hydrogen and the metallization of hydrogen at high pressure[16,23⇓⇓⇓~27]. In addition, due to the contradictory conclusions of several studies on Ar(H2)2, the experimental study of Ar(H2)2 under ultra-high pressure and the calculation results of density functional theory have resolved these contradictory conclusions about crystal structure and metallization[28⇓⇓⇓⇓~33][34].
At pressures above 4.2 GPa, Ar(H2)2 was observed to crystallize out of a mixture of Ar and H2 in DAC, consistent with previous experimental studies[28,35]. The insulating Ar(H2)2 with MgZn2-type structure is stable in the pressure range of at least 265 GPa based on XRD and 358 GPa based on the continuity of spectroscopic observations. The XRD spectra can be well indexed with the MgZn2 model up to 265 GPa (Fig. 1A). Synchrotron XRD measurements show that the predicted phase with CeCu2structure at 55 ~ 70 GPa and the predicted phase transition to AlB2 structure (hexagonal lattice) at 250 GPa are not found[30⇓~32]. The cell volume and cell parameters of Ar(H2)2 change smoothly with increasing pressure (Fig. 1b and C), indicating no change in stoichiometry. It is worth mentioning that at pressures above 13.9 GPa, the volume of Ar(H2)2 becomes slightly larger than the sum of the volumes of Ar and H2 (Fig. 1b), which is in contrast to SiH4(H2)2, and the doping of Ar produces a negative chemical pressure on the H2 molecule[27]. Raman spectroscopy measurements show that the Ar(H2)2 at low pressure exhibits a rotor mode characteristic of a freely rotating H2 molecule[35].
图1 (A) 在 265 GPa 下旋转 DAC ±10° 收集的X 射线衍射图的 LeBail 全谱拟合。插图为Ar(H2)2 结晶的原始图像。掩蔽区域(半透明蓝色)是来自金刚石的饱和衍射峰;(B) Ar(H2)2 的 EOS;(C) Ar(H2)2 在压力下的晶胞参数。六个样品用于确定晶胞参数a。六个样品中的三个用于确定 a、c 和晶胞体积。这是由于在压力下形成的 Ar(H2)2 晶体的优选取向[34]

Fig.1 (A) LeBail full-profile fitting of XRD pattern collected with rotating DAC by ±10° at 265 GPa. (Inset) Caked raw image. Masked regions (semitransparent blue) are saturated diffraction peaks from diamond. (B) EOS of Ar(H2)2. (C) Cell parameters of Ar(H2)2 at pressures. Six samples were used to determine cell parameter a. Three of the six samples were used to determine both a, c, and unit cell volume. This is due to preferred orientations of Ar(H2)2 crystals formed under pressure[34].Copyright (2017) National Academy of Sciences, U.S.A

The splitting of the oscillator at 216 GPa with increasing pressure indicates a considerable difference in the intramolecular distance between the two H2 molecules present in the Ar(H2)2 unit cell, which is related to the rotational ordering of the H2 molecules, and the splitting of the H2 oscillator points to a molecular orientational ordering transition. Similar oscillator frequency discontinuity observations can also be found in the direction transitions of H2, D2, and N2[36⇓~38].
Although the quasi-one-dimensional anisotropic sublattice of H2 in Ar(H2)2 is believed to favor the insulator-to-metal transition, the coordination number of H2 molecules in Ar(H2)2 is reduced and the intermolecular distance is increased compared with that in I-phase H2[30]. The band gap of Ar(H2)2 is 2. 0 eV at 358 GPa, and the linear extrapolation shows that the band gap closes at 1 020 ± 40 GPa, which is much higher than the measured value of pure H2(495 GPa). This indicates that Ar is not conducive to the molecular dissociation and band gap closing of H2. "Ar doping" introduces negative chemical pressure to hydrogen molecules, consumes intermolecular interactions and causes the band gap closing to shift to high pressure[39]. The results of charge analysis based on the Ar4H36 cluster model show that the valence orbitals of Ar and H are partially mixed but no new bonds are formed under ultra-high pressure.

2.2 Metallization of xenon under high pressure

Xenon is one of the inert gases and is also a colorless and odorless gas. Because the outer layer of the atom is a closed shell structure, the chemical properties of xenon are stable under ambient conditions. Matter can be compressed by more than an order of magnitude in volume at pressures greater than 100 GPa, and the interaction between atoms at such great pressures will eventually be completely changed, leading to the conversion of all matter into metals[40][41]. Rare gases are the simplest substances for such studies, and xenon is predicted to metallize at the lowest pressure, so xenon is widely studied at high pressures[42⇓⇓⇓⇓⇓⇓⇓⇓⇓~52]. With increasing pressure, xenon transforms from a face-centered cubic (fcc) structure at room temperature to an intermediate close-packed phase at 14 GPa, and then completely transforms to a hexagonal close-packed (hcp) structure at greater than 75 GPa[43,47]. Reichlin et al. Even suggested that xenon maintains the hcp structure at a pressure of at least 172 GPa[49]. At pressures greater than 100 GPa, xenon becomes metallic[43]. Some have directly measured the electrical transport properties of solid xenon at pressures up to 155 GPa and temperatures from 300 K to 27 mK[15]. It is found that the temperature dependence of its resistance changes from a semiconductor to a metal at pressures from 121 to 138 GPa, which directly proves the metallization phenomenon of xenon solid. A conductivity anomaly is observed at low temperatures near the transition. Metallic behavior can be observed from room temperature to low temperature at a pressure of 138 GPa. However, at temperatures below 25 K, the resistance increases significantly. Pure metallization behavior of xenon was observed at a pressure of 155 GPa.

2.3 Unique structure of Xe-H2 compound under high pressure

Although xenon has low chemical activity, it can carry out chemical reactions, especially when induced by pressure, the reactivity of xenon is greatly increased, and it can form weakly bound stoichiometric compounds. Therefore, pressure is an effective method for the synthesis of xenon compounds. With the increase of pressure, a series of compounds can be synthesized through chemical reactions involving xenon. For example, the Xe-H2 binary system can form stable compounds under high pressure[16]. At 4.1 GPa, xenon and hydrogen form a xenon-rich solid. At 4.8 GPa, xenon and hydrogen form a unique hydrogen-rich structure, which is interpreted as a triple solid hydrogen lattice modulated by xenon layers, composed of xenon dimers. The stoichiometry of the xenon-rich solid changes at 4.4, 4.9, and 5.4 GPa, respectively. There are two distinct sets of Xe-Xe distances for the prominent characteristic sublattice of xenon. The 6 xenon atoms in the unit cell are aligned into three Xe-Xe pairs oriented along the c-axis of the unit cell and create a dimer array. The distance of Xe-Xe in the dimer at 4.9 GPa is close to the dimer in the neutral gas phase, while it is also close to the nearest-neighbor Xe-Xe distance in solid fcc xenon at 5 GPa and room temperature[53][54]. Five vibrational modes of H2 were observed in the Raman spectrum, indicating that the H2 molecule is intact. The low-frequency Raman spectrum is indistinguishable from that of pure solid H2 under the same conditions, indicating rotational disorder of the molecule. No signature of Xe — H bonding is observed in the vibrational spectrum[55]. The infrared and Raman spectra indicate a weakening of the intramolecular covalent bonds of the compound and persistence of semiconducting behavior up to at least 255 GPa.

2.4 Chemical reaction of xenon and fluorine at high pressure

Since the first noble gas containing Xe compound XePtF6 was synthesized by chemical reaction, a lot of theoretical and experimental studies on noble gas compounds have been carried out[56][57⇓~59]. Among them, xenon fluoride has been studied more extensively, such as XeF2, XeF4 and Xe F 6[60⇓~62]. Applying pressure is able to convert the molecular solid into an extended solid with more mobile electrons to soften the repulsive interatomic interactions in the narrow space. New two-dimensional and three-dimensional extended non-molecular phases of solid XeF2 and their metallation have been found[63]. At about 50 GPa, the transparent linear insulating XeF2 transforms into a reddish two-dimensional graphite-like hexagonal layered structure of semiconducting XeF4. When the pressure is higher than 70 GPa, it further transforms into a black three-dimensional fluorite-like structure of metal XeF8 polyhedron. Pressure-induced chemical bonding of non-bonded lone pairs through all eight valence electrons in Xe, thus satisfying octet rule delocalization to the sp3d2 hybrid orbital of 2D XeF4 and the p3d5 hybrid orbital of 3D XeF8 at high pressure. This enables simultaneous molecular to nonmolecular and insulator to metal transitions of XeF2. XeF2 is one of the most stable noble gas fluorides and has a linear symmetric molecule with a body-centered tetragonal structure (I4/mmm, Z = 2, I phase)[64]. The bonding properties of XeF2 can be described in terms of three-center four-electron bonding, i.e., two electrons of two F atoms and two electrons of a Xe atom hybridized with sp3d, with mixed covalent and ionic character. The linear molecule XeF2 is unstable and transforms into a new extended phase at pressures between 40 and 50 GPa. As the pressure increases, the XeF2 undergoes a series of changes. Transition to a two-dimensional graphite-layer-like structure at 50 GPa, followed by a continuous electronic insulator-to-metal transition at 50 ∼ 70 GPa, and further to a three-dimensional fluorite-like structure at pressures above 70 GPa. The variation of resistance shows that the pressure of metallization is far lower than that of Xe or F2, from typical insulators below 25 GPa to semiconductors in the range of 25 to 50 GPa, and finally to metals above 70 GPa. In this pressure range, the sample also exhibits a unique color. The sample is transparent below 40 GPa, yellow at 40 ~ 50 GPa, red at 50 ~ 60 GPa, and black at 60 ~ 70 GPa. It is worth noting that the change of resistance and color is reversible under pressure change, although there is some hysteresis. Phase Ⅰ (I4/mnm) of XeF2, which undergoes phase transition at high pressure :XeF2, transforms to phase Ⅱ (Immm) at 7 GPa, to phase Ⅲ (Pnnm-1) at 13 GPa, to phase Ⅳ (Pnnm-2) at 23 GPa, and finally to phase Ⅴ (Fmmm) at 70 GPa. The Ⅰ-Ⅱ phase transition occurs abruptly with a volume collapse of 4.3% at 7 GPa, while the Ⅱ-Ⅲ-Ⅳ phase transition is quite smooth without any significant volume change. At the transition pressure of 70 GPa, the density of phase Ⅴ is 2.0% higher than that of phase Ⅳ.

3 Gas with superconducting properties at high pressure

For more than a hundred years, the unremitting research on superconductivity has achieved brilliant results. In this process, people continue to discover the superconductivity of various substances, from various elemental simple substances, copper oxide ceramic materials, iron-based compounds to binary hydrides, and then to ternary hydrides. At present, the Tc up to 260 K has been achieved under high pressure. In addition, various theories of superconductivity have been developed for the study of superconductivity. Due to the limitation of experimental conditions, theoretical calculations have played a great role in the study of superconductivity. The combined use of superconducting theory, computational tools and high-pressure experiments has made great progress in the study of superconducting phenomena in matter under high-pressure conditions.

3.1 Overview of Superconducting Phenomena

Superconductivity refers to the phenomenon that the resistance of a material becomes zero below a certain temperature, which is called the critical transition temperature (Tc). In 1933, Walther Meissner and Robert Ochsenfeld discovered the complete diamagnetism of superconductors, that is, when the superconductor is in the superconducting state, the internal magnetic field of the superconductor is zero and the magnetic field is completely repelled, that is, the Meissner effect. But when the external magnetic field is larger than the critical value, the superconductivity is destroyed[65]. Therefore, zero resistance and zero magnetic field are the two main characteristics of superconducting materials.
Since Onnes discovered the superconductivity of Hg at low temperature in 1911, scientists have begun to study various elements at low temperature to observe whether there is superconductivity and the Tc of superconductivity (Fig. 2)[66]. The earliest theory to explain superconductivity was the London equation proposed by F. London and H. London in 1935[67]. This set of equations is based on classical electromagnetic theory and can effectively explain the Meissner effect. In 1957, American physicists proposed the BCS theory, named after their initials, to explain the microscopic mechanism of superconductivity[68,69]. BCS theory holds that the vibration of the lattice, called Phonon, causes two electrons with opposite spin and momentum to form a Cooper pair with zero momentum and zero total spin, which is called electron-Phonon interaction. Because the total spin of the Cooper pair is zero, the theory of bosons in quantum statistical mechanics is applicable, and the Cooper pair, like a superfluid, can flow around lattice defects and impurities to form superconducting current without hindrance. However, BCS theory cannot successfully explain the phenomena of unconventional superconductors or high temperature superconductivity.
图2 上图:超导元素固体及其实验临界温度 (Tc) 周期表。下图:超导二元氢化物周期表(0~300 GPa)。蓝色表示理论预测,红色表示实验结果[66]

Fig.2 Top: Periodic table of superconducting elemental solids and their experimental critical temperature(Tc). Bottom: Periodic table of superconducting binary hydrides (0~300 GPa). Theoretical predictions indicated in blue and experimental results in red[66]. Copyright 2020, Elsevier Science Direct

In 1986, Bednorz and M Müller discovered a new superconducting material, a copper oxide ceramic material with a Tc of 35 K, which opened the era of copper-based high-temperature superconductors, and they increased the superconducting critical temperature by 50%[70]. Zhu et al. Quickly repeated and confirmed the Zurich laboratory's findings, and also found a copper oxide ceramic material with a Tc of up to 93 K[71]. In 1993, copper oxides increased the Tc to 133 K at ambient pressure[72]. In the same year, a T c of 153 K was observed HgBa2Ca2Cu3O8+δ at a high pressure of 15 GPa by applying pressure to copper oxides to increase the Tc[73]. Subsequently, the Tc of cuprates was increased to 164 K under high pressure[74]. In 2008, Hosono et al. Discovered a new class of superconductors, iron-based superconductors, whose Tc is 26 K[75]. It is worth noting that both Cu-based superconductors and Fe-based superconductors are unconventional superconductors, that is, they are not BCS superconductors, and the electron-phonon coupling cannot explain the superconducting phenomena of these two systems. This kind of superconductors has a completely new mechanism, which is beyond the scope of BCS theory. In the process of research on superconductivity, two kinds of superconductors, copper oxide ceramics and iron-based compounds, have been found one after another, but their Tc is too low and they are still very far away from room temperature. By contrast, the BCS theory of conventional superconductivity provides guidance for achieving high Tc without a theoretical upper limit — all that is required is a favorable combination of high-frequency phonons, strong electron-phonon coupling, and high-density States[76]. In addition, heavier elements can be beneficial as they contribute to the low frequency of enhanced electron-phonon coupling. Multiple hydrides require lower pressures than pure hydrogen. Importantly, this hydrogen-rich material formed by chemical pre-compression can achieve high Tc, which makes a great step forward in the study of superconductivity under high pressure and prepares for the ultimate realization of room temperature superconductivity.

3.2 Predicted high temperature superconductor under high pressure

Because it is very challenging to obtain higher Tc under experimental conditions, scientists predict the Tc of various compounds under high pressure by theoretical calculation in order to find compounds with higher Tc. These compounds with high Tc predicted by theoretical calculation were confirmed in later experiments, such as H3S[77,78].
The structural information of hydrogen-containing molecular systems at high temperature and high pressure is closely related to many problems in physics, chemistry and other related sciences[79]. Various fascinating physical phenomena of hydrogen-containing molecules have been predicted or observed due to the modification of interatomic interactions and the redistribution of electron density. H2S is an analog of H2O at the molecular level. However, the phase diagram of solid H2S up to 100 GPa is fundamentally different from that of H2O[80⇓⇓~83]. At ambient pressure, H2S crystallizes as three typical molecular solids (phases Ⅰ-Ⅲ, depending on temperature)[84,85]. As the pressure increases, the H2S transforms into three high-pressure phases (Ⅳ, Ⅴ and Ⅵ phases)[80⇓⇓~83]. H2S changes from yellowish to black at 27 GPa, a transition that is complicated by the partial dissociation of H2S and the appearance of elemental sulfur above 27 GPa at room temperature and at higher pressures at lower temperatures[86]. Metallization at pressures above 96 GPa[78,83]. It is important to note that metallation refers to the metallation of elemental sulfur rather than compounds, as H2S predicts decomposition to sulfur and hydrogen under metal pressure[87]. In addition, elemental sulfur has been found to become metallic above 95 GPa[88]. However, it is shown by first-principles structure prediction that H2S is thermodynamically stable and does not decompose at least up to 200 GPa. H2S is expected to metallize at 130 GPa with band gap closure, and the predicted metallization pressure is higher than the experimentally observed value[78]. Because the H2S metal structure remains elusive, the understanding of its metallicity and potential superconductivity is greatly hindered. An extensive structural study of solid H2S in the pressure range from 10 to 200 GPa was carried out by an unbiased structure prediction method based on particle swarm optimization algorithm. The study reveals the superconducting potential of metallic H2S, which is predicted to have a maximum transition temperature of 82 K at 160 GPa, higher than that predicted for most typical hydrogen-containing compounds, for example, SnH4(80 K),GeH4(64 K)[89][90].
It is necessary to confirm the pressure and composition of H2S metallization by high pressure experiment because the pressure of H2S metallization predicted by theoretical calculation is not consistent with that obtained by infrared spectroscopy and there are two statements about the composition of metallization: H2S and S+H2. The experiment was pressurized at a temperature of 200 K[77]. With increasing pressure, H2S begins to conduct and transforms into a semiconductor at 50 GPa, and into a metal at 90 – 100 GPa. However, no photoconductive response was observed in this state and it is an inferior metal. In a further experiment, the sample was heated above room temperature at 150 GPa, and the signature of superconductivity was observed during cooling: the resistivity dropped sharply to zero and the transition temperature decreased with magnetic field, and susceptibility measurements confirmed that Tc was 203 K. Furthermore, the apparent isotope shift of Tc in sulfur deuteride by isotope comparison indicates that the electron-phonon mechanism of superconductivity is consistent with BCS theory. Although the superconductivity of H2S at high pressure is confirmed by experiments, it is not clear that this specific chemical composition has a high Tc. Because H2S decomposes under high pressure, the Tc of elemental sulfur is low, and the existence of H2 molecules is not observed in the experiment, so H2S may be converted into hydrides with higher hydrogen content, and H3S may lead to high temperature superconductivity in this system[91⇓⇓~94]. Because of the different reaction conditions, mainly the low temperature parameters used in the calculation, the chemical composition and Tc of superconductors obtained by theoretical calculation may be slightly different from the experimental results, but the theoretical calculation is still an effective method to find compounds with high Tc under high pressure.
The Tc of H3S under high pressure is 203 K, which indicates the prospect and potential of hydrogen-rich materials to realize room temperature superconductivity under high pressure[77]. In fact, superconductivity was first discovered in polyhydrides as early as 1970, such as Th4H15 with a Tc of 8 K at ambient pressure[95]. Since the hydrogen content plays a key role in the superconductivity of compounds, the search for high temperature superconductors in polyhydrides has become an important direction[95]. First-principles structure search for stable hydrogen-rich clathrate structures in rare earth hydride at high pressure revealed that these hydride structures do not contain H2 molecules but rather unusual H cages with stoichiometries of H24, H29, and H32 (Fig. 3), in which the H atoms are weakly covalently bonded to each other and the rare earth atom occupies the center of the cage. High-temperature superconductivity is closely related to the H clathrate structure, which originates from the large H-derived electronic density of States at the Fermi level and the strong electron-phonon coupling associated with the motion of H atoms within the cage. YH10 with a stoichiometric H32 clathrate structure is predicted to be a potential room-temperature superconductor, and its Tc is expected to be as high as 303 K at 400 GPa[96]. In another study, YH10 is also predicted to be a room temperature superconductor with a high Tc of 326 K at 250 GPa[97].
图3 REH6 (a)、REH9 (b) 和 REH10 (c) 的包合物结构。小球和大球分别代表 H 和 RE 原子。图片分别描绘了 REH6、REH9 和 REH10 的以 RE 为中心的 H24、H29 和 H32 笼。每个具有 Oh 或 D4h 对称性的 H24 或 H32 笼包含六个正方形和八个六边形或六个正方形和十二个六边形。一个 H29 笼子由6个不规则正方形、6个五边形和6个六边形组成[96]

Fig.3 Clathrate structures of REH6 (a), REH9 (b), and REH10 (c). The small and large spheres represent H and RE atoms, respectively. The middle panel depicts the RE-centered H24, H29, and H32 cages of REH6, REH9, and REH10, respectively. Each H24 or H32 cage with Oh or D4h symmetry contains six squares and eight hexagons or six squares and twelve hexagons. One H29 cage consists of six irregular squares, six pentagons, and six hexagons[96]. Copyright 2017, American Physical Society

Almost all binary hydrides have been studied theoretically by structure search simulations, however, superconductivity in ternary hydrides at high pressure has been poorly investigated[66,98]. This is because crystal structure prediction simulations of ternary systems are computationally demanding and their structure search is challenging as the number of atoms and atomic species increases. But the search for room temperature or even higher temperature superconductors in ternary hydrides is still an urgent and challenging task. A strategy to engineer high-temperature superconductors has recently been proposed by introducing additional electrons into known hydrogen-rich binary systems via metal doping[99]. Since MgH16 contains H2 molecules, it is not a good superconductor. Using ternary compounds to mimic lithium or electron-doped binary hydrides MgH16, the additional electrons introduced destroy the H2 molecule, increasing the number of atomic hydrogens compared with MgH16, which is necessary to stabilize the clathrate structure or other high Tc structures. Through the study of the selected Li-Mg-H ternary system, a ternary compound Li2MgH16 with a unique clathrate structure is identified, which is predicted to have a high Tc of 473 K at 250 GPa and a metallization at 300 GPa, making it possible to find superconductors at room temperature or even higher temperatures. The superconducting phenomenon of Li2MgH16 provides a feasible strategy for tuning the superconductivity of polyhydrides by donating electrons to the hydrides via metal doping. This strategy may be an effective way to search for high temperature superconductors in various ternary hydrides.

3.3 High temperature superconductivity in lanthanide polyhydrides under high pressure

Polyhydrides have been predicted to have high Tc at high pressure by simulation. However, it is very challenging to achieve the predicted high pressure in experiments, and there is little experimental progress. With the increasing pressure that can be achieved under experimental conditions, many promising high temperature superconductors predicted before can be explored.
Since the non-hydrogen atoms inside the polyhydrides may provide a chemical pre-compression effect to reduce the pressure required for metallization, it is possible to discover new polyhydride high temperature superconductors at the lowest possible pressure[20]. In addition, the synthesis of polyhydrides at lower pressure can also be used to further study their superconductivity with various probes, and the study of the synthesis path and structure of polyhydrides can also help to deepen the understanding of polyhydride. Some people found that cerium and hydrogen directly reacted to form CeH9 ( P 6 3 / m m c) by laser heating to 2000 K under the pressure of 80 ~ 100 GPa in DAC and synchrotron X-ray diffraction[100]. Initially, cerium and hydrogen react to form CeH2 ( F m 3 - m) at 9 GPa and room temperature. Cubic CeH3 ( P m 3 - n) obtained at 36 GPa and after laser heating with increasing pressure has not been found before and is isostructural with β-UH3 ( β - P m 3 - n). The Ce-H phases formed experimentally at different pressures and temperatures are shown in Fig. 4. The pressure-composition phase diagram of the stable phase of the Ce-H system predicted based on the evolutionary structure prediction method is shown in Fig. 5. Electron-phonon coupling (EPC) calculations show that CeH9 ( P 6 3 / m m c) is a high temperature superconductor with Tc estimated to be as high as 117 K at 200 GPa. In another study, a series of cerium polyhydrides were synthesized by direct reaction of Ce and H2 only at high pressure (cold pressure treatment), and the hydrogen content of cerium polyhydrides increased with pressure and underwent four phase transitions at high pressure[101]. After decompression to ambient conditions, the hexagonal phase reverts to the initial fcc phase. It is found that the CeH9 formed at 159 GPa exhibits a three-dimensional hydrogen network composed of cage-like H29 cages. The electron localization function indicates weak electron localization between H-H atoms, and the band structure confirms its metallic character.
图4 各种 Ce-H 相合成和稳定性的压力温度路径。a 从 9 GPa 开始,铈与氢反应形成 F m 3 - m - C e H 2,在高达 33 GPa 时保持稳定。b 在 33 GPa 激光加热下,H2 介质中的 F m 3 - m - C e H 2 反应生成 β - P m 3 - n - C e H 3 。 β - P m 3 - n - C e H 3 在高达 80 GPa 时保持稳定。c β - P m 3 - n - C e H 3 在 H2 介质中 80~100 GPa 的激光加热导致 P 6 3 / m m c - C e H 9 超氢化物的出现。发现超氢化物相在我们研究中达到的最大压力(100 GPa)下是稳定的。d 完全减压后, β - P m 3 - n - C e H 3 和 I 4 1 m d - C e 2 H 5 在环境条件下被回收[100]

Fig.4 Pressure temperature path for the synthesis and stability of various Ce-H phases. a Starting at 9 GPa, cerium reacts with hydrogen to form F m 3 - m - C e H 2, which remained stable up to 33 GPa. b At 33 GPa with laser heating, F m 3 - m - C e H 2 in H2 medium reacted to form β - P m 3 - n - C e H 3 . β - P m 3 - n - C e H 3 remained stable up to 80 GPa. c Laser heating of β - P m 3 - n - C e H 3 in H2 medium at 80~100 GPa resulted in the occurrence of the P 6 3 / m m c - C e H 9 superhydride. The superhydride phase was found to be stable up to the maximum pressure reached in our studies i.e. 100 GPa. d After complete decompression, β - P m 3 - n - C e H 3 and I 4 1 m d - C e 2 H 5 were recovered at ambient conditions[100]. Copyright 2019, Nature Communications

图5 理论上预测的高压下 Ce-H 系统中稳定相的压力组成相图。红色横条表示各相的稳定性范围;该相图是在进化结构预测方法 USPEX 的基础上创建的。实验发现的 P63/mmc-CeH9 预计在 78 GPa 到至少 250 GPa 时保持稳定[100]

Fig.5 Pressure-composition phase diagram of theoretically predicted stable phases in the Ce-H system at high pressures. Red horizontal bars show the range of stability of each phase; this phase diagram was created on the basis of the evolutionary structure prediction method USPEX. The experimentally discovered P63/mmc-CeH9 is predicted to be stable from 78 GPa up to at least 250 GPa[100]. Copyright 2019, Nature Communications

The realization of Tc up to 203 K in H3S means that the study of room temperature superconductivity is a step forward. However, there is still a big gap between this temperature and room temperature. Therefore, superconductors with higher Tc are waiting for further experimental studies. Recently, the first lanthanum polyhydrides were synthesized at temperatures above 160 GPa and around 1000 K, with similar stoichiometry and LaH10 as La H 10 ± x (-1 < X < 2)[102][96,97]. Further experiments show that when the initial temperature is 300 K, the resistance decreases at about 275 K, and the resistance decreases significantly at 260 K. On heating, the resistance increases sharply at 245 K, indicating that this change is reversible[103]. However, these studies do not provide evidence of a zero-resistance state or any other evidence that would confirm superconductivity. Drozdov et al. Used La or LaH3 and hydrogen to synthesize LaH10 ( F m 3 - m) under high pressure, which reached a maximum of 252 K in Tc at a pressure of 170 GPa, followed by a sudden drop in Tc at higher pressures[104]. Upon application of an external magnetic field to it, its critical transition temperature decreases, proving the existence of LaH10 superconductivity, and the upper critical field at zero temperature is found to be about 136 T. In addition, when comparing the Tc of LaH10 and LaD10 with the same fcc crystal structure,The Tc of the LaD10 was found to be significantly reduced compared to the LaH10, which demonstrates the presence of an isotope effect. The zero resistance, the isotope effect and the decrease of the critical transition temperature under external magnetic field together prove the existence of superconductivity in LaH10. After the discovery of the high Tc of H3S, LaH10 has increased the Tc by about 50 K, which is very close to the goal of room temperature superconductivity. In another study, lanthanum polyhydride achieved Tc values up to 260 K[103]. This is an encouraging step towards the goal of room temperature superconductivity in the near future. In the future, we can believe that we will find more room temperature superconductors and higher T c in ternary compound systems under high pressure. In addition, with the continuous improvement of Tc, reducing the huge pressure required to achieve Tc may become a new problem[105].

3.4 "Second island" of superconductivity in lanthanide polyhydrides under high pressure

Rare earth polyhydrides formed under high pressure are potential high temperature superconductors, therefore, a systematic computational study of the structural and superconducting properties of rare earth polyhydrides formed under pressure for the entire lanthanide series is necessary[96]. REH4 is found to be a stable structure for all lanthanides by using first-principles DFT calculations combined with the results of structure search studies[106]. For almost all lanthanides, REH10 is stable at high pressures (with the exception of Ho, Er, Tm, and Lu), and in cubic or hexagonal structures, REH9 generally becomes stable at slightly lower pressures (with the exception of La, where LaH10 is already very stable at relatively low pressures). REH8 appears as the preferred stable clathrate structure in the hydrides of the late-stage lanthanides Nd, Ho, Er, Tm, and Lu (> 250 GPa). The stoichiometric ratio of the most hydrogen-rich rare earth hydrides (at least up to 400 GPa) is either REH8, REH9, or REH10. The most hydrogen-rich clathrate hydride, REH10, is stable in the range RE = Nd, Pm, Sm, Eu, Gd, Tb and Dy. For later lanthanides Er, Tm, Yb and Lu,ErH8, TmH8, YbH10 and LuH8 are stable at pressures above 250 GPa. REH8 ( F m 3 - m )、REH9 ( F 4 - 3 m )、REH9 ( P 6 3 / m m c) and REH10 ( F m 3 - m) are four stable hydrogen-rich clathrate structures. The analysis of the electronic and dynamic properties and electron-phonon coupling interaction of the most hydride-rich compound REHn(n=8,9,10), which is stabilized below 400 GPa, shows that the increase of Tc is associated with a high density of H-s States and a low number of RE-f States at the Fermi level[107,108]. A "second island" of superconductivity appears in Yb and Lu polyhydrides ”,YbH10 at 250 GPa Tc is 102 K(YbH9 is 80 K),LuH8 at 300 GPa Tc is 86 K. By comparing the superconducting trend of the whole lanthanide hydride series, it is found that the Tc of LaH10 is still the highest, but YbH10 and LuH8 should also be considered as potential high-temperature superconductors. Later-stage lanthanide polyhydrides may become a "second island" for lanthanide superconductivity exploration. In addition, studies of the late-stage lanthanides may provide further insight into the limits of BCS superconductivity and the ultimate goal of achieving room-temperature superconductivity.

4 Extreme high energy material

Pressure is an ideal tool to find breakthrough energetic materials because it can greatly change the properties of materials and significantly affect the research of materials science. N2 and H2 are gases at ambient conditions and can form materials with extremely high energy density after being compressed under high pressure, and singly bonded polymeric nitrogen and atomic metallic hydrogen are generally considered to be energetic materials[109]. Although the synthesis of single-bonded polymeric nitrogen and atomic metallic hydrogen usually requires high pressures of hundreds of GPa, which makes it impossible to be directly applied in practice, the study of their stability, metastability and basic characteristics is still valuable for finding extremely energetic materials through alternative synthesis routes.

4.1 Nitrogen under high pressure

Energetic materials are defined as materials that release a large amount of heat and/or a large amount of gas in a very short time by heat, collision, impact, spark, etc[110]. Energetic materials are traditionally C, H, N, O compounds with low crystal symmetry. Energetic materials have potential uses in explosives, pyrotechnics, and propellants due to their rapid energy release[111,112]. Energetic materials synthesized under high pressure for use in propellants and explosives are of research interest to scientists. Polymer solids composed of molecular units composed of low atomic number elements forming extended three-dimensional structures are potential energetic materials,For example, polynitrides (containing only nitrogen atoms) have potential applications in rocket engineering as high-energy materials, and polynitrides can release a large amount of energy in the process of transforming from a high-energy polymer phase to a thermodynamically energetically favorable molecular phase[113].
Nitrogen is unique in chemistry because the triple bond of the nitrogen molecule is one of the strongest, with a triple bond energy of 954 kJ/mol, while the single bond energy is weaker, only 160 kJ/mol[113]. Due to the large difference in the average bond energy of the single bond (160 kJ/mol), the double bond (418 kJ/mol), and the triple bond (954 kJ/mole), a large amount of energy will be released during the transition from singly bonded nitrogen to triple bonded nitrogen of diatomic molecules, accompanied by a large amount of gas production[111]. Because nitrogen is the main decomposition product, which avoids pollution to the environment, this polynitride can be considered as a green energy storage material[111,112].
At ambient conditions, nitrogen is a diatomic molecule with a nitrogen-nitrogen triple bond. Nitrogen solidifies at low temperatures or moderate pressures with weak van der Waals interactions between molecules. Since the phase boundary between the low-pressure nitrogen molecular phase and the high-pressure polymer phase has attracted the interest of scientists, the search for polymeric nitrides began as early as the 1980s. Cubically deflected polymeric nitrogen (cg-N) was the first nitrogen polymer phase predicted[114]. Eremets et al. Studied N2 at 110 GPa and 2000 K using a laser-heated DAC and found the theoretically predicted cg-N[115,116]. The existence of cg-N has also been observed in other high temperature and high pressure experiments[117⇓~119]. After cg-N, the theory predicts many other polymer phases, such as nitrogen fullerene N 60[120]. The theoretical prediction shows that at high enough pressure, high pressure can effectively break the nitrogen-nitrogen triple bond in the nitrogen molecule and synthesize polymeric nitrogen, and the nitrogen molecule will be transformed into a three-dimensional network structure of polymeric nitrogen, in which each nitrogen atom forms a single bond with three other nitrogen atoms[114]. It is worth noting that the cleavage of the triple bond of the nitrogen molecule in this transformation will store a large amount of energy, which has great potential in the manufacture of explosives. Therefore, the study of high nitrogen compounds has promoted the development of modern explosive materials[112]. At 300 K and pressures above 150 GPa, nitrogen transforms into an amorphous non-molecular state (η phase) and becomes opaque and conducting[121]. This pressure-induced amorphous η phase is metastable, with the nitrogen-nitrogen triple bond broken and replaced by a polymeric single bond in the η phase. It is worth mentioning that the nitrogen transition to the η phase exhibits a clear hysteresis. When the polymer phase is formed, the phase remains metastable even after decompression to 50 GPa. Since the reaction kinetics decrease exponentially with temperature, the hysteresis increases at lower temperatures, and polymeric nitrogen is recoverable at zero pressure at 100 K[122]. Therefore, the use of high temperature to accelerate the transformation to the desired phase and low temperature to suppress the reverse transformation and maintain the metastable phase is a reasonable strategy for the synthesis of novel materials[123]. In another experimental study, single-bonded crystalline nitrogen with a layered polymer structure was synthesized at 120 ∼ 180 GPa and at pressures above 240 GPa with a hexagonal layered polymer structure[119].

4.2 Hydrogen at high pressure

As the first element in the periodic table, as well as the simplest and most abundant element, the study of hydrogen has been promoting the development of energy materials and even new energy applications. Low-density gaseous and liquid hydrogen has long been used in large quantities as an important energy material, with a wide range of applications involving rocket propellants and automotive fuel cells[124,125]. Hydrogen is a gas at room temperature, but solid hydrogen can be formed at temperatures below its melting point of 14.01 K. The density of solid hydrogen is 0.086 g/cm3, which is one of the least dense solids. Solid hydrogen is a molecular crystal at low pressure and is an insulator. Solid hydrogen can become metallic hydrogen under a high pressure of several million atmospheres, and metallic hydrogen is a state in which hydrogen is compressed and then transformed into metallic properties by phase transfer. When hydrogen is in the metallic state, the hydrogen molecule will split into single hydrogen atoms, the hydrogen chemical bond will be broken, and the bound electrons in the molecule will be squeezed into common electrons and can move freely. This free movement of electrons makes metallic hydrogen conductive. Metallic hydrogen is an extremely dense substance and can even be considered the most efficient nuclear fusion fuel. Metallic hydrogen is a high-energy material second only to nuclear energy materials, which may have an important impact on energy and rockets[39]. At present, the study of hydrogen under high pressure is still in its infancy in practical material applications, but the study of metallic hydrogen has shown great potential and promising prospects in many aspects such as high-energy materials.
In 1935, Wigner and Huntington predicted that molecular hydrogen would decompose into atomic hydrogen metal at a pressure of 25 GPa, but because they incorrectly used zero pressure compressibility for all pressures, there was a large gap between the predicted 25 GPa and the pressure required for actual hydrogen metallization[41]. In 1968, Ashcroft further predicted that metallic hydrogen might be a high-temperature superconductor[107]. There are two routes to the conversion to metallic hydrogen: increasing pressure at low temperature or increasing temperature to cross the plasma phase transition[39]. At present, Dias et al. Discovered metallic hydrogen at low temperature and 495 GPa by increasing pressure at low temperature[39]. However, some people are skeptical about this. On the one hand, Loubeyre and others believe that the pressure of the experiment may be overestimated, and the experiment may not reach such a high pressure; On the other hand, it is not enough to determine metallic hydrogen by reflectivity, and Alexander Goncharov believes that the reflective material may be alumina[126]. It is worth mentioning that the route to increasing pressure at low temperatures found multiple phases that had not been envisioned in the simple phase diagram predicted by Wigner and Huntington. In the I phase at low temperature and low pressure, the molecule is in a spherically symmetric quantum state, forming a hcp structure. XRD measurements show that phase I remains at least up to 119 GPa at 300 K. Phases II, III and IV are phases with structural changes and molecular orientation order[127⇓⇓⇓~131]. At 110 GPa and below 150 K, phase I transforms into the symmetry-broken phase II. After that, phase Ⅱ transforms into phase Ⅲ at 150 GPa, and phase Ⅲ can be maintained at least up to 300 GPa. The dense hydrogen Ⅳ phase is found at 300 K and above 230 GPa. A slightly different phase V was found in Raman spectroscopy studies at two different pressures up to 360 GPa and 385 GPa[132]. The new phase in hydrogen observed at liquid helium temperatures is believed to precede the metallic phase and is referred to as the H2-PRE phase (and at higher temperatures as the Ⅵ phase)[39].

5 Applications in Planetary Science

High pressure is closely related to the study of planetary science, because most of the matter in the universe is in a state of high pressure. For example, more than 95% of the material in the Earth's interior is at a high pressure of at least 10 GPa, and the pressure in the Earth's core ranges from 330 to 360 GPa, while the pressure in Jupiter's core is estimated to be above 1 TPa[133][134]. High-pressure studies of some iconic materials on Earth and other planets can help understand their internal structure. New phases of compounds and new compounds have potential applications as key materials for Earth and planetary science research.

5.1 Application of Helium in Planetary Science

It is well known that helium is an electrical insulator at room temperature and pressure and has the largest known band gap. At the same time, helium is the second most abundant element in the universe, and it makes up a large fraction of the giant gaseous planets, such as Saturn and Jupiter. In the past, helium was thought to metallize at a pressure of about 10 TPa, which is far greater than the pressure of Jupiter's core (which is estimated to be above 1 TPa), so helium may still be an insulator in giant planets[135][134]. However, the results of high pressure studies show that temperature plays a vital role in the metallization of helium, especially in the fluid state[136]. First-principles molecular dynamics simulations indicate that the energy gap of liquid helium depends strongly on temperature (Fig. 6)[137]. As shown in Fig. 6, the band gap of helium closes at 20 000 K and 6.6 g·cm-3, and the pressure at this time is 3 TPa, which can be achieved in Jupiter[134]. As the density increases, the fluid becomes increasingly metallized, which may occur at slightly higher densities and temperatures than band gap closing due to the localization of electrons at the band edges (mobility edges). Further insight into the liquid structure can be gained by comparison with a one-component plasma model[138]. Because the structure of the simulated liquid is different from that of the one-component plasma, the effective ion-ion interaction in the simulated liquid is weakened, which is caused by electron screening.
图6 在 0 K(静态条件)(黑色)、10 000 K(蓝色)、20 000 K(绿色)和 50 000 K(红色)以及沿着 ρ1/ρ0 = 4 的预压缩 Hugoniot 计算的电子能隙,其中 ρ1 是预压缩的 密度和 ρ0 = 0.1233 g·cm-3(灰色)[137]

Fig.6 Calculated electronic energy gap at 0 K (static conditions) (black), 10 000 K (blue), 20 000 K (green), and 50 000 K (red) and along a precompressed Hugoniot with ρ1/ρ0 = 4, where ρ1 is the precompressed density and ρ0 = 0.1233 g·cm-3 (gray)[137]. Copyright 2008 National Academy of Sciences, U.S.A

The large effect of temperature on the electronic structure of helium suggests that helium rain is unlikely to occur in Jupiter or Saturn today. For the Saturn evolution model, if helium rain occurs, it will occur at pressures of 100 to 1000 GPa and temperatures of 5000 to 10 000 K. Jupiter, as well as exoplanets larger and older than Jupiter, are calculated to have higher pressures and temperatures, in which the band gap may close completely. It has been suggested that the exosolution and gravitational separation of helium and hydrogen after planetary cooling may be the cause of Saturn's overbrightness[139,140]. So far, this argument comes from calculations on the hydrogen-helium miscibility band gap performed at low temperatures. The miscibility of helium in hydrogen may be enhanced compared to the predictions of low temperature calculations, as temperature transforms dense fluid helium from an insulator to a semiconductor and eventually to a metal. An increase in solubility would lower the critical temperature of miscibility below which hydrogen and helium are immiscible, but this temperature is lower than that of Saturn's thermal evolution model. Therefore, other more plausible mechanisms are needed to explain Saturn's over-brightness and the helium deficiency of Jupiter's and Saturn's atmospheres[141]. The electronic structure of helium may also have an important influence on the generation of magnetic fields[142]. The temperature-induced band gap closure tends to enhance the conductivity, thereby reducing the magnetic diffusivity and increasing the magnetic Reynolds number.

5.2 The "lack of xenon paradox"

The amount of xenon in the atmosphere of the sun, the earth, asteroids and comets is very low, and the earth's atmosphere contains only trace amounts of xenon. In 1902, the amount of xenon in the Earth's atmosphere was estimated to be one part in 20 million[143]. It has been speculated that the low xenon content on Earth may be due to the covalent bonding of xenon and oxygen in quartz, which reduces the amount of xenon released into the atmosphere[144]. However, Jupiter's atmosphere contains an unusually high amount of xenon, about 2.6 times that of the sun[145]. The reason for this phenomenon is unknown, but it may be caused by the rapid accumulation of planetesimals before the temperature of the solar nebula rose in the early formation of the solar system[146]. In response to the difference in xenon content in the planet, some people have proposed that xenon is stored in the depths of the earth to deal with this "xenon shortage paradox". The most abundant oxygen element in the earth's mantle can react with xenon under the huge geological pressure to fix xenon in the interior of the earth[147]. Therefore, the possibility of studying the reaction of xenon and oxygen at high pressure helps to confirm this view and to understand xenon chemistry at geological pressure. Experimental studies have shown that the reaction produces two oxides at pressures below 100 GPa, namely, Xe2O5 under oxygen-rich conditions and Xe3O2 under oxygen-poor conditions[147]. The synthesis of xenon oxide is a strong evidence against the "xenon shortage paradox". In another theoretical study, the Xe-Ni and Xe-Fe structures are stable under Earth's core conditions, suggesting that xenon may also react with other elements in the Earth's interior and be preserved in the Earth's interior[148]. In any case, these conclusions suggest that xenon may be abundant in the Earth's interior.

6 Conclusion and prospect

At present, the study of gases and their chemical reactions under high pressure has achieved fruitful results. The superconducting critical transition temperature of the gas has been raised to 260 K, close to the room temperature level, which not only means a big step towards the realization of room temperature superconductivity, but also will further promote the development of superconductivity theory. In addition, there have been many studies on binary hydrides, but only a few literatures have explored selected phases and compositions for ternary, quaternary and complex hydrides, and there is still a huge space for room temperature superconductors to be explored. In the future, with the emergence of new research methods and theories, more sufficient high-pressure research can be carried out. The synthesis of polynitrides and the discovery of metallic hydrogen show that the research on energy storage materials under high pressure has achieved some results. The development of planetary science has been further promoted by the study of noble gases at high pressure.

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