TY - JOUR
AU - Cui, Tian
AB - Abstract Hydrogen atoms can provide high phonon frequencies and strong electron–phonon coupling in hydrogen-rich materials, which are believed to be potential high-temperature superconductors at lower pressure than metallic hydrogen. Especially, recently both of theoretical and experimental reports on sulfur hydrides under pressure exhibiting superconductivity at temperatures as high as 200 K have further stimulated an intense search for room-temperature superconductors in hydrides. This review focuses on crystal structures, stabilities, pressure-induced transformations, metallization, and superconductivity of hydrogen-rich materials at high pressures. superconductivity, metallization, crystal structure, high pressure, hydride INTRODUCTION Since superconductivity of 4.2 K in mercury was first discovered by Onnes in 1911 [1], searching for high critical temperature superconductors has been one of the major activities in condensed matter physics. There is a great progress in the study of unconventional superconductors such as copper oxides and iron-based compounds which often show high superconducting transition temperature (Tc). Although their superconducting mechanisms are still a controversy, the highest Tc of copper oxides and iron-based superconductors reach 133 K [2] and 56 K [3] at ambient pressure, respectively. But the situation is not optimistic for the conventional superconductors. Magnesium diboride—the best known conventional superconductor with Tc of 39 K [4] observed in 2001 is far lower than that of copper oxide superconductor. Pressure is one of the thermodynamic parameters that controls the structures and properties of condensed matter. High pressure can effectively reduce the distance between atoms, shorten the bonds, and change electronic structure, leading to new phases with unusual structures and properties that hardly occur at atmospheric pressure. For example, high pressure can make the insulator transform to a metal state [5], increase Tc of superconductors (Tc of copper oxides is raised to 164 K at 31 GPa) [6], etc. To note, superconductivity under pressure has been extended to most elements of periodic table. Back in the year 1935, Wigner and Huntington theoretically predicted that solid hydrogen would be metallized at high pressure, that is metallic hydrogen [7], which is believed to be a room-temperature superconductor [8]. Therefore, searching for metallic phase of solid hydrogen becomes a very important topic in physics. There are so many experimental research works on the hydrogen under high pressures. But these works reveal that the metallization of hydrogen is very difficult. Up to date, there is no experimental evidence for the predicted metallic state in the pressure range up to 388 GPa [9]. One of the important topics is how to reduce the metallic pressure of hydrogen system. In 2004, Aschroft [10] proposed a great idea that hydrogen-rich materials can be metallized at much lower pressures due to ‘chemical pre-compression’. Because these materials are dominated by hydrogen elements, which can provide high phonon frequencies and strong electron–phonon coupling (EPC), high temperature superconductivity can be found after metallization. Therefore, they are considered as good candidates to search for high Tc superconductors within the reach of the experimental diamond anvil cell (DAC). Based on this idea, scientists began to search for likely the high-temperature superconductors in hydrogen-rich materials. Because there are some difficulties in the experimental studies of the metallization and superconductivity of hydrogen-rich materials under high pressure, theoretical research works have been at the forefront of this field and made outstanding contributions. The carbon group hydrides with highest content of hydrogen in naturally existed hydrides were first widely studied. Theoretical investigations revealed that silane (SiH4), germane (GeH4), stannane (SnH4), and disilane (Si2H6) are metallized at much lower pressure than pure hydrogen with high Tc. For example, SiH4 is predicted to be 20–75 K [11]. These theoretical predictions really excite the research works on the hydrogen-rich systems under high pressures. Then, can the other hydrogen-rich compounds be expected to be potential candidates for high-Tc superconductivity? Mixed closed-shell systems and H2 at high pressure can form new H2-containing van der Waals compounds, such as SiH4(H2)2 and GeH4(H2)2, which were predicted to be superconductors with high Tc values of 107 K [12] and 90 K [13] at 250 GPa, respectively. Superconductivity was also reported in alkali metal and alkaline-earth metal hydrides KH6 [14] and CaH6 [15], where Tc is 82 K at 300 GPa and 235 K at 150 GPa, respectively. However, only a low Tc ∼ 17 K of silane at 90 and 120 GPa in these hydrides has been observed through experiment [16], though debates remained. Little experimental research has made the studies of hydrogen-rich materials in depression, but the step has never stopped. Recently, the novel sulfur hydrides H3S [17,18] with Im-3m symmetry was predicted theoretically by our group to be a high-temperature superconductor with Tc reaching as high as 191–204 K at 200 GPa. Subsequently, Drozdov et al. observed a high Tc = 203 K in H2S sample at 155 GPa based on the resistant transition, isotope effect, and Meissner effect [19]. The observed Tc values and its pressure dependences are close to our theoretical predicted, suggesting that high Tc in H2S sample mainly comes from H3S. Moreover, the Im-3m structure was confirmed experimentally by synchrotron X-ray diffraction (XRD) more recently [20]. In addition, Mazin specially emphasizes [21] ‘this is the first time that a previously unknown material predicted to be a high-temperature superconductor has been experimentally confirmed to be one.’ The discovery of Tc ∼ 200 K at high pressure in sulfur hydrides by theoretical prediction and experimental measurement reveals that high-Tc in hydrogen-rich materials indeed can be achieved. Therefore, many theoretical predicted stable hydrogen-rich materials are potential candidates for high-temperature superconductors. This review is organized as a systematic summary of the superconducting behavior of the stable hydrides. Beginning from alkali metal hydrides, and ending with halogen hydrides, we discuss structures, metallization, and superconductivity of hydrogen-rich materials at high pressures. ALKALI METAL HYDRIDES Up to now, among the alkali metals (Li, Na, K, Rb, Cs) hydrides, only LiHn and KHn (n > 1) systems were predicted to be superconductors under pressure [14,22]. Except for LiHn, linear H3− units emerge in alkali metals (Na, K, Rb, Cs) hydrides. Moreover, LiH2, LiH6, NaH3, and NaH7 have been synthesized experimentally [23,24]. Lithium hydrides LiHn (n = 2–8) was systematically investigated under high pressure in theory [25]. LiH2 with P4/mbm symmetry is energetically stable relative to LiH and H2 at ∼120 GPa, which contains two types of hydrogen: H− and H2 units. LiH6 (space group R-3m) has the lowest enthalpy of formation above 150 GPa, and it possesses Li and H2 units. By analyzing the density of states (DOS) for LiH2 and LiH6, both of them are insulators at 0 GPa and metalize at 100 GPa. However, the metallization nature of LiH2 and LiH6 is different. For the former, pressure-induced band-gap closed between the H− (‘impurity donor band’) and H2 σu* bands, while for the latter, electron transfers from Li to H2 σu* levels. For LiH8 with I422 symmetry, its stable pressure range is 100–200 GPa. The superconductivity of three stoichiometries (LiH2, LiH6, and LiH8) was extensively studied in another theoretical work [22]. LiH2 is not a superconductor, while LiH6 and LiH8 are superconductors. The critical temperatures (Tc) for LiH6 and LiH8 are 38 K (150 GPa) and 31 K (100 GPa), respectively, which are mostly due to the intermolecular vibrations of H2 units. And the Tc of LiH6 increases with increasing pressure, while the value of Tc for LiH8 keeps almost no change. The theoretical prediction promotes people to synthesize the lithium hydrides in experiment. In 2012, Howie et al. synthesized LiH at 50 MPa, but LiHx (x > 1) was not found [26]. Recently, LiH2 and LiH6 were observed above 130 GPa at 300 K. However, when pressure is up to 215 GPa, they both kept insulating [23]. There are some differences in metallization between experiment and theory. Therefore, more research works are needed to study the Li-H system at high pressure. Sodium hydrides It was theoretically reported that NaHn (n > 1) can be stable under pressure [27]. With respect to Li, the ionization potentials of Na is lower, results in that NaHn can be synthesized at lower pressure than LiHn. At ∼25 GPa, the NaH9 becomes thermodynamically stable. Moreover, the phase Cmc21-NaH9 adopts H− and H2 units and its metallization occurs at 250 GPa. For NaH7 and NaH11, their stable pressure ranges are 25–100 and 25–150 GPa, respectively. The metallization mechanism and structures between odd and even NaHn are different. By comparing, the odd NaHn possesses H− and H2 units, and the Na-p deters their metallization, although some can metalize. But the even NaHn only contains H2 units, and the H2 σu* bands are partially filled by Na-s elections which induce metallic behavior. After a few years, it is reported that NaH3 and NaH7 can be synthesized in a laser-heated DAC above 40 GPa and 2000 K [24]. Furthermore, H3− units were found in NaH7. Potassium hydrides KH6 was predicted energetically stable above 70 GPa by our group, which possesses two-layered structures C2/m and C2/c [14], as shown in Fig. 1. The former owns H2 and H3 units, forming a 1D network. The latter only contains H2 units, and its arrangements of H atoms are similar to that of solid hydrogen (Cmca phase). For C2/m, it metalizes at 145 GPa, while C2/c keeps metallic all the time that is a 1D conductor attributed to hydrogen-bonded network. Based on the BCS theory, the calculation showed that the EPC parameter λ and Tc for C2/c reach 0.91 and 58.66–69.84 K at 166 GPa. By further analysis, the potassium atom vibration makes mainly contribution to λ. In addition, C2/c has a negative pressure dependence of Tc in the pressure range from 166 to 300 GPa. In the same year, another group also studied the KHn (n > 1) under pressure by first-principles calculation [28]. The KH5 consists of H3− molecules which become stable at 3 GPa, and it keeps the lowest enthalpy and remains insulator in the studied pressure range. But some metastable phases exhibit metallic properties. Figure 1. Open in new tabDownload slide The structures of KH6 with space group (a) C2/m at 100 GPa and (b) C2/c at 166 GPa [14]. Copyright 2012 American Physical Society. Figure 1. Open in new tabDownload slide The structures of KH6 with space group (a) C2/m at 100 GPa and (b) C2/c at 166 GPa [14]. Copyright 2012 American Physical Society. Rubidium hydrides and cesium hydrides Rubidium hydrides and cesium hydrides were also studied under pressure [29,30]. For RbHn (n > 1), some species begin to stabilize at 2 GPa, which contain Rb+, H2 and linear H3− units [29]. Below 100 GPa, two compounds RbH9 (Pm) and RbH5 (Cmcm) are most preferred. RbH3 is stable above 100 GPa consisting of Rb+ and H3−. While for RbH6, the polymeric chains (H3−)∞ emerge. Moreover, RbH3 and RbH6 are good metals. CsHn (n > 1) are similar to RbHn, which become stable at 2 GPa and also possess H3− units [30]. The compound CsH3 is preferred in the pressure range of 30 to 200 GPa, while CsH7 is stable up to 150 GPa. Meanwhile, the author indicated that the metallic phases Cmmm and Cmma of CsH3 are attributed to the hydrogen formation. ALKALINE-EARTH METAL HYDRIDES For the alkaline-earth metal (Be, Mg, Ca, Ba, and Sr) hydrides, except for SrHn (n > 2), they are predicted to have high Tc. Especially, the Tc of CaH6 reaches 220–235 K (150 GPa), which is mostly due to the ‘H4’ units [15]. Unfortunately, there is no experimental study up to now. Beryllium hydrides Beryllium hydrides have been extensively explored by first-principles calculation. Except the well-known BeH2, no stoichiometry was found to be thermodynamically stable up to 400 GPa [31,32]. Then, the structures and properties of BeH2 at high pressure were studied [32,33]. The phase transition sequence at high pressure is Ibam → P-3m1 → R-3m → Cmcm → P4/nmm. The first three phases are insulator and the last two phases are metallic. Moreover, the Tc of metallic Cmcm and P4/nmm phases were calculated with 32–44 and 46–62 K at 250 and 400 GPa, respectively. For both phases, the tendency of Tc with pressure is first increased and then decreased. Magnesium hydrides Magnesium hydrides under pressure were theoretically searched [34]. MgH4, MgH12, and MgH16 were found to have lower enthalpy than MgH2 and H2 at 92, 122, and 117 GPa, respectively. MgH4 possesses H−, H2, and Mg2+, while MgH12,16 adopts H2δ−. The metallic phases Cmcm-MgH4 and R3-MgH12 show superconductivity with Tc of 29–37 and 47–60 K at 100 and 140 GPa, respectively. Although they both have comparative EPC λ, the average logarithmic frequency of MgH12 is larger than that of MgH4. Therefore, the Tc value of MgH12 is higher than that of MgH4. Calcium hydrides Recent theoretical work indicated that by compression, three stoichiometries CaH4, CaH6, and CaH12 become energetically stable [15]. In addition, the formation of hydrogen in CaH4, CaH6, and CaH12 is different, which is present for H atoms and H2 units, H4 units, and only H2 units, respectively. Owing to the exotic ‘H4’ units which are susceptible to a Jahn-Teller distortion, the electronic property, the bonding feature, and superconductivity of CaH6 were investigated. And the covalent bond in H4 units is obvious by calculated electron localization function (ELF). The ‘H4’ contributes more to EPC. Further calculation shows that the λ and Tc are 2.69 and 220–235 K at 150 GPa, respectively. And the Tc shows a decreasing trend with pressure. Although CaH6 has been predicted as a good superconductor, it has not been observed in experiment. Strontium hydrides and barium hydrides Sr-H and Ba-H systems at high pressure were investigated theoretically in detail. For Sr-H system, SrH4, SrH6, SrH10, and SrH12 become thermodynamically stable under compression [35,36]. While for Ba-H system, pressure induced BaH6, BaH8, and BaH12 are stable, in which the hydrogen sublattices are H−, H3,− and H2 units [31]. In addition, P4/mmm-BaH6 possesses lower enthalpy and the calculated Tc is 30–38 K at 100 GPa. BORON GROUP HYDRIDES Except for thallium element, the other boron group hydrides have been intensively studied. For boron hydrides, the estimated Tc of B2H6 and BH are 125 and 14.1–21.4 K at high pressure [37,38]. There are an amount of research works on AlH3, however, the argument about its superconductivity is existent. GaH3 is stable above 160 GPa, and its calculated Tc reaches 86 K [39]. For indium hydrides, the Tc of R-3-InH3 and P21/m-InH5 at 200 and 150 GPa are 34.1–40.5 and 22.4–27.1 K, respectively [40]. Boron hydrides For boron hydrides, the well-known stoichiometry is diborane B2H6. But theoretical prediction revealed that B2H6 (P21/c) became unstable and decomposed into BH (Ibam) and H2 at 153 GPa [37]. Moreover, BH can coexist with B2H6 between 50 and 153 GPa. It is interesting that B2H6 re-stabilizes again with Pbcn symmetry beyond 350 GPa, and the calculated Tc is 125 K at 360 GPa [38]. For BH, it has two structures, a semimetallic Ibam and a metallic P6/mmm. The calculated Tc of P6/mmm reaches 14.1–21.4 K at 175 GPa and decreases with increasing pressure. Aluminium hydrides Aluminum hydride (AlH3) at high pressure gains people's attention and has been widely explored both in theory and experiment. In 2007, the phase transition at high pressure was given as R-3c (phase I) → Pnma (phase II) → Pm-3n (phase III) at 24 and 73 GPa, respectively. Moreover, both R-3c and Pnma are insulators, whereas Pm-3n becomes metallic [41]. The calculated Tc of Pm-3n was 24 K at 110 GPa, but its superconductivity was not observed in experiment [42]. Later, a theoretical work provided an explanation about why the experimental and theoretical data on Tc of AlH3 is distinct [43]. Considering anharmonic self-energy, the calculated value of Tc reduces to as low as 2 K. Therefore, the distinction between experimental and theoretical data is attributed to the anharmonicity that suppresses the EPC parameter. In addition, the metallic phase Pm-3n can be stabilized at ambient condition once a finite electronic temperature is considered [44]. AlH3 (H2) was also investigated from 25 to 300 GPa in theory by our group [45]. It undergoes two phase transitions, from insulating P1 to semiconductor P-1, then to metallic P21/m at 75 and 250 GPa, respectively. In addition, H2 units were found in P21/m phase. The calculated Tc of metallic P21/m is 132–146 K at 250 GPa, having a large λ of 1.625. It is noted that the total λ is mostly from mediate modes, while contribution of H2 is little. Gallium hydrides and indium hydrides As we know, unlikely BH3 and AlH3, GaH3 and InH3 are thermodynamically unstable at ambient conditions. Up to now, it is theoretically reported that GaH3 with Pm-3n symmetry is thermodynamically stable relative to Ga and H2 above 160 GPa [39]. The calculated Tc reaches 86 K at 160 GPa, and H atoms play an important role in superconductivity. The theoretical study of indium hydrides in a wide pressure range of 0–300 GPa was performed by our group [40]. Two stoichiometries InH3 and InH5 were found to be stable by compressing. The InH5 can be synthesized at 120 GPa by 2In + 5H2 → 2InH5. And then the decomposition happened by InH5 → InH3 + H2, which demonstrated that InH3 becomes favorite stoichiometry till 300 GPa. The lowest-enthalpy stable phases of InH3 and InH5 are R-3 and P21/m symmetry, respectively. And they both possess H2 or H3 units. In addition, electrons move from In atoms to H atoms by Bader analysis. The EPC calculations show that the metallic R-3-InH3 and P21/m-InH5 have a large λ of 0.92 and 0.85, resulting in the estimated Tc of 34.1–40.5 K at 200 GPa and 22.4–27.1 K at 150 GPa, respectively. CARBON GROUP HYDRIDES The concept of ‘chemical pre-compression’ first appeared by doping the IVa group elements to hydrogen [10] and the hydrides with carbon group elements were widely studied even now. For the first element with the lowest mass, CH4, the results were not very satisfactory [46]. The metallization of CH4 did not occur up to 520 GPa by first-principle calculation [47]. But many works suggested that the other carbon group (Si, Ge, Sn, and Pd) hydrides may realize the metallization and become superconductors under pressure. Silicon hydrides For silane (SiH4), a great deal of theoretical and experimental works [11,16,48–55] were carried out to explore the crystal structures, metallization, and superconductivity under high pressure. It is found that there are at least 10 high-pressure phases (Pman, C2/c, P63, P-1, Cmca, P4/nbm, Pbcn, P-3, P21/c, and C2/m) of SiH4 in the pressure range of 50 and 606 GPa, as shown in Fig. 2. Among them, one phase (P63) was found in the experiment and others were from the theoretical prediction [16]. All of these structures could achieve metallization and exhibit superconductivity with Tc of 16–166 K at lower pressure than pure hydrogen. Except for extreme pressures (383–606 GPa) [55], there still exist controversies on the most stable structures of SiH4 at the corresponding pressures. Specifically, Degtyareva et al. explained that the metallization of P63-SiH4 might be due to the decomposition of SiH4 and the reaction between H and the environment materials like Pt or Re [56]. Figure 2. Open in new tabDownload slide The selected high-pressure crystal structures of silane. Adapted from [11,53,55]. Copyright 2008 and 2009 American Physical Society. Figure 2. Open in new tabDownload slide The selected high-pressure crystal structures of silane. Adapted from [11,53,55]. Copyright 2008 and 2009 American Physical Society. Based on the above explorations, our group performed more extensive study on disilane (Si2H6) [57], and found three crystalline structures P-1 (135–275 GPa), Pm-3m (275–300 GPa), and C2/c (300–400 GPa). Furthermore, the estimated Tc of P-1 at 200 GPa, Pm-3m at 275 GPa, C2/c at 300 GPa is 92, 153, 42 K, respectively. Afterwards, Flores-Livas et al. proposed a new metallic phase of disilane with Cmcm symmetry [58] at 190–280 GPa with Tc ∼ 13 K at 220 GPa. Besides the silicon hydrides naturally existed, a new molecular compound SiH4(H2)2 was observed from silane-hydrogen mixtures at ∼6.8 GPa by XRD and Raman experiments [59,60]. And a possible group space F-43m [59] of SiH4(H2)2 was provided. Later, theoretical works [61–64] were attempted to study structures and superconductivity at high pressure. They proposed three phases F-43m [61], I-4m2 [62–64], Pmn21 [63] at ∼6.8 GPa, and two new metallic phases with P1 (125 GPa) [64] and Ccca (248 GPa) [12] at higher pressures. Furthermore, calculation showed that Ccca phase possesses a Tc of 98–107 K at 250 GPa [12]. Germanium hydrides The research works on GeH4 were mainly concentrated on theoretical predictions [65–67]. It is found that there are at least seven phases (I4/mmm, P-43m, I-42m, Pman, C2/m, Ama2, and C2/c) in the pressure range of 0 and 500 GPa and all phase can become metal with the increasing pressure. Moreover, three metallic phases (C2/m, Ama2, and C2/c) exhibited superconductivity with Tc of 40–84 K at high pressures. In addition, our group study revealed that GeH3 with Cccm symmetry is stable above 280 GPa, and has a Tc of 80 K at 300 GPa [68]. A new molecular compound GeH4(H2)2 can be synthetized at 7.5 GPa by experiment [69]. Furthermore, the metallic phase P21/c of GeH4(H2)2 was proposed to have the Tc of 76–90 K at 250 GPa [13]. Stannum hydrides For SnH4, Tse, Yao and Tanaka proposed a P6/mmm structure with Tc ∼ 80 K at 120 GPa [70]. They claimed that soft phonons and Kohn anomalies play an important role in improving the electron–phonon interaction. Later, Gao et al. [71] predicted two metallic phase of SnH4 with Ama2 symmetry at 96–180 GPa and P63/mmc symmetry above 180 GPa. Ama2 phase has a Tc of 15–22 K at 120 GPa and the Tc in P63/mmc is 52–62 K at 200 GPa. Plumbum hydrides For PbH4, two-layered metallic phases Imma (132–296 GPa) and Ibam (above 296 GPa) were predicted [72]. In both phase, electrons near Fermi level are analogous with free electrons which exhibit not only metallic but also diffusive or liquid-like properties. Another theoretical work explored the high-pressure crystal structure, metallic and superconductivity of PbH4(H2)2 [73]. Their calculations revealed that the PbH4(H2)2 with C2/m symmetry could become metal above 133 GPa and exhibit Tc of 107 K at 230 GPa. For carbon group hydrides, some potential regular patterns can be summarized. First, H2 units gradually prefer to appear in MH4 (Ge, Sn and Pb) with the increasing radius of impurity elements (Si, Ge, Sn and Pb). Second, the EPC parameter in SiH4 is mainly contributed by H-Si-H vibration. While in GeH4, SnH4, GeH4(H2)2, and SnH4(H2)2, the EPC parameter are mainly derived from the intermediate frequency region, mostly corresponding to the vibration of H-M-H or the interaction of M and H2 units. Third, for MH4(H2)2 (Ge, Sn and Pb) systems, with the increasing pressure, charges transform from intra-molecular of MH4 units to two sites: one is H2 units, and the other is the interstitial between MH4 and H2 units. Besides, their superconducting transition temperatures are always higher than the corresponding MH4, which means the additive hydrogen plays an important role in improving the superconductivity of hydrides. PNICTOGEN HYDRIDES As the pnictogen element, NH3 have been studied widely, but they have difficulty in achieving metallization under high pressure. Unluckily, the hydrides of phosphorus, arsenic, and antimony are seldom to be studied under high pressure. The H3S-Im-3m with a high superconducting transition temperature values of 191–204 K at 200 GPa predicted by our group [17] and proved by Drozdov et al. [19] has been broadly accepted. Later, our group executed a series of systemic research works to explore the high pressure crystal structures and superconductivity in antimony and bismuth hydrides [74,75]. We found that Tc of SbH4 at 150 GPa reaches 118 K and Tc of BiH5 at 300 GPa reaches 119 K. Phosphorus hydrides More recently, Drozdov, Eremets and Troyan performed resistivity measurements on covalent phosphine (PH3) samples which metallize at 40 GPa, show superconductivity at 83 GPa with Tc ∼ 30 K and the maximum Tc reaches 103 K at ∼207 GPa [76]. But the exact composition and crystal structure of the superconducting phase are not determined. Subsequently, density functional theory calculations show that PH2 [77] with C2/m symmetry consisting of two formula units in the primitive cell could be a candidate hydride with Tc ∼ 82 K at 200 GPa. But other theoretical calculations show that all the phosphorus hydrides are thermodynamically unstable with respect to P and H2 in the pressure range 100–300 GPa [78,79]. The calculated Tc of PH2 with I4/mmm symmetry is 40 K at 100 GPa, and reaches a maximum value of 78 K at 220 GPa. In addition, the Tc dependence on pressure of PH is in agreement with experiment. Although the calculated Tc of PH, PH2, and PH3 with respect to pressure are comparable to experiments, the superconducting phase remains unclear. Arsenic hydrides and antimony hydrides For arsenic hydrides, there is only AsH and AsH8 stable above 300 GPa and AsH8 is predicted to exhibit Tc values of ∼150 K above 350 GPa [80]. The metallic SbH4-P63/mmc phase with nontrivial binding stable above 127 GPa was uncovered to have the most negative enthalpy among all of the researched antimony hydrides [74]. In P63/mmc-SbH4, there exists unequivalent hydrogen marked H1 and H2 which occupy on 4e and 4f sites. One kind of hydrogen (H1) is bound with strong covalent interaction to form H2 units and the other kind of hydrogen (H2) binds with the nearest Sb atom to form weak covalent bonds. Startlingly, the apparent charge localizes between Sb and H2 atoms when the isosurface value of 0.7 is selected but the value of ELF fades in 0.75, implying the nearest Sb and H2 atoms form startling weak polar covalent bonds. Besides, the Tc of 118 K was obtained in P63/mmc-SbH4 at 150 GPa. With the increasing pressure, Tc decreases (118 K at 150 GPa and 86 K at 300 GPa for μ* = 0.10) at a descendant gradient (dTc/dP) of −0.21 K/GPa, which means Tc has only a weak dependence on pressure. Subsequently, the same high-symmetry structure of SbH4 was found by using two different structural search techniques [80]. Moreover, they obtained the Tc of 102 K at 150 GPa, which confirms our theoretical prediction. Bismuth hydrides For BiHn (n = 1–6), except for BiH3, all stoichiometries are stable at high pressure [75]. The remarkable structural feature is the emergence of H2 units in BiH2, BiH4, and BiH6. BiH5 adopts a startling layered structure intercalated by H2 and the linear H3 units. The ionic bonding feature in Bi-H system is quite different from the same group covalent hydride of SbH4, and the discrepancy of bonding feature depends on the existing form of hydrogen atoms and individual crystalline structures. In SbH4, except for H2 units, it also contains Sb-H weak interaction. However, except for BiH, only H2 or H3 units were found in Bi-H system and no excrescent hydrogen can bind with Bi atoms. On the other hand, the elemental nature could determine the discrepancy. In pnictogen group, nitrogen (N) and phosphorus (P) are nonmetal, and arsenic (As) and antimony (Sb) are regarded as semimetal, while bismuth (Bi) is the sole ‘real metal’. Compared with other pnictogen elements, bismuth has the heaviest atomic mass and the weakest electronegativity. In Bi-H system, H atoms may have a stronger ability in attracting electrons than Bi and then form the ionic bonding. Furthermore, the calculated Tc of BiH, BiH2, BH4, BiH5, and BiH6 at 300 GPa are 20, 65, 75, 119, and 113 K, respectively, which indicates all these stable bismuth hydrides are potential high-temperature superconductors. It also can be seen that the Tc increases with the increasing hydrogen content with the maximum value of 119 K in BiH5. The results of Bi-H system verify the idea that hydrogen content has a great influence on the superconducting transition temperature. In addition, the Tc of BiH and BiH4 decrease with the increasing pressure, while the Tc increases with the increasing pressure in BiH2, BiH5, and BiH6. CHALCOGEN HYDRIDES As the lightest chalcogen hydrides, H2O has a rich phase diagram: amorphous, hydrogen bond symmetry, ionic phase and so on [81]. Unfortunately, it has difficulty in achieving metallization up to 2 TPa [82]. For other chalcogen hydrides, they show metallic properties and high-temperature superconductivity at lower pressure; especially for new sulfur trihydrides H3S, it sets a record high critical temperature of 200 K [17–19]. Sulfur hydrides In 1990s, the structures and molecular dissociation of solid H2S at high pressure have been studied extensively, but this remains controversial. In 1997, infrared-absorption spectral measurement at room temperature showed that H2S molecules dissociate and metalize at 46 and 96 GPa, respectively [83]. In 2010, our group theoretically predicted that protons are delocalized and can move back and forth along the S–S separation in phase V of H2S [84]. It is indicating that the decomposition of H2S under high pressure is not simple. In 2014, it was theoretically predicted that H2S cannot decompose into sulfur and hydrogen up to 200 GPa [85]. Moreover, two metallic phase P-1 and Cmca were proposed with the highest Tc ∼ 80 K at 160 GPa. In 2011, it is reported that mixing H2S and H2 can form a new compound (H2S)2H2 (H3S, the stoichiometric ratio of H and S atoms is 3:1) near 3.5 GPa [86]. Unfortunately, the highest pressure in this study is 30 GPa. In 2014, we have extensively explored the high-pressure structures and superconductivity of H3S through ab initio calculations [17]. The low pressure crystal structure with P1 symmetry (below 37 GPa) has been determined. Moreover, the XRD, Raman, and equation of states for P1 are in excellent agreement with experimental results. Furthermore, three new high-pressure phases were firstly proposed: orthorhombic Cccm (37–111 GPa), hexagonal R3m (111–180 GPa), cubic Im-3m (180–300 GPa). Interestingly, there are H2 units in insulating phase P1 and Cccm, whereas H2 units disappear in metallic phases R3m and Im-3m, and hydrogen atoms cooperating with sulfur atoms form strong covalent bonding, as shown in Fig. 3. Further EPC calculations predict that the Tc of the R3m phase at 130 GPa is 155–166 K. Remarkably, we predicted the Tc of the Im-3m phase to achieve 191–204 K at 200 GPa for the first time, reaching an order of 200 K. In addition, Tc increases with increasing pressure in the R3m phase (135–145 K at 110 GPa and 165–175 K at 150 GPa), whereas Tc decreases nearly linearly with increasing pressure in the Im-3m phase (184–200 K at 250 GPa and 179–196 K at 300 GPa). Further analysis shows that H atoms play a significant role in superconductivity of H3S. In addition, we performed the high-pressure stability of different stoichiometric HnS (n > 1) using ab initio calculations [18] and determined two main ways to form H3S crystal: 3H2S → 2H3S + S, 2H2S + H2 → 2H3S, as depicted in Fig. 3. That is, H3S can be obtained by directly compressing pure H2S above 43 GPa [18] or mixing H2S and H2 at lower pressure [18,86]. The other theoretical research works also confirmed our results in succession [87,88]. Excitingly, superconductivity in an H2S sample with high Tc = 203 K above 155 GPa has been observed based on the resistant transition, isotope effect and Meissner effect [19], as shown in Fig. 4. Moreover, Shimizu et al. in cooperation with Eremets further confirm that the H2S decomposes H3S and S at high pressure by XRD measurements [20]. In addition, they also confirm that the superconducting (SC) phase is mostly in good agreement with our theoretically predicted body-centered cubic structure [17]. Figure 3. Open in new tabDownload slide (A) Predicted formation enthalpies of HnS with respect to decomposition into S and H2 under pressure [18]. Dashed lines connect data points, and solid lines denote the convex hull. Copyright 2015 American Physical Society. (B) Crystal structures of H3S-R3m and its corresponding ELF [17]. (C) Crystal structures of H3S-Im-3m and its corresponding ELF [17]. Figure 3. Open in new tabDownload slide (A) Predicted formation enthalpies of HnS with respect to decomposition into S and H2 under pressure [18]. Dashed lines connect data points, and solid lines denote the convex hull. Copyright 2015 American Physical Society. (B) Crystal structures of H3S-R3m and its corresponding ELF [17]. (C) Crystal structures of H3S-Im-3m and its corresponding ELF [17]. Figure 4. Open in new tabDownload slide (a) Changes of resistance and Tc of sulfur hydride with temperature at constant pressure—the annealing process. (b) Typical superconductive steps for sulfur hydride (blue trace) and sulfur deuteride (red trace). (c) Temperature dependence of the magnetization of sulfur hydride at a pressure of 155 GPa in zero-field cooled (ZFC) and 20 Oe field cooled (FC) modes (black circles). Adapted from [19]. Copyright 2015 Nature Publishing Group. (d) Dependence of Tc on pressure for experimental and theoretical results [17,19,20,85]. Figure 4. Open in new tabDownload slide (a) Changes of resistance and Tc of sulfur hydride with temperature at constant pressure—the annealing process. (b) Typical superconductive steps for sulfur hydride (blue trace) and sulfur deuteride (red trace). (c) Temperature dependence of the magnetization of sulfur hydride at a pressure of 155 GPa in zero-field cooled (ZFC) and 20 Oe field cooled (FC) modes (black circles). Adapted from [19]. Copyright 2015 Nature Publishing Group. (d) Dependence of Tc on pressure for experimental and theoretical results [17,19,20,85]. There is a widespread consensus that the superconducting samples are composed of sulfur trihydride (H3S) Im-3m phase proposed by our group and is considered to be conventional in nature. Therefore, there are a lot of theoretical research works using the Im-3m structure to study the superconducting mechanism of H3S by analysis of the electronic structure and bonding characteristics. Bernstein et al. underlined that the high-temperature superconductivity of H3S is attributed to strong covalent bonds giving rise to large EPCs [87], which is similar to the MgB2. In addition, a more detailed study on Van Hove singularity of H3S proposes that increasing the electron DOS near the Fermi surface will increase the superconducting transition temperature [89]. Heil and Boeri constructed hypothetical alchemical atoms by partially replacing sulfur with chalcogen group element and showed that the critical temperatures of H3S could be improved by increasing the ionic character of the relevant bonds [90]. Besides the substitution of S by elements in the same main group, further substitutions by phosphorus and halogen group element, and in the optimized case of H3S0.925P0.075, the Tc may reach a record high value of 280 K at 250 GPa [91]. Selenium hydrides For selenium hydrides, H3Se is stable above 166 GPa which possess Im-3m symmetry isostructural to that of H3S [92]. But the predicted Tc of 116 K at 200 GPa is lower than that of H3S, which is mainly due to the reduced λ. In addition, the other selenium hydrides, the Se-rich HSe2 (C2/m), and HSe (P4/nmm) are stable above 124 and 249 GPa, respectively. Tellurium hydride For H-Te system, H4Te, H5Te2, HTe, and HTe2 were found to be stable above 140 GPa [93]. H4Te with P6/mmm symmetry is stable above 162 GPa, and then transforms to R-3m at 234 GPa, and both phases contain H2 units. H5Te2 adopts C2/m symmetry with intriguing linear H3 units. For HTe, P4/nmm phase is stable above 140 GPa which is isostructural to HSe. Then it transforms to P63/mmc phase near 286 GPa. The Tc for H4Te, H5Te2, and HTe are estimated to be 99, 58, and 19 K at 200 GPa, respectively. Polonium hydrides For H-Po system, several stoichiometries (PoH, PoH2, PoH4, and PoH6) are stable with respect to Po and H2 under pressure [94]. Moreover, except PoH, PoH2, PoH4, and PoH6 contain H2 units. PoH adopts P63/mmc symmetry and it is isostructural to HTe. The Tc of PoH4 (space group C2/c) is predicted to be 47 K at 200 GPa. HALOGEN HYDRIDES The consensus for HX (X = F, Cl, Br) is that Cmc21 phase transforms to hydrogen bond symmetry phase Cmcm at high pressure [95–98]. For HI, a complex triclinic phase with P1 in a different symmetry appears at low temperatures [98]. Interestingly, triangular H3+ species are unexpectedly found in H2F, H3F, H5F, H5Cl, and H5Br above 100 GPa, while there is no H3+ species in HnI (n = 1–6). With decreasing the electronegativity of halogen, HX (X = F, Cl, Br, and I) becomes unstable and decomposes into other stoichiometries at lower pressure. In addition, the metallization pressure is gradually decreased with decreasing the electronegativity. The electronegativity of bromine is between chlorine and iodine; the structure and stability of HnBr is not only resemble with HnCl but also similar with HnI. Fluorine and chlorine hydrides HnF (n = 1–5) and HnCl (n = 1–9) have been extensively studied by our group through ab initio calculations [99]. The results show that HF is the only stable stoichiometry up to 300 GPa, whereas other stoichiometries H2,3,4,5F are metastable at high pressure. It is interesting that triangular H3+ species are unexpectedly found in stoichiometries H2F with [H3]+[HF2]−, H3F with [H3]+[F]−, and H5F with [H3]+[HF2]−[H2]3. A perfect equilateral triangle with H–H bond length 0.854 Å is formed in H5F. HCl is stable up to 300 GPa with the high pressure phase sequence Cmc21 → Cmcm → P-1 → C2/m [95–97]. And stoichiometries H2Cl, H3Cl, H5Cl, and H7Cl were found to become stable at high pressure [99–101]. H2Cl with C2/c symmetry is stable in a wide pressure range from 12 to 341 GPa, then it transforms to R-3m phase. H3Cl adopts three high pressure phases with symmetry Cc (12–40 GPa), C2/c (40–60 GPa), and P212121 (60–120 GPa). H5Cl prefers a Cc symmetry between 120 and 250 GPa. H7Cl (P21 symmetry) with the highest hydrogen content is found to be stable in a narrow pressure range from 70 to 110 GPa. Besides the H-rich compounds, a Cl-rich compound H4Cl7 was also found [101]. It possesses two high pressure phase C2/m (90–278 GPa) and C2/c (278–445 GPa). For the hydrogen-rich H-Cl compounds, the stable phases contain covalent HCl zigzag chain which resembles pure HCl of Cmc21 or Cmcm phase and H2 molecules units. Instead of covalent HCl molecular units, there exists H3+ molecular cation and Cl− anion in H5Cl. Importantly, we clearly see the formation process of H3+ through bond length, bond overlap population, ELF, and Bader charge as a function of pressure, as shown in Fig. 5. Moreover, formation of H3+ species is mainly due to charges in hydrogen atoms transfer into chlorine atoms at high pressure. It is also reported that the Tc of HCl- C2/m at 250 GPa and H2Cl-R-3m at 450 GPa are 20 and 45 K, respectively [96,101]. Figure 5. Open in new tabDownload slide (a) Electron localization function (ELF) maps of H5Cl-Cc with increasing pressure [99]. Copyright 2015 American Chemical Society. (b) ELF maps of H2I-Pnma for (010) plane. (c) ELF maps of H2I-R-3m for (110) plane. Adapted from [104] with permission from the PCCP Owner Societies. Figure 5. Open in new tabDownload slide (a) Electron localization function (ELF) maps of H5Cl-Cc with increasing pressure [99]. Copyright 2015 American Chemical Society. (b) ELF maps of H2I-Pnma for (010) plane. (c) ELF maps of H2I-R-3m for (110) plane. Adapted from [104] with permission from the PCCP Owner Societies. Bromine hydrides Different from HF and HCl, our calculations show that HBr is unstable above 64 GPa and decomposes into new hydrides H2Br and Br2 [102]. In previous experimental research on HBr at high pressure, the Raman spectra of Br2 molecules were observed after hydrogen-bond symmetrization, but Raman signals of H2 molecules were not detected [103]. Combining the theoretical calculation results, it is suggested that HBr decompose into H2Br and Br2 at high pressure via the reaction 4HBr → 2H2Br + Br2. This phenomenon reminds us that H2S decomposes into new hydrides H3S and S at high pressure [18]. H2Br with C2/c symmetry is stable in the pressure range from 30 to 140 GPa. Then, up to 240 GPa, a Cmcm phase is formed. H3Br with P212121 symmetry can be synthesized via the reaction HBr + H2 → H3Br above 8 GPa. It transforms into low symmetry P-1 phase at about 100 GPa. Different from H-Cl compounds, H4Br with hexagonal P63/mmc structure was observed at 240 GPa. H5Br contains three high pressure phases with symmetry C2/c (70–100 GPa), Cc (100–140 GPa), and Pmn21 (140–280 GPa). H7Br with P21/m symmetry was found to be stable from 30 to 60 GPa. For the stable lower pressure phases, they contain covalent HBr zigzag chain which resembles to pure HBr of Cmc21 or Cmcm phase and H2 molecules units. In the high pressure phase Cc and Pmn21 of H5Br, they consist of H3+ molecular cation, Cl− anion, and H2 molecules units. The structure of H5Br-Cc is very similar to H5Cl-Cc. For H5Br-Pmn21 at 140 GPa, H3 unit forms an approximate equilateral triangle with H–H length of 0.903, 0.903, and 0.911 Å. H2Br-Cmcm consists of monatomic lattices of bromine and H2 molecules units, which is a good metal. For the case of H4Br-P63/mmc, H-Br forms 3D network that traps H2 molecules arranged in a straight chain, which is also a metal. Further, EPC calculations show that hydrogen-rich H2Br and H4Br are superconductors with Tc of 12.1 K and 2.4 K at 240 GPa, respectively. Iodine hydrides For HI with the smallest electronegativity of iodine, it decomposes into solids H2 and I2 at lower pressure of 6.7 GPa [104]. Different from the other halogen hydrides, there is no H3+ species in hydrogen-rich HnI (n = 1–6). Increasing pressure to 20 GPa, mixed solids H2 and I2 can synthetize H5I with P2221 symmetry, which consist of zigzag rectangular chains built from [I–H–I] units and H2 molecular units. At about 114 GPa, H5I decomposes into H4I (P6/mmm) and H2. In addition, H2I with Pnma is stable in a wide pressure range of 100–246 GPa, then transforms to R-3m phase. In H4I-P6/mmm and H2I-Pnma, iodine forms a monatomic tube network that traps H2 molecules units [104]. In the case of H2I-R-3m, iodine monatomic still exists, but H2 molecular units disappear forming an atomic phase, as depicted in Fig. 5. Further EPC calculations show that the Tc of H4I-P6/mmm is 17.5 K at 100 GPa [105] and 9.9 K at 120 GPa [104]. In addition, the Tc of Pnma and R-3m phases for H2I are 3.8 and 33 K at 240 GPa, respectively [104]. Significantly, the Tc of atomic phase R-3m is increased eight times that of Pnma, which is mainly attributed to increment EPC λ, logarithmic average phonon frequency ωlog, and electric DOS values at Fermi level. CONCLUSIONS AND PROSPECTS In summary, the 33 classes of main group elements hydrides at high pressure are surveyed above and covered in detail in this review. They exhibit a rich variety stoichiometry and new structures, and show novel superconductivity at high pressure. Above all, the recent discovery of record Tc ∼ 200 K in sulfur hydrides under high pressure injects fresh energy into the field of searching high temperature superconductor in hydrogen-rich materials. For example, a high Tc ∼ 100 K of pnictogen hydrides at high pressure was discovered by experimental and theoretical studies. The extensive study on room temperature superconductors in hydrogen-rich materials at high pressure is an exciting long-term research frontier. Finally, more experimental research works are needed to prove the theoretical prediction of novel high-temperature superconductors under high pressure, although experimental challenges are significant. Table 1. Summary of the maximum superconducting transition temperature of hydrogen-rich materials at high pressures. . . . . . . Main group . . Structure (stable pressure . Maximum Tc . Pressure . . hydrides . Hydrides . range GPa) . (K) . (GPa) . Reference . Alkali metal LiH6 R-3m (>140) 38 150 [22,26] LiH8 I422 (100–200) 31 100 [22,26] KH6 C2/c (166–300) 70 166 [14] Alkaline-earth BeH2 Cmcm (202–400) 62 400 [32,33] MgH4 Cmcm (>92) 37 100 [34] MgH12 R3 (>122) 60 140 [34] CaH6 Im-3m (127.7–200) 235 150 [15] BaH6 P4/mmm (50–100) 38 100 [31] Boron group BH3 Pbcn (350–400) 125 360 [38] AlH5 P21/m (75–250) 146 250 [45] GaH3 Pm-3n (160–300) 86 160 [39] InH3 R-3 (223–300) 40 200 [40] InH5 P21/m (120–223) 27 150 [40] Carbon group SiH4 Pman (> 96) 166 202 [48] SiH3 Pm-3m (275–300) 153 300 [57] SiH8 Ccca (24–300) 107 250 [63] GeH4 C2/c (196–300) 64 196 [66] GeH3 Cccm (280–400) 80 300 [68] GeH8 P21/c (220–350) 90 250 [13] SnH4 P63/mmc (180–300) 62 200 [71] PbH8 C2/m (133–300) 107 230 [73] Pnictogen PH3(exp.) Uncertain 103 207 [76] PH2 I4/mmm 78 220 [77] AsH8 C2/c (350–450) 151 450 [80] SbH4 P63/mmc (150–300) 118 150 [74] BiH2 P21/m (150–300) 65 300 [75] BiH4 Pmmn (150–300) 93 150 [75] BiH5 C2/m (200–300) 119 300 [75] BiH6 P-1 (200–300) 113 300 [75] Chalcogen H2S Cmca (160–200) 80 160 [85] H3S Im-3m (180–300) 191–204 200 [17] H3S (exp.) Im-3m 203 155 [19,20] H3Se Im-3m (166–300) 116 200 [92] H5Te2 C2/m (170–300) 58 200 [93] H4Te P63/mmc (162–300) 99 200 [93] H4Po C2/c (137–300) 47 200 [94] Halogen H2Cl R-3m (341–400) 45 450 [101] H2Br Cmcm (240–300) 12 240 [102] H4Br P63/mmc (240–300) 2.4 240 [102] H2I R-3m (246–300) 33 240 [104] H4I P6/mmm (114–300) 20 150 [105] . . . . . . Main group . . Structure (stable pressure . Maximum Tc . Pressure . . hydrides . Hydrides . range GPa) . (K) . (GPa) . Reference . Alkali metal LiH6 R-3m (>140) 38 150 [22,26] LiH8 I422 (100–200) 31 100 [22,26] KH6 C2/c (166–300) 70 166 [14] Alkaline-earth BeH2 Cmcm (202–400) 62 400 [32,33] MgH4 Cmcm (>92) 37 100 [34] MgH12 R3 (>122) 60 140 [34] CaH6 Im-3m (127.7–200) 235 150 [15] BaH6 P4/mmm (50–100) 38 100 [31] Boron group BH3 Pbcn (350–400) 125 360 [38] AlH5 P21/m (75–250) 146 250 [45] GaH3 Pm-3n (160–300) 86 160 [39] InH3 R-3 (223–300) 40 200 [40] InH5 P21/m (120–223) 27 150 [40] Carbon group SiH4 Pman (> 96) 166 202 [48] SiH3 Pm-3m (275–300) 153 300 [57] SiH8 Ccca (24–300) 107 250 [63] GeH4 C2/c (196–300) 64 196 [66] GeH3 Cccm (280–400) 80 300 [68] GeH8 P21/c (220–350) 90 250 [13] SnH4 P63/mmc (180–300) 62 200 [71] PbH8 C2/m (133–300) 107 230 [73] Pnictogen PH3(exp.) Uncertain 103 207 [76] PH2 I4/mmm 78 220 [77] AsH8 C2/c (350–450) 151 450 [80] SbH4 P63/mmc (150–300) 118 150 [74] BiH2 P21/m (150–300) 65 300 [75] BiH4 Pmmn (150–300) 93 150 [75] BiH5 C2/m (200–300) 119 300 [75] BiH6 P-1 (200–300) 113 300 [75] Chalcogen H2S Cmca (160–200) 80 160 [85] H3S Im-3m (180–300) 191–204 200 [17] H3S (exp.) Im-3m 203 155 [19,20] H3Se Im-3m (166–300) 116 200 [92] H5Te2 C2/m (170–300) 58 200 [93] H4Te P63/mmc (162–300) 99 200 [93] H4Po C2/c (137–300) 47 200 [94] Halogen H2Cl R-3m (341–400) 45 450 [101] H2Br Cmcm (240–300) 12 240 [102] H4Br P63/mmc (240–300) 2.4 240 [102] H2I R-3m (246–300) 33 240 [104] H4I P6/mmm (114–300) 20 150 [105] Open in new tab Table 1. Summary of the maximum superconducting transition temperature of hydrogen-rich materials at high pressures. . . . . . . Main group . . Structure (stable pressure . Maximum Tc . Pressure . . hydrides . Hydrides . range GPa) . (K) . (GPa) . Reference . Alkali metal LiH6 R-3m (>140) 38 150 [22,26] LiH8 I422 (100–200) 31 100 [22,26] KH6 C2/c (166–300) 70 166 [14] Alkaline-earth BeH2 Cmcm (202–400) 62 400 [32,33] MgH4 Cmcm (>92) 37 100 [34] MgH12 R3 (>122) 60 140 [34] CaH6 Im-3m (127.7–200) 235 150 [15] BaH6 P4/mmm (50–100) 38 100 [31] Boron group BH3 Pbcn (350–400) 125 360 [38] AlH5 P21/m (75–250) 146 250 [45] GaH3 Pm-3n (160–300) 86 160 [39] InH3 R-3 (223–300) 40 200 [40] InH5 P21/m (120–223) 27 150 [40] Carbon group SiH4 Pman (> 96) 166 202 [48] SiH3 Pm-3m (275–300) 153 300 [57] SiH8 Ccca (24–300) 107 250 [63] GeH4 C2/c (196–300) 64 196 [66] GeH3 Cccm (280–400) 80 300 [68] GeH8 P21/c (220–350) 90 250 [13] SnH4 P63/mmc (180–300) 62 200 [71] PbH8 C2/m (133–300) 107 230 [73] Pnictogen PH3(exp.) Uncertain 103 207 [76] PH2 I4/mmm 78 220 [77] AsH8 C2/c (350–450) 151 450 [80] SbH4 P63/mmc (150–300) 118 150 [74] BiH2 P21/m (150–300) 65 300 [75] BiH4 Pmmn (150–300) 93 150 [75] BiH5 C2/m (200–300) 119 300 [75] BiH6 P-1 (200–300) 113 300 [75] Chalcogen H2S Cmca (160–200) 80 160 [85] H3S Im-3m (180–300) 191–204 200 [17] H3S (exp.) Im-3m 203 155 [19,20] H3Se Im-3m (166–300) 116 200 [92] H5Te2 C2/m (170–300) 58 200 [93] H4Te P63/mmc (162–300) 99 200 [93] H4Po C2/c (137–300) 47 200 [94] Halogen H2Cl R-3m (341–400) 45 450 [101] H2Br Cmcm (240–300) 12 240 [102] H4Br P63/mmc (240–300) 2.4 240 [102] H2I R-3m (246–300) 33 240 [104] H4I P6/mmm (114–300) 20 150 [105] . . . . . . Main group . . Structure (stable pressure . Maximum Tc . Pressure . . hydrides . Hydrides . range GPa) . (K) . (GPa) . Reference . Alkali metal LiH6 R-3m (>140) 38 150 [22,26] LiH8 I422 (100–200) 31 100 [22,26] KH6 C2/c (166–300) 70 166 [14] Alkaline-earth BeH2 Cmcm (202–400) 62 400 [32,33] MgH4 Cmcm (>92) 37 100 [34] MgH12 R3 (>122) 60 140 [34] CaH6 Im-3m (127.7–200) 235 150 [15] BaH6 P4/mmm (50–100) 38 100 [31] Boron group BH3 Pbcn (350–400) 125 360 [38] AlH5 P21/m (75–250) 146 250 [45] GaH3 Pm-3n (160–300) 86 160 [39] InH3 R-3 (223–300) 40 200 [40] InH5 P21/m (120–223) 27 150 [40] Carbon group SiH4 Pman (> 96) 166 202 [48] SiH3 Pm-3m (275–300) 153 300 [57] SiH8 Ccca (24–300) 107 250 [63] GeH4 C2/c (196–300) 64 196 [66] GeH3 Cccm (280–400) 80 300 [68] GeH8 P21/c (220–350) 90 250 [13] SnH4 P63/mmc (180–300) 62 200 [71] PbH8 C2/m (133–300) 107 230 [73] Pnictogen PH3(exp.) Uncertain 103 207 [76] PH2 I4/mmm 78 220 [77] AsH8 C2/c (350–450) 151 450 [80] SbH4 P63/mmc (150–300) 118 150 [74] BiH2 P21/m (150–300) 65 300 [75] BiH4 Pmmn (150–300) 93 150 [75] BiH5 C2/m (200–300) 119 300 [75] BiH6 P-1 (200–300) 113 300 [75] Chalcogen H2S Cmca (160–200) 80 160 [85] H3S Im-3m (180–300) 191–204 200 [17] H3S (exp.) Im-3m 203 155 [19,20] H3Se Im-3m (166–300) 116 200 [92] H5Te2 C2/m (170–300) 58 200 [93] H4Te P63/mmc (162–300) 99 200 [93] H4Po C2/c (137–300) 47 200 [94] Halogen H2Cl R-3m (341–400) 45 450 [101] H2Br Cmcm (240–300) 12 240 [102] H4Br P63/mmc (240–300) 2.4 240 [102] H2I R-3m (246–300) 33 240 [104] H4I P6/mmm (114–300) 20 150 [105] Open in new tab Acknowledgments The authors would thank the collaborations and discussions with A. R. Oganov, Yanming Ma, Tao Xiao, M. I. Eremets, K. Shimizu and Lanyu Liu. 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TI - Structure and superconductivity of hydrides at high pressures
JF - National Science Review
DO - 10.1093/nsr/nww029
DA - 2017-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/structure-and-superconductivity-of-hydrides-at-high-pressures-crIya71d0R
SP - 121
EP - 135
VL - 4
IS - 1
DP - DeepDyve
ER -