Polyhydride

A polyhydride or superhydride is a compound that contains an abnormally large amount of hydrogen. This can be described as high hydrogen stoichiometry. Examples include iron pentahydride FeH5, LiH6, and LiH7. The more well known lithium hydride only has one hydrogen atom.[1]

Polyhydrides are only known to be stable under high pressure.[1]

Polyhydrides are important because they can form substances with a very high density of hydrogen. They may resemble the elusive metallic hydrogen, but can be made under lower pressures. One possibility is that they could be superconductors. Hydrogen sulfide under high pressures forms SH3 units, and can be a superconductor at 203 K (−70 °C) and a pressure of 1.5 million atmospheres.[1]

Structures

Unit cell diagram showing the structure of NaH7, which contains H3− complexes. The coloured balls it the isosurface, plotted at the level of 0.07 electrons*Å−3. One of H2 molecules is bonded to a hydrogen atom in the NaH unit with a bond length of 1.25 Å, forming a H3− linear anion.

The polyhydrides of alkaline earth and alkali metals contain cage structures. Also hydrogen may be clustered into H−, H3−, or H2 units. Polyhydrides of transition metals may have the hydrogen atoms arranged around the metal atom. Computations suggest that increasing hydrogen levels will reduce the dimensionality of the metal arrangement, so that layers form separated by hydrogen sheets.[1] The H3− substructure is linear.[2]

H3+ would form triangular structures in the hypothetical H5Cl.[2]

Compounds

When sodium hydride is compressed with hydrogen NaH3 and NaH7. These are formed at 30 GPa and 2,100 K.[2]

formula name temperature

°C

 pressure

GPa

 crystal structure space group a Å b c β cell volume formulae per unit cell refs
LiH2 lithium dihydride 27 130 [3]
LiH6 Lithium hexahydride [1]
LiH7 Lithium heptahydride [1]
NaH3 sodium trihydride orthorhombic Cmcm 3.332 Å 6.354 Å 4.142 Å 90 87.69 4 [2]
NaH7 sodium heptahydride monoclinic Cc 6.99 3.597 5.541 69.465 130.5 [2]
CaHx 500 22 double hexagon [4]
CaHx 600 121 [4]
FeH5 iron pentahydride 1200 66 tetragonal I4/mmm [1]
H3Se Selenium trihydride 10 [5]
LaH10 Lanthanum decahydride 1000 170 cubic Fm3m 5.09 5.09 5.09 132 4 [6]
LaH10 Lanthanum decahydride 25 121 Hexagonal R3m 3.67 3.67 8.83 1 [6]
LaH7 Lanthanum heptahydride 25 109 monoclinic C2/m 6.44 3.8 3.69 135 63.9 2 [6]
UH7 Uranium heptahydride 2000 63 fcc P63/mmc [7]
UH8 Uranium octahydride 300 1-55 fcc Fm3m [7]
UH8 Uranium nonahydride 40-55 fcc P63/mmc [7]

Predicted

Using computational chemistry many other polyhydrides are predicted, including LiH8,[8] LiH9,[9] LiH10,[9] CsH3,[10] KH5 RbH5,[11] RbH9,[8] NaH9, BaH6,[11] CaH6,[12] MgH4, MgH12, MgH16,[13] SrH4, SrH6,[14] SrH10, SrH12,[8] ScH4, ScH6, ScH8,[15] YH4 and YH6,[16] YH24, LaH8, LaH10,[17] YH9, LaH11, CeH8, CeH9, CeH10, PrH8, PrH9,[18] ThH6, ThH7 and ThH10,[19] U2H13, UH7, UH8, UH9,[7] AlH5,[20] GaH5, InH5,[8] SnH8, SnH12, SnH14,[21] PbH8,[22] SiH8 (subsequently discovered),[8] GeH8,[23] (although Ge3H11 may be stable instead)[24] AsH8, SbH4,[25] BiH4, BiH5, BiH6,[26] H3Se,[27] H3S,[28] Te2H5, TeH4,[29] PoH4, PoH6,[8] H2F, H3F,[8] H2Cl, H3Cl, H5Cl, H7Cl,[30] H2Br, H3Br, H4Br, H5Br, H5I,[8] XeH2, XeH4,.[31]

Among the transition elements, VH8 in a C2/m structure around 200 Gpa is predicted to have a superconducting transition temperature of 71.4 K. VH5 in a P63/mmm space group has a lower transition temperature.[32]

Properties

Superconduction

Under suitably high pressures polyhydrides may become superconducting. Characteristics of substances that are predicted to have high superconducting temperatures are a high phonon frequency, which will happen for light elements, and strong bonds. Hydrogen is the lightest and so will have the highest frequency of vibration. Even changing the isotope to deuterium will lower the frequency and lower the transition temperature. Compounds with more hydrogen will resemble the predicted metallic hydrogen. However superconductors also tend to be substances with high symmetry, and also need the electrons not to be locked into molecular subunits, and require large numbers of electrons in states near the Fermi level. There should also be electron-phonon coupling which happens when the electric properties are tied to the mechanical position of the hydrogen atoms.[18][33] For example Lithium hexahydride is predicted to lose all electrical resistance below 38 K at a pressure of 150 Gpa. The hypotheical LiH8 has a predicted superconducting transition temperature at 31 K at 200 GPa.[34] MgH6 is predicted to have a Tc of 400 K around 300 Gpa.[35] CaH6 could have a Tc of 260 K at 120 GPa. PH3 doped H3S is also predicted to have a transition temperature above the 203 K measured for H3S (contaminated with solid sulfur).[36] Rare earth and actinide polyhydrides may also have highish transition temperatures, for example ThH10 with Tc = 194 K.[19] UH8, which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K.[19] AcH10, if it could be ever made, is predicted to superconduct at temperatures over 204 K, and AcH10 would be similarly conduction under lower pressures (150 GPa).[37]

H3Se actually is a van der Waals solid with formula 2H2Se•H2 with a measured Tc of 105K under a pressure of 135 GPa.[5]

References

  1. 1 2 3 4 5 6 7 Pépin, C. M.; Geneste, G.; Dewaele, A.; Mezouar, M.; Loubeyre, P. (27 July 2017). "Synthesis of FeH5 : A layered structure with atomic hydrogen slabs". Science. 357 (6349): 382–385. Bibcode:2017Sci...357..382P. doi:10.1126/science.aan0961. PMID 28751605.
  2. 1 2 3 4 5 Struzhkin, Viktor V.; Kim, Duck Young; Stavrou, Elissaios; Muramatsu, Takaki; Mao, Ho-kwang; Pickard, Chris J.; Needs, Richard J.; Prakapenka, Vitali B.; Goncharov, Alexander F. (28 July 2016). "Synthesis of sodium polyhydrides at high pressures". Nature Communications. 7: 12267. Bibcode:2016NatCo...712267S. doi:10.1038/ncomms12267. PMC 4974473. PMID 27464650.
  3. ↑ Pépin, Charles; Loubeyre, Paul; Occelli, Florent; Dumas, Paul (23 June 2015). "Synthesis of lithium polyhydrides above 130 GPa at 300 K". Proceedings of the National Academy of Sciences. 112 (25): 7673–7676. Bibcode:2015PNAS..112.7673P. doi:10.1073/pnas.1507508112. PMC 4485130.
  4. 1 2 Mishra, Ajay Kumar; Ahart, Muhtar; Somayazulu, Maddury; Park, C. Y; Hemley, Russel J (2017-03-13). "Synthesis of Calcium polyhydrides at high pressure and high temperature". Bulletin of the American Physical Society. 62 (4): B35.008. Bibcode:2017APS..MARB35008M.
  5. 1 2 Mishra, A. K.; Somayazulu, M.; Ahart, M.; Karandikar, A.; Hemley, R. J.; Struzhkin, V. (9 March 2018). "Novel Synthesis Route and Observation of Superconductivity in the Se-H System at Extreme Conditions". Bulletin of the American Physical Society.
  6. 1 2 3 Geballe, Zachary M.; Liu, Hanyu; Mishra, Ajay K.; Ahart, Muhtar; Somayazulu, Maddury; Meng, Yue; Baldini, Maria; Hemley, Russell J. (15 January 2018). "Synthesis and Stability of Lanthanum Superhydrides". Angewandte Chemie International Edition. 57 (3): 688–692. doi:10.1002/anie.201709970. PMID 29193506.
  7. 1 2 3 4 Kruglov, Ivan A.; Kvashnin, Alexander G.; Goncharov, Alexander F.; Oganov, Artem R.; Lobanov, Sergey; Holtgrewe, Nicholas; Yanilkin, Alexey V. (17 August 2017). "High-temperature superconductivity of uranium hydrides at near-ambient conditions". arXiv:1708.05251 [cond-mat.mtrl-sci].
  8. 1 2 3 4 5 6 7 8 Duan, Defang; Liu, Yunxian; Ma, Yanbin; Shao, Ziji; Liu, Bingbing; Cui, Tian (28 April 2016). "Structure and superconductivity of hydrides at high pressures". National Science Review: nww029. doi:10.1093/nsr/nww029.
  9. 1 2 Chen, Yangmei; Geng, Hua Y.; Yan, Xiaozhen; Sun, Yi; Wu, Qiang; Chen, Xiangrong (2017). "Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures". Inorganic Chemistry. 56 (7): 3867–3874. doi:10.1021/acs.inorgchem.6b02709. PMID 28318270.
  10. ↑ Shamp, Andrew; Hooper, James; Zurek, Eva (3 September 2012). "Compressed Cesium Polyhydrides: Cs+ Sublattices and H3- Three-Connected Nets". Inorganic Chemistry. 51 (17): 9333–9342. doi:10.1021/ic301045v.
  11. 1 2 Zurek, Eva (6 June 2016). "Hydrides of the Alkali Metals and Alkaline Earth Metals Under Pressure". Comments on Inorganic Chemistry. 37 (2): 78–98. doi:10.1080/02603594.2016.1196679.
  12. ↑ Wang, H.; Tse, J. S.; Tanaka, K.; Iitaka, T.; Ma, Y. (6 April 2012). "Superconductive sodalite-like clathrate calcium hydride at high pressures". Proceedings of the National Academy of Sciences. 109 (17): 6463–6466. arXiv:1203.0263. Bibcode:2012PNAS..109.6463W. doi:10.1073/pnas.1118168109.
  13. ↑ Lonie, David C.; Hooper, James; Altintas, Bahadir; Zurek, Eva (19 February 2013). "Metallization of magnesium polyhydrides under pressure". Physical Review B. 87 (5): 054107. arXiv:1301.4750. Bibcode:2013PhRvB..87e4107L. doi:10.1103/PhysRevB.87.054107.
  14. ↑ Hooper, James; Terpstra, Tyson; Shamp, Andrew; Zurek, Eva (27 March 2014). "Composition and Constitution of Compressed Strontium Polyhydrides". The Journal of Physical Chemistry C. 118 (12): 6433–6447. doi:10.1021/jp4125342.
  15. ↑ Qian, Shifeng (2017). "Theoretical study of stability and superconductivity of". Physical Review B. 96 (9): 094513. Bibcode:2017PhRvB..96i4513Q. doi:10.1103/physrevb.96.094513.
  16. ↑ Li, Yinwei; Hao, Jian; Liu, Hanyu; Tse, John S.; Wang, Yanchao; Ma, Yanming (5 May 2015). "Pressure-stabilized superconductive yttrium hydrides". Scientific Reports. 5 (1): 9948. Bibcode:2015NatSR...5E9948L. doi:10.1038/srep09948.
  17. ↑ Liu, Hanyu; Naumov, Ivan I.; Hoffmann, Roald; Ashcroft, N. W.; Hemley, Russell J. (3 July 2017). "Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure". Proceedings of the National Academy of Sciences. 114 (27): 6990–6995. Bibcode:2017PNAS..114.6990L. doi:10.1073/pnas.1704505114.
  18. 1 2 Peng, Feng; Sun, Ying; Pickard, Chris J.; Needs, Richard J.; Wu, Qiang; Ma, Yanming (8 September 2017). "Hydrogen Clathrate Structures in Rare Earth Hydrides at High Pressures: Possible Route to Room-Temperature Superconductivity" (PDF). Physical Review Letters. 119 (10): 107001. Bibcode:2017PhRvL.119j7001P. doi:10.1103/PhysRevLett.119.107001. PMID 28949166.
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  20. ↑ Hou, Pugeng; Zhao, Xiusong; Tian, Fubo; Li, Da; Duan, Defang; Zhao, Zhonglong; Chu, Binhua; Liu, Bingbing; Cui, Tian (2015). "High pressure structures and superconductivity of AlH3(H2) predicted by first principles". RSC Adv. 5 (7): 5096–5101. doi:10.1039/C4RA14990D.
  21. ↑ Mahdi Davari Esfahani, M.; Wang, Zhenhai; Oganov, Artem R.; Dong, Huafeng; Zhu, Qiang; Wang, Shengnan; Rakitin, Maksim S.; Zhou, Xiang-Feng (11 March 2016). "Superconductivity of novel tin hydrides (Snn Hm) under pressure". Scientific Reports. 6 (1): 22873. arXiv:1512.07604. Bibcode:2016NatSR...622873M. doi:10.1038/srep22873.
  22. ↑ Cheng, Ya; Zhang, Chao; Wang, Tingting; Zhong, Guohua; Yang, Chunlei; Chen, Xiao-Jia; Lin, Hai-Qing (12 November 2015). "Pressure-induced superconductivity in H2-containing hydride PbH4(H2)2". Scientific Reports. 5 (1): 16475. Bibcode:2015NatSR...516475C. doi:10.1038/srep16475.
  23. ↑ Szcze¸śniak, R.; Szcze¸śniak, D.; Durajski, A.P. (April 2014). "Thermodynamics of the superconducting phase in compressed GeH4(H2)2". Solid State Communications. 184: 6–11. Bibcode:2014SSCom.184....6S. doi:10.1016/j.ssc.2013.12.036.
  24. ↑ Davari Esfahani, M. Mahdi; Oganov, Artem R.; Niu, Haiyang; Zhang, Jin (10 April 2017). "Superconductivity and unexpected chemistry of germanium hydrides under pressure". Physical Review B. 95 (13): 134506. arXiv:1701.05600. Bibcode:2017PhRvB..95m4506D. doi:10.1103/PhysRevB.95.134506.
  25. ↑ Fu, Yuhao; Du, Xiangpo; Zhang, Lijun; Peng, Feng; Zhang, Miao; Pickard, Chris J.; Needs, Richard J.; Singh, David J.; Zheng, Weitao; Ma, Yanming (22 March 2016). "High-Pressure Phase Stability and Superconductivity of Pnictogen Hydrides and Chemical Trends for Compressed Hydrides". Chemistry of Materials. 28 (6): 1746–1755. arXiv:1510.04415. doi:10.1021/acs.chemmater.5b04638.
  26. ↑ Ma, Yanbin; Duan, Defang; Li, Da; Liu, Yunxian; Tian, Fubo; Yu, Hongyu; Xu, Chunhong; Shao, Ziji; Liu, Bingbing; Cui, Tian (17 November 2015). "High-pressure structures and superconductivity of bismuth hydrides". arXiv:1511.05291 [cond-mat.supr-con].
  27. ↑ Zhang, Shoutao; Wang, Yanchao; Zhang, Jurong; Liu, Hanyu; Zhong, Xin; Song, Hai-Feng; Yang, Guochun; Zhang, Lijun; Ma, Yanming (22 October 2015). "Phase Diagram and High-Temperature Superconductivity of Compressed Selenium Hydrides". Scientific Reports. 5 (1): 15433. arXiv:1502.02607. Bibcode:2015NatSR...515433Z. doi:10.1038/srep15433. PMC 4614537.
  28. ↑ Durajski, Artur P.; Szczęśniak, Radosław (30 June 2017). "First-principles study of superconducting hydrogen sulfide at pressure up to 500 GPa". Scientific Reports. 7 (1): 4473. Bibcode:2017NatSR...7.4473D. doi:10.1038/s41598-017-04714-5. PMID 28667259.
  29. ↑ Zhong, Xin; Wang, Hui; Zhang, Jurong; Liu, Hanyu; Zhang, Shoutao; Song, Hai-Feng; Yang, Guochun; Zhang, Lijun; Ma, Yanming (4 February 2016). "Tellurium Hydrides at High Pressures: High-Temperature Superconductors". Physical Review Letters. 116 (5): 057002. arXiv:1503.00396. Bibcode:2016PhRvL.116e7002Z. doi:10.1103/PhysRevLett.116.057002. PMID 26894729.
  30. ↑ Duan, Defang; Huang, Xiaoli; Tian, Fubo; Liu, Yunxian; Li, Da; Yu, Hongyu; Liu, Bingbing; Tian, Wenjing; Cui, Tian (12 November 2015). "Predicted Formation of H3+ in Solid Halogen Polyhydrides at High Pressures". The Journal of Physical Chemistry A. 119 (45): 11059–11065. Bibcode:2015JPCA..11911059D. doi:10.1021/acs.jpca.5b08183.
  31. ↑ Yan, Xiaozhen; Chen, Yangmei; Kuang, Xiaoyu; Xiang, Shikai (28 September 2015). "Structure, stability, and superconductivity of new Xe–H compounds under high pressure". The Journal of Chemical Physics. 143 (12): 124310. Bibcode:2015JChPh.143l4310Y. doi:10.1063/1.4931931. PMID 26429014.
  32. ↑ Li, Xiaofeng; Peng, Feng (2 November 2017). "Superconductivity of Pressure-Stabilized Vanadium Hydrides". Inorganic Chemistry. 56 (22): 13759. doi:10.1021/acs.inorgchem.7b01686. PMID 29094931.
  33. ↑ Pietronero, Luciano; Boeri, Lilia; Cappelluti, Emmanuele; Ortenzi, Luciano (9 September 2017). "Conventional/unconventional superconductivity in high-pressure hydrides and beyond: insights from theory and perspectives". Quantum Studies: Mathematics and Foundations. 5: 5. doi:10.1007/s40509-017-0128-8.
  34. ↑ Xie, Yu; Li, Quan; Oganov, Artem R.; Wang, Hui (31 January 2014). "Superconductivity of lithium-doped hydrogen under high pressure". Acta Crystallographica Section C. 70 (2): 104–111. doi:10.1107/S2053229613028337.
  35. ↑ Szczȩśniak, R.; Durajski, A. P. (13 July 2016). "Superconductivity well above room temperature in compressed MgH6". Frontiers of Physics. 11 (6): 117406. Bibcode:2016FrPhy..11k7406S. doi:10.1007/s11467-016-0578-1.
  36. ↑ Eremets, M I; Drozdov, A P (30 November 2016). "High-temperature conventional superconductivity". Physics-Uspekhi. 59 (11): 1154–1160. Bibcode:2016PhyU...59.1154E. doi:10.3367/UFNe.2016.09.037921.
  37. ↑ Semenok, Dmitrii V; Kvashnin, Alexander G; Kruglov, Ivan A; Oganov, Artem R (2018). "Actinium hydrides AcH10, AcH12, AcH16 as high-temperature conventional superconductors". arXiv:1802.05676 [cond-mat.supr-con].
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