He Reactivity under High Pressure: Chemistry Without Chemical Bond
Update: 2019-10-22 09:15:19      Author: yangjuan@csrc.ac.cn

In 1933, Pauling predicted that noble gas (NG) elements could become chemically reactive under certain circumstances. In 1962, Bartlett synthesized the first NG compound, Xe+[PtF6]-.1 Since then, numerous NG compounds have been found.2-8 Due to a high ionization energy of 24.59 eV, Helium (He) has been thought to be chemically inert for a long time. In 2017, Dong et al. reported He could form compounds with Sodium and Sodium oxide under high pressure, in a combined experimental and computational study.9 The new compounds, Na2He and Na2OHe, showed no charge transfer between the He atoms and the other elements. Na2He can be regarded as a high-pressure electride, in which localized electrons exist in the interstitial space. Na2He is stabilized so in the form of Na+2E-2He, where E represents the electrons localized in interstitial sites. Sun et al. study the He reactivity with alkali oxide and sulfide compounds under high pressure. 10 Finally, H2O was reported to be reactive with He above around 300 GPa by Liu et al. 11 In all the known He compounds, there is no electron transfer between He and other elements. The stability of these He compounds is still not well understood.

Zhen Liu, Jorge Botana, Haiqing Lin of CSRC, Dadong Yan of Beijing Normal University, Steven Valdez and Maosheng Miao of California State University Northridge, Andreas Hermann of The University of Edinburgh, Eva Zurek of State University of New York at Buffalo, compared density functional theory and Madelung energy calculations to find out the mechanism that drives the stability of He compounds under high pressure. In Fig. 1, we show the 1D model, where the blue circles stand for anions, and the red ones stand for cations; big and small circles indicate doubly and singly charged ions, respectively. The white circles stand for NG atoms (He in our case). The insertion of He within both AB and A2B chains will increase the distance between ions. For the AB chain, He inserts between A-B repetitive units, increasing the distance between cations and anions, and increasing the Madelung energy. This weakens the stability of the systems. For A2B chain, He is inserted between A-B-A unit. The distance between two cations increases, which reduces the Madelung energy. This strengthens the stability of the chain after the He insertion. In this manner, A2B or AB2 compounds tend to form stable systems with He under high pressure. To test this model, the authors chose MgF2, MgO, Li2O, LiF, CaF2 and Na as candidate compounds to react with He. In Fig. 2, all the AB2-type compounds form stable structures with He, while MgO and LiF would not be able to. The Madelung energy and the internal energy are compared in Fig. 3. They follow similar trends in the studied pressure interval, for both the AB2 type and AB type compounds. For AB2 type compounds (MgF2, Li2O, CaF2, Na2+E2-), the Madelung energy change after the He insertion is greatly reduced at high pressure, stabilizing the insertion. For AB type compounds, the Madelung energy change stays positive for all pressures, as the internal energy does, inhibiting the He insertion in AB type compounds. This study unveiled the mechanism that drives the stability of He compounds with ionic crystals and predicts that there is a large family of He compounds that can be synthesized.

Their work has been published on Nature Communication. Also, their work has been reported by many professional websites such as X-mol, Scientific American, C&EN, and Chemistry World.

1.png

Fig. 1: 1D model of He insertion in chain-like AB- and A2B-type ionic compounds.

1.png

Fig. 2: Reaction enthalpies as defined by the above formula, between Helium and (a) MgF2, (b) Li2O, (c) CaF2, (d) MgO, (e) LiF, and (f) Na, plotted as a function of pressure. The pressure range in (a)-(e) is 0 to 300 GPa, and in (f) is 0 to 400 GPa. The reaction enthalpies are calculated respect to the He inserted compounds (dashed lines). Shaded areas indicate the intervals where He inserted compounds are stable.

1.png

Fig. 3. Relative changes per formula unit of the mechanical work PV, the internal energy E for the He-inclusion reaction, and the numerically determined Madelung energies (EM) of the He-included compounds, as a function of pressure. Shaded areas indicate the intervals where He inserted compounds are stable.


References:

[1]      Bartlett N. Xenon Hexafluoroplatinate(V) Xe+[Ptf6]-. Proc. Chem. Soc. London 6, 197-236 (1962).

[2]      Chernick C. L., Claassen H. H., Fields P. R., Hyman H. H., Malm J. G., Manning W. M., Matheson M. S., Quarterman L. A., Schreiner F., Selig H. H., Sheft I., Siegel S., Sloth E. N., Stein L., Studier M. H., Weeks J. L. & Zirin M. H. Fluorine Compounds of Xenon and Radon. Science 138, 136-138 (1962).

[3]      Smith D. F. Xenon Trioxide. J. Am. Chem. Soc. 85, 816-817 (1963).

[4]      Templeton D. H., Williamson M., Forrester J. D. & Zalkin A. Crystal and Molecular Structure of Xenon Trioxide. J. Am. Chem. Soc. 85, 817-817 (1963).

[5]      Khriachtchev L., Pettersson M., Runeberg N., Lundell J. & Rasanen M. A Stable Argon Compound. Nature 406, 874-876 (2000).

[6]      Brock D. S. & Schrobilgen G. Synthesis of the Missing Oxide of Xenon, XeO2, and Its Implications for Earth's Missing Xenon. J. Am. Chem. Soc. 133, 6265-6269 (2011).

[7]      Wang Q. & Wang X. F. Infrared Spectra of Ngbes (Ng = Ne, Ar, Kr, Xe) and BeS2 in Noble-Gas Matrices. J. Phys. Chem. A 117, 1508-1513 (2013).

[8]      Wang X. F., Andrews L., Brosi F. & Riedel S. Matrix Infrared Spectroscopy and Quantum-Chemical Calculations for the Coinage-Metal Fluorides: Comparisons of Ar-Auf, Ne-Auf, and Molecules MF2 and MF3. Chem. Eur. J 19, 1397-1409 (2013).

[9]      Dong X., Oganov A. R., Goncharov A. F., Stavrou E., Lobanov S., Saleh G., Qian G. R., Zhu Q., Gatti C., Deringer V. L., Dronskowski R., Zhou X. F., Prakapenka V. B., Konopkova Z., Popov I. A., Boldyrev A. I. & Wang H. T. A Stable Compound of Helium and Sodium at High Pressure. Nat. Chem. 9, 440-445 (2017).

[10]  Sun J., Pickard C. J. & Needs R. J. Formation of Noble Gas Compounds with Alkali Oxides and Sulfides under Pressure. arXiv:1409.2227, 5 (2014).

[11]  Liu H. Y., Yao Y. S. & Klug D. D. Stable Structures of He and H2O at High Pressure. Phys. Rev. B 91, 014102 (2015).

 

External Links:

https://www.nature.com/articles/s41467-018-03284-y.

http://www.x-mol.com/news/12324

https://www.scientificamerican.com/article/a-noble-gas-surprise-helium-can-form-weird-compounds/

https://cen.acs.org/physicalChemistry/theoreticalChemistry/Model-explains-stability-extreme-pressure/96/i14

https://www.chemistryworld.com/news/hundreds-of-helium-compounds-could-be-hiding-in-earths-mantle-/3008895.article


CSRC 新闻 CSRC News CSRC Events CSRC Seminars CSRC Divisions 孙昌璞院士个人主页