A stable compound of helium and sodium at high pressure

Abstract

Recent developments in crystal structure prediction, in particular, the powerful evolutionary algorithm USPEX [1,2], enable reliable prediction of stable compounds formed by given elements. At normal conditions such calculations produce the well-known stable compounds: e.g., NaCl as the only compound of Na and Cl, or MgO as the only stable compound of Mg and O. At high pressures and in low-dimensional materials, unexpected phenomena have been predicted-then experimentally verified. I will discuss several recent examples: 1. Discovery of two new stable high-pressure compounds of helium, Na2He and Na2HeO (Na2He has been synthesized experimentally) [3]. This discovery has implications for both fundamental chemistry and planetary sciences. 2. Formation of new stable sodium chlorides: Na3Cl, Na2Cl, Na3Cl2, NaCl3, NaCl7 [4], Na4Cl3 [5], and a large number of new stable potassium chlorides [6]. These predictions were verified experimentally [5,6] and are still not fully understood. 3. New stable magnesium oxides: Mg3O2 and MgO2 [7] and MgO3 [8], and silicon oxides SiO and SiO3 [8]. Among these predictions, stability of MgO2 has already been experimentally confirmed [9]. These predictions may have implications for planetary chemistry. 4. USPEX-based prediction of the Cui group [10] and experimental verification of Eremets group [11] of a new high-temperature superconductor-cubic H3S. This discovery opens new hopes for room-temperature superconductivity. 5. Prediction [12] that dominant silicon oxide nanoparticles at normal conditions (ambient P-T, and normal air) will be oxygen-enriched and magnetic: e.g. Si7O19. This may explain well-documented carcinogenic activity of fine silica dust. Future avenues for explanation and generalization of these phenomena will be discussed. 1] Oganov A.R. et al, J.

On the position of helium and neon in the Periodic Table of Elements

“…Consequently, with over half a thousand of currently known compounds of noble gases, their chemistry has been extensively reviewed during the last three decades (Laszlo and Schrobilgen 1988;Lehmann et al 2002;Schrobilgen and Moran 2003;Grochala 2007;Grochala et al 2011;Brock et al 2013;Hope 2013;Haner and Schrobilgen 2015). Moreover, the experimental and theoretical chemistry and physics of noble gas compounds at high pressure started to flourish (Kim et al 2010;Kurzydłowski et al 2011;Dong et al 2017;Zhu et al 2013Zhu et al , 2014 Table of… and Schwerdtfeger 2014; Li et al 2015;Dewaele et al 2016;Kurzydłowski and ZaleskiEjgierd 2016), following the preliminary exploration a decade ago (Grochala 2007). One particularly interesting case is that of Na 2 He at very high pressures, which is called ''stable'' and ''compound'' by the authors (Dong et al 2017), despite the fact that (1) high pressure is necessary to keep this species intact, and that (2) diverse bonding analyses point out to the lack of chemical bonding in this system.…”

Novel magnesium borides and their superconductivity

“…This method has the capability of finding possible compositions and the corresponding stable and metastable structures at given pressures, and successfully predicted new phases of MgB 2 at high pressure 18 and new stable phases of different systems like Na-Cl, boron and Na-He 16,28,29 . High-temperature superconductivity in hydrogen-rich compounds, e.g., Sn-H 30 and Ge-H 31 were also studied.…”

Primary Research Data and Scholarly Communication

“…However, if the negative results of the past were not questioned, would we have had publications on arsenic-based life [19] (subsequently proved to be an invalid conclusion), or on compounds of helium? [20] On the other hand, despite these cautions, the benefi ts of access to both primary research data and big data analytics will be great. We are moving beyond the use of technology to simply collect, store, process, and preserve data to using it to analyze and interpret data.…”