Electrostatic force driven helium insertion into ammonia and water crystals under pressure

Abstract

Helium, ammonia and ice are among the major components of giant gas planets, and predictions of their chemical structures are therefore crucial in predicting planetary dynamics.Here we demonstrate a strong driving force originating from the alternation of the electrostatic interactions for helium to react with crystals of polar molecules such as ammonia and ice. We show that ammonia and helium can form thermodynamically stable compounds above 45 GPa, while ice and helium can form thermodynamically stable compounds above 300 GPa. The changes in the electrostatic interactions provide the driving force for helium insertion under high pressure, but the mechanism is very different to those that occur in ammonia and ice. This work extends the reactivity of helium into new types of compounds and demonstrates the richness of the chemistry of this most stable element in the periodic table.

Materials by design at high pressures

“…These compounds are therefore essential to the understanding of the interior structure of planets. 4,[170][171][172][173][174][175][176][177] Here, we summarize the theory-guided synthesis of noble gas compounds, paying particular attention to Xe and He compounds (Table 4). 8a and b).…”

“…Recent theoretical works have explored possible stable compounds formed by helium and the interior components of ice giants (e.g., CH 4 , NH 3 , and H 2 O). CSP methods have predicted many such compounds at high pressure and temperature conditions corresponding to those in planet interiors, such as He(NH 3 ) 2 , 172 He(H 2 O) 2 , 173,174 a superionic characteristic. Although these predicted compounds have yet to be conrmed, these theoretical results might be helpful for the understanding of the planet interior models and planets' evolution.…”

Noble Gases in Solid Compounds Show a Rich Display of Chemistry With Enough Pressure

“…The driving force of He insertion reactions is electrostatic (Liu et al, 2018;Bai et al, 2019). The key point is that all the above ionic compounds involved in He insertion possess unequal numbers of cations and anions, although the overall charge is neutral.…”

Seven-coordinated Silicon in a SiO2He Compound Formed under the Extreme Conditions of Planetary Interiors

“…The near complete absence of charge transfer between SiO 2 and He indicates that He atoms do not chemically bond with neighboring atoms. Other He-containing systems behave similarly: for example, FeO 2 He [24], H 2 O-He [26][27][28], NH 3 -He [28][29][30], HeN 4 [34], and Na 2 He [35]. Electronic band structure and DOS calculations show that P nma SiO 2 He is an insulator with a band gap of 7.59 eV at 610 GPa (Fig.…”

“…Ice giants are mainly composed of H 2 O, NH 3 , and CH 4 ; their atmospheres are rich in He [21,25]. Theoretical studies have shown that H 2 O [26][27][28], NH 3 [28][29][30], and CH 4 [31] can form stable compounds with He at high pressure, and firstprinciples molecular dynamics suggests that both H 2 O-He and NH 3 -He may exist in a superionic state in the interiors of icy planets, in addition to the plastic state of NH 3 -He and the plastic or partially diffusive state of CH 4 -He at lower pressures and temperatures. Theoritical study reported that He can form stable crystalline compounds with iron at terapascal pressures [32].…”