Formation of ammonia–helium compounds at high pressure

Abstract

Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres. Here, we report the possibility of helium being fixed in the upper mantles of these planets in the form of NH 3-He compounds. Structure predictions reveal two energetically stable NH 3-He compounds with stoichiometries (NH 3) 2 He and NH 3 He at high pressures. At low temperatures, (NH 3) 2 He is ionic with NH 3 molecules partially dissociating into (NH 2) − and (NH 4) + ions. Simulations show that (NH 3) 2 He transforms into intermediate phase at 100 GPa and 1000 K with H atoms slightly vibrate around N atoms, and then to a superionic phase at~2000 K with H and He exhibiting liquid behavior within the fixed N sublattice. Finally, (NH 3) 2 He becomes a fluid phase at temperatures of 3000 K. The stability of (NH 3) 2 He at high pressure and temperature could contribute to update models of the interiors of Uranus and Neptune.

The Helium Elemental and Isotopic Compositions of the Earth's Core Based on Ab Initio Simulations

“…The He atom essentially exhibits a spherical ELF distribution (Figure S7a) and there are no local ELF maxima along the directions He–Fe, He–O, and He–S, ruling out covalent interactions between He and other species. We examine the charge transfer based on Bader's topological analysis of the electron density (Bader, 1985). Bader charge analysis suggests a negligible charge transfer from the metallic aggregates to the He atom (Fe 50 : 0.13 e − ; Fe 40 O 10 : 0.09 e − ; and Fe 45 S 5 : 0.13 e − ) (Table S3), comparable to those in Na 2 He (Dong et al., 2017), FeO 2 He (J. Zhang et al., 2018), MgF 2 He (Z. Liu et al., 2018), NH 3 –He (Shi et al., 2020), H 2 O–He (H. Liu et al., 2015), and HeN 4 (Li et al., 2018) where the He atom does not form any local chemical bonds (i.e., keeps its inertness) but rather is located between like‐charged ions to shield their repulsive Coulomb interactions. We compute the projected electronic density of states (pDOS) to confirm the inertness of He in these metallic phases. We compare the two pDOSs for each phase with and without the He atom (Figure S7b).…”

“…Zhang et al, 2018), MgF 2 He (Z. Liu et al, 2018), NH 3 -He (Shi et al, 2020), H 2 O-He (H. Liu et al, 2015), and HeN 4 (Li et al, 2018) where the He atom does not form any local chemical bonds (i.e., keeps its inertness) but rather is located between like-charged ions to shield their repulsive Coulomb interactions. (iii) We compute the projected electronic density of states (pDOS) to confirm the inertness of He in these metallic phases.…”

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.…”

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

“…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].…”