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Stabilization of Metallic, Excitonic Insulator, and Superionic Phases in Helium-Rare Gas Compounds at Sub-Terapascal Pressures

Cong Liu, Jordi Boronat, Claudio Cazorla

TL;DR

The study addresses how inert helium and rare gases can form stable compounds under extreme pressures and how these phases influence planetary interiors. The authors apply crystal-structure prediction and first-principles calculations to map the zero-temperature phase diagram of binary He–RG systems up to $1$ TPa, assessing thermodynamic and vibrational stability as well as electronic and ionic transport properties. They predict multiple stable stoichiometries, including ArHe, ArHe2, KrHe, KrHe2, XeHe, and XeHe2, with ground-state structures such as $I4/mcm$, $P6/mmm$, $Pnma$, and $P6/mmm$, and show that Xe-containing systems can become metallic around $510$–$585$ GPa and host excitonic insulator phases, while XeHe and XeHe2 also exhibit superionic helium diffusion at high $P$–$T$. This work demonstrates that mixing helium with heavier rare gases provides a practical route to realize metallic, excitonic-insulator, and superionic phases at experimentally accessible pressures, offering new insights for condensed-matter physics and planetary science.

Abstract

Helium and rare gases (RG: Ne, Ar, Kr, Xe) are typically considered chemically inert, yet under the extreme pressures of planetary interiors they may form compounds with unexpected properties. Using crystal structure prediction and first-principles calculations, we mapped the phase diagram of binary He-RG systems up to $1$ TPa. We identify several previously unknown stoichiometric compounds that are both thermodynamically and vibrationally stable at sub-terapascal pressures, within the reach of modern high-pressure experiments. In particular, AHe$_{2}$ and AHe (A: Ar, Kr, Xe) adopt previously unreported orthorhombic, hexagonal and cubic phases that remain stable over wide pressure ranges. We further find that He-Xe systems host metallic and excitonic insulator phases at pressures nearly an order of magnitude lower than those required for pure helium, offering a pathway to realize these exotic quantum states experimentally. Finite-temperature simulations also reveal superionic He-Xe phases, in which helium ions diffuse either anisotropically or isotropically depending on the host lattice. These findings constitute the first prediction of helium-based systems that combine metallicity and superionicity, with profound implications for energy transport and planetary dynamo processes. Overall, our results demonstrate that mixing helium with heavier rare gases provides an effective strategy to stabilize metallic, excitonic insulator, and superionic phases at experimentally accessible pressures, opening new research directions for condensed matter physics and planetary science.

Stabilization of Metallic, Excitonic Insulator, and Superionic Phases in Helium-Rare Gas Compounds at Sub-Terapascal Pressures

TL;DR

The study addresses how inert helium and rare gases can form stable compounds under extreme pressures and how these phases influence planetary interiors. The authors apply crystal-structure prediction and first-principles calculations to map the zero-temperature phase diagram of binary He–RG systems up to TPa, assessing thermodynamic and vibrational stability as well as electronic and ionic transport properties. They predict multiple stable stoichiometries, including ArHe, ArHe2, KrHe, KrHe2, XeHe, and XeHe2, with ground-state structures such as , , , and , and show that Xe-containing systems can become metallic around – GPa and host excitonic insulator phases, while XeHe and XeHe2 also exhibit superionic helium diffusion at high –. This work demonstrates that mixing helium with heavier rare gases provides a practical route to realize metallic, excitonic-insulator, and superionic phases at experimentally accessible pressures, offering new insights for condensed-matter physics and planetary science.

Abstract

Helium and rare gases (RG: Ne, Ar, Kr, Xe) are typically considered chemically inert, yet under the extreme pressures of planetary interiors they may form compounds with unexpected properties. Using crystal structure prediction and first-principles calculations, we mapped the phase diagram of binary He-RG systems up to TPa. We identify several previously unknown stoichiometric compounds that are both thermodynamically and vibrationally stable at sub-terapascal pressures, within the reach of modern high-pressure experiments. In particular, AHe and AHe (A: Ar, Kr, Xe) adopt previously unreported orthorhombic, hexagonal and cubic phases that remain stable over wide pressure ranges. We further find that He-Xe systems host metallic and excitonic insulator phases at pressures nearly an order of magnitude lower than those required for pure helium, offering a pathway to realize these exotic quantum states experimentally. Finite-temperature simulations also reveal superionic He-Xe phases, in which helium ions diffuse either anisotropically or isotropically depending on the host lattice. These findings constitute the first prediction of helium-based systems that combine metallicity and superionicity, with profound implications for energy transport and planetary dynamo processes. Overall, our results demonstrate that mixing helium with heavier rare gases provides an effective strategy to stabilize metallic, excitonic insulator, and superionic phases at experimentally accessible pressures, opening new research directions for condensed matter physics and planetary science.

Paper Structure

This paper contains 6 sections, 9 figures.

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Generalized zero-temperature phase diagram
  4. Electronic properties
  5. Ionic transport properties
  6. Conclusions

Figures (9)

  • Figure 1: Generalized zero-temperature phase diagram of He--RG compounds under pressure.a. Pressure--composition phase diagram of He--RG alloys; all compounds are thermodynamically stable against decomposition into secondary and elemental RG phases. Cyan shaded regions denote metallic behaviour. Crystal structures of b. Ne$_2$He in the $I4/mcm$ phase, c. NeHe$_2$ in the $P6_{3}/mmc$ phase, d. NeHe$_2$ in the $Fd\overline{3}m$ phase, e. ArHe$_2$ in the $P6/mmm$ phase, f. ArHe in the $Pnma$ phase, g. Ar$_2$He in the $I4/mcm$ phase, h. ArHe in the $Cmcm$ phase, i. KrHe in the $Cmcm$ phase, j. KrHe in the $Pnma$ phase, k. KrHe$_{2}$ in the $P6/mmm$ phase, l. XeHe in the $Pnma$ phase, m. XeHe in the $P6_{3}/mmc$, n. XeHe in the $Fm\overline{3}m$ phase, and o. XeHe$_{2}$ in the $P6/mmm$ phase.
  • Figure 2: Phonon spectra of highly compressed He--RG compounds. Phonon dispersion relations are well-behaved in all cases, that is, are real and positively defined, demonstrating vibrational stability of the reported crystal structures.
  • Figure 3: Formation enthalpy of He--Ne compounds under pressure. The NeHe composition, not shown in the figure, is thermodynamically unstable against elemental decomposition into Ne and He. a. Ne$_{2}$He composition. b. NeHe$_{2}$ composition. Ball-stick representation of the c. tetragonal $I4/mcm$, d. hexagonal $P6_{3}/mmc$, and e. cubic $Fd\overline{3}m$ phases. Ne and He atoms are represented by magenta and white spheres, respectively.
  • Figure 4: Formation enthalpy of He--Ar compounds under pressure.a. Ar$_{2}$He composition. b. ArHe$_{2}$ composition. c. ArHe$_{2}$ composition. Ball-stick representation of the d. orthorhombic $Cmcm$, e. orthorhombic $Pnma$, and f. hexagonal $P6/mmm$ phases. Ar and He atoms are represented by green and white spheres, respectively.
  • Figure 5: Formation enthalpy of He--Kr compounds under pressure. The Kr$_{2}$He composition, not shown in the figure, is thermodynamically unstable against elemental decomposition into Kr and He. a. KrHe composition. b. KrHe$_{2}$ composition. Ball-stick representation of the c. orthorhombic $Cmcm$, and d. hexagonal $P6/mmm$ phases. Kr and He atoms are represented by violet and white spheres, respectively.
  • ...and 4 more figures