Formation mechanism of chemically precompressed hydrogen clathrates in metal superhydrides

TL;DR

This paper uses first-principles calculations to reveal how interstitial excess electrons in metal frameworks facilitate the formation of chemically precompressed hydrogen clathrates in superhydrides, explaining their stability at lower pressures.

Contribution

It uncovers the role of interstitial excess electrons in metal frameworks in promoting chemical precompression of hydrogen cages in superhydrides, a novel insight into their formation mechanism.

Findings

Interstitial excess electrons are more abundant in ThH$_{10}$ and ThH$_9$ frameworks.

Stronger Coulomb attraction leads to larger chemical precompression in ThH$_{10}$ and ThH$_9$.

The formation mechanism applies to CeH$_9$, explaining its high-pressure stability.

Abstract

Recently, the experimental discovery of high-$T_c$ superconductivity in compressed hydrides H$_3$S and LaH$_{10}$ at megabar pressures has triggered searches for various superconducting superhydrides. It was experimentally observed that thorium hydrides, ThH$_{10}$ and ThH$_9$, are stabilized at much lower pressures compared to LaH$_{10}$. Based on first-principles density-functional theory calculations, we reveal that the isolated Th frameworks of ThH$_{10}$ and ThH$_9$ have relatively more excess electrons in interstitial regions than the La framework of LaH$_{10}$. Such interstitial excess electrons easily participate in the formation of anionic H cage surrounding metal atom. The resulting Coulomb attraction between cationic Th atoms and anionic H cages is estimated to be stronger than the corresponding one of LaH$_{10}$, thereby giving rise to larger chemical precompressions in ThH$_{10}$ and ThH$_9$. Such a formation mechanism of H clathrates can also be applied to another experimentally synthesized superhydride CeH$_9$, confirming the experimental evidence that the chemical precompression in CeH$_9$ is larger than that in LaH$_{10}$. Our findings demonstrate that interstitial excess electrons in the isolated metal frameworks of high-pressure superhydrides play an important role in generating the chemical precompression of H clathrates.