Origin of enhanced chemical precompression in cerium hydride CeH$_{9}$

TL;DR

This study uses density-functional theory to explain how the unique electronic interactions in cerium hydride CeH$_9$ lead to its high-pressure stability and lower synthesis pressure, highlighting the role of Ce 4f electrons in chemical precompression.

Contribution

It reveals the electronic origin of enhanced chemical precompression in CeH$_9$, showing the hybridization and delocalization of Ce electrons influence its structural stability at lower pressures.

Findings

Ce 5p semicore and 4f/5d valence states hybridize with H 1s.

Electron transfer occurs from Ce to H atoms.

Delocalized Ce 4f electrons aid chemical precompression.

Abstract

The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-$T_{\rm c}$ superconductivity at high pressure. Recently, cerium hydride CeH$_9$ composed of Ce-encapsulated clathrate H cages was synthesized at much lower pressures of 80$-$100 GPa, compared to other experimentally synthesized rare-earth hydrides such as LaH$_{10}$ and YH$_6$. Based on density-functional theory calculations, we find that the Ce 5$p$ semicore and 4$f$/5$d$ valence states strongly hybridize with the H 1$s$ state, while a transfer of electrons occurs from Ce to H atoms. Further, we reveal that the delocalized nature of Ce 4$f$ electrons plays an important role in the chemical precompression of clathrate H cages. Our findings not only suggest that the bonding nature between the Ce atoms and H cages is characterized as a mixture of ionic and covalent, but also have important implications for understanding the origin of enhanced chemical precompression that results in the lower pressures required for the synthesis of CeH$_9$.