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

The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-$$T_{\rm{c}}$$ T c superconductivity at high pressure. Recently, cerium hydride $$\hbox {CeH}_9$$ 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 $$\hbox {LaH}_{{10}}$$ LaH 10 and $$\hbox {YH}_6$$ YH 6 . Based on density-functional theory calculations, we find that the Ce 5p semicore and 4f/5d valence states strongly hybridize with the H 1s state, while a transfer of electrons occurs from Ce to H atoms. Further, we reveal that the delocalized nature of Ce 4f 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 $$\hbox {CeH}_9$$ CeH 9 .