Bonding insights for hydrogen-based superconductors: Nearly free electrons from rare earth 4f orbitals

Abstract

The moderate synthesis conditions of CeH9 under cold compression demonstrate its remarkable chemical precompression effect, while the single occupied 4f electron configuration of Ce contributes to the rich chemical and physical properties observed in cerium-bearing compounds. In collaboration with machine-learning-accelerated crystal structure prediction, we predicted a thermodynamically stable superconducting phase CeSc2H24 at 200 GPa. With considering more accurate quantum anharmonic effects, the thermodynamically stable pressure of CeSc2H24 is further decreased to 117 GPa, at which pressure it exhibits a high superconducting critical temperature (Tc) of ∼210 K. We found a different framework of bonding theory for hydrogen-based superconductors where strong metallic bonds in hydrides originate from near-free electronic states between the highest effective energy level and the Fermi level. Specifically, the delocalization of Ce-4f orbitals in CeSc2H24 provided a number of near-free electrons, thereby greatly enhancing metallic bonds and making the system energetically much more favorable. Our theory for CeSc2H24 emphasizes the non-negligible role of metallic bonding in hydrides, which paves the way for developing high-temperature superconductors under experimentally accessible pressures.

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