A novel metastable structure and superconductivity of hydrogen-rich compound CdH₆ under pressure

The particle swarm optimization algorithm has predicted a series of binary cadmium hydrides that could be dynamically stable at pressures between 100 GPa and 300 GPa. These low-energy phases are composed of both Cd atoms and H₂ molecules. Here, we propose a hitherto unknown metastable Cmcm-CdH₆ phase, consisting of one-dimensional zigzag graphite-like hydrogenic H₆ chains, quasimolecular H₂ units and Cd atoms, which is metallic above 290 GPa. Due to H₂σ → Cd d donation and Cd d → H₂σ^* back-donation, the electrons occupy antibonding orbitals for both types of hydrogen atoms. This results in weakened chemical bonds in the Cmcm-CdH₆ phase via a Kubas-like mechanism, promoting the emergence of high superconductivity, which is estimated to be up to ∼ 60 K at 290 GPa. This work will inspire the search for superconductivity in materials based on group IIB hydrides under pressure.