The Electronic State of Hydrogen in the α Phase of the Hydrogen‐Storage Material PdH(D) x : Does a Chemical Bond Between Palladium and Hydrogen Exist?

The palladium–hydrogen system is one of the most famous hydrogen‐storage systems. Although there has been much research on β‐phase PdH(D) x , we comprehensively investigated the nature of the interaction between Pd and H(D) in α‐phase PdH(D) x ( x <0.03 at 303 K), and revealed the existence of Pd−H(D) chemical bond for the first time, by various in situ experimental techniques and first‐principles theoretical calculations. The lattice expansion, magnetic susceptibility, and electrical resistivity all provide evidence. In situ solid‐state 1 H and 2 H NMR spectroscopy and first‐principles theoretical calculations revealed that a Pd−H(D) chemical bond exists in the α phase, but the bonding character of the Pd−H(D) bond in the α phase is quite different from that in the β phase; the nature of the Pd−H(D) bond in the α phase is a localized covalent bond whereas that in the β phase is a metallic bond.