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 ¹H and ²H 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.