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Metallic palladium
Palladium hydride is palladium metal with hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdH. At room temperature, palladium hydrides may contain two crystalline phases, α and β (also called α′). Pure α phase exists at x < 0.017 while pure β phase exists at x > 0.58; intermediate x values correspond to α β mixtures. Hydrogen absorption by palladium is reversible and therefore has been investigated for hydrogen storage. Palladium electrodes have been used in some cold fusion experiments, under the theory that hydrogen can be "squeezed" between palladium atoms to help it fuse at lower temperatures than normal.

The absorption of hydrogen gas by palladium was first noted by T. Graham in 1866 and absorption of electrolytically produced hydrogen, where hydrogen was absorbed into a palladium cathode, was first documented in 1939. Hydrogen can be absorbed into the metal hydride and then desorbed back out for thousands of cycles. Researchers look for ways to extend the useful life of palladium storage.

The absorption of hydrogen produces two different phases, both of which contain palladium metal atoms in a face centered cubic (fcc, rocksalt) lattice, which is the same structure as pure palladium metal. At low concentrations up to PdH the palladium lattice expands slightly, from 388.9 pm to 389.5 pm. Above this concentration the second phase appears with a lattice constant of 402.5 pm. Both phases coexist until a composition of PdH when the alpha phase disappears. The absorption of hydrogen is reversible, and hydrogen rapidly diffuses through the metal lattice. Metallic conductivity reduces as hydrogen is absorbed, until at around PdH the solid becomes a semiconductor. Additionally, empty Pd states, that are below the fermi energy, are also lowered with the presence of H.

PdH is a superconductor with a transition temperature T of about 9 K for x = 1. (Pure palladium is not superconducting.) Drops in resistivity vs. temperature curves were observed at higher temperatures (up to 273 K) in hydrogen rich (x ~ 1), nonstoichiometric palladium hydride and interpreted as superconducting transitions. These results have been questioned and have not been confirmed thus far. A great advantage of palladium hydride over many other hydride systems is that palladium hydride does not need to be highly pressurized to become superconducting. This makes measurements easier and gives more opportunity for different kinds of measurements (many superconducting materials require extreme pressure in order to superconduct, on the order of 100 GPa). The reason for such a behaviour and the particular structure of trimers has been analyzed.