Editing Hydride (section)

===Interstitial hydrides or metallic hydrides===
[[File:Metal Hydride for Hydrogen Storage-Ovonic.jpg|thumb|Metal hydride for hydrogen storage applications]]
Interstitial hydrides most commonly exist within metals or alloys. They are traditionally termed "compounds" even though they do not strictly conform to the definition of a compound, more closely resembling common alloys such as steel. In such hydrides, hydrogen can exist as either atomic or diatomic entities. Mechanical or thermal processing, such as bending, striking, or annealing, may cause the hydrogen to precipitate out of solution by degassing. Their bonding is generally considered [[metallic bonding|metallic]]. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usually [[Non-stoichiometric compound|non-stoichiometric]], with variable amounts of hydrogen atoms in the lattice. In materials engineering, the phenomenon of [[hydrogen embrittlement]] results from the formation of interstitial hydrides. Hydrides of this type form according to either one of two main mechanisms. The first mechanism involves the adsorption of dihydrogen, succeeded by the cleaving of the H-H bond, the delocalisation of the hydrogen's electrons, and finally the diffusion of the protons into the metal lattice. The other main mechanism involves the electrolytic reduction of ionised hydrogen on the surface of the metal lattice, also followed by the diffusion of the protons into the lattice. The second mechanism is responsible for the observed temporary volume expansion of certain electrodes used in electrolytic experiments.

[[Palladium]] absorbs up to 900 times its own volume of hydrogen at room temperatures, forming [[palladium hydride]]. This material has been discussed as a means to carry hydrogen for vehicular [[fuel cell]]s. Interstitial hydrides show certain promise as a way for safe [[hydrogen storage]]. Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice (in an fcc lattice there is one octahedral hole per metal atom). The limit of absorption at normal pressures is PdH0.7, indicating that approximately 70% of the octahedral holes are occupied.<ref>[[Palladium hydride]]</ref>

Many interstitial hydrides have been developed that readily absorb and discharge hydrogen at room temperature and atmospheric pressure. They are usually based on [[intermetallic]] compounds and solid-solution alloys. However, their application is still limited, as they are capable of storing only about 2 weight percent of hydrogen, insufficient for automotive applications.<ref>{{cite journal |title=Materials for hydrogen storage|journal= Materials Today|volume= 6|issue= 9|year=2003|pages=24–33|doi=10.1016/s1369-7021(03)00922-2|last1= Züttel|first1= Andreas|doi-access= free}}</ref>
[[File:PAHCRU.png|thumb|Structure of {{chem2|[HRu6(CO)18]-}}, a metal cluster with an interstitial hydride ligand (small turquoise sphere at center).<ref>{{cite journal |title=Direct location of the interstitial hydride ligand in [HRu6(CO)18]– by both X-ray and neutron analyses of [Ph4As][HRu6(CO)18] by Both X-ray and Neutron Analyses of [Ph4As][HRu6(CO)18]|journal=Journal of the Chemical Society, Chemical Communications|issue=7|year=1980|page=295|doi=10.1039/c39800000295|last1=Jackson|first1=Peter F.|last2=Johnson|first2=Brian F. G.|last3=Lewis|first3=Jack|last4=Raithby|first4=Paul R.|last5=McPartlin|first5=Mary|last6=Nelson|first6=William J. H.|last7=Rouse|first7=Keith D.|last8=Allibon|first8=John|last9=Mason|first9=Sax A.}}</ref> ]]