What are interstitial hydrides?
Well, first of all, a hydride is basically any compound that hydrogen forms with another element.
Hydrides are classified into three categories, according to the nature of their bonding. Saline hydrides have an ionic bond. Covalent hydrides have a covalent bond. Interstitial hydrides have a metallic bond. These compounds are non-stoichiometric (which means the number of atoms of the elements presented cannot be represented by a ratio of small whole numbers), and their composition varies with temperature and pressure.
One of the most common interstitial hydrides is palladium - which has been considered as a means to carry hydrogen for fuel cells.