Interstitials are a variety of crystallographic defects where atoms assume a normally unoccupied site in the crystal structure. In interstitial defects two or more atoms may share one lattice site, thereby increasing its total energy. Alternatively small atoms in some crystals may occupy interstitial sites in energetically favorable configurations, such as hydrogen in palladium. Interstitials can be produced by bombarding a crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium.