In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those already present they are known as a self-interstitial defect. Alternatively, small atoms in some crystals may occupy interstitial sites, 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. The presence of interstitial defects can modify the physical and chemical properties of a material. History The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Hägg. Transition metals generally crystallise in either the hexagonal close packed or face centered cubic structures, both of which can be considered to be made up of layers of hexagonally close packed atoms. In both of these very similar lattices there are two sorts of interstice, or hole: Two tetrahedral holes per metal atom, i.e. the hole is between four metal atoms One octahedral hole per metal atom, i.e. the hole is between six metal atoms It was suggested by early workers that: the metal lattice was relatively unaffected by the interstitial atom the electrical conductivity was comparable to that of the pure metal there was a range of composition the type of interstice occupied was determined by the size of the atom These were not viewed as compounds, but rather as solutions, of say carbon, in the metal lattice, with a limiting upper “concentration” of the smaller atom that was determined by the number of interstices available. Current A more detailed knowledge of the structures of metals, and binary and ternary phases of metals and non metals shows that: generally at low concentrations of the small atom, the phase can be described as a solution, and this approximates to the historical description of an interstitial compound above. at higher concentrations of the small atom, phases with different lattice structures may be present, and these may have a range of stoichiometries. One example is the solubility of carbon in iron. The form of pure iron stable between 910 °C and 1390 °C, γ-iron, forms a solid solution with carbon termed austenite which is also known as steel. Self-interstitials