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.

The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Gunnar 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:

It was suggested by early workers that:

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.

A more detailed knowledge of the structures of metals, and binary and ternary phases of metals and non metals shows that:

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-interstitial defects are interstitial defects which contain only atoms which are the same as those already present in the lattice.

The structure of interstitial defects has been experimentally determined in some metals and semiconductors.