Understanding Interstitial Defects
An interstitial defect is a type of crystal imperfection where an atom occupies a site within the crystal lattice that is not a regular atomic position. This 'interstitial site' is typically much smaller than the atom, causing localized distortion and strain on the surrounding lattice atoms.
Formation and Classification
Interstitial defects can form as either 'self-interstitials' or 'impurity interstitials'. A self-interstitial occurs when an atom of the host material moves from its regular lattice site to an interstitial position. An impurity interstitial, conversely, involves a foreign atom lodging itself into an interstitial space within the host lattice.
Practical Example: Carbon in Steel
A prime example of an interstitial defect is carbon atoms in the iron crystal lattice of steel. Small carbon atoms fit into the interstitial spaces between larger iron atoms. This intentional introduction of interstitial carbon dramatically increases the hardness and strength of iron by hindering the movement of dislocations, which are another type of crystal defect.
Impact on Material Properties
The presence of interstitial defects significantly alters a material's properties. While they can enhance mechanical properties like tensile strength and hardness (as seen in steel), they often reduce ductility, electrical conductivity (due to electron scattering caused by lattice distortion), and thermal conductivity.