Interstitial defect

Interstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites. They are generally high energy configurations. Small atoms in some crystals can occupy interstitial sites in an energetically favourable configuration, such as hydrogen in palladium.

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Self-interstitials

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.

Contrary to what one might intuitively expect, most self-interstitials in metals with a known structure have a so called 'split' structure, in which two atoms share the same lattice site (Schilling 1978, Erhart 1991). Typically the center of mass of the two atoms is at the lattice site, and they are displaced symmetrically from it along one of the principal lattice directions. For instance, in several common FCC metals such as Cu, Ni and Pt the ground state structure of the self-interstitial is the split 100 interstitial structure, where two atoms are displaced in a positive and negative 100 direction from the lattice site. In BCC Fe the ground state interstitial structure is similarly a 110 split interstitial.

Curiously enough, these split interstitials are often called dumbbell interstitials, because plotting the two atoms forming the interstitial with two large spheres and a thick line joining them makes the structure resemble a dumbbell weight-lifting device.

In other BCC metals than Fe, the ground state structure is believed to be the so called 111 crowdion interstitial (although the issue is still not well established), which can be understood as a long chain (typically some 10-20) of atoms along the 111 lattice direction, compressed compared to the perfect lattice such that the chain contains one extra atom.

In semiconductors the situation is more complex, since defects may be charged and different charge states may have different structures. For instance, in Si the interstitial may either have a split 110 structure or a tetrahedral truly interstitial one (Watkins 1997).

Impurity interstitials

Small impurity interstitials atoms are usually on true off-lattice sites between the lattice atoms. Such sites can be characterized by the symmetry of the interstitial atom position with respect to its nearest lattice atoms. For instance, an impurity atom I with 4 nearest lattice atom A neighbours (at equal distances) in a FCC lattice is in a tetrahedral symmetry position, and thus can be called a tetrahedral interstitial.

Large impurity interstitials can also be in split interstitial configurations together with a lattice atom, similar to those of the self-interstitial atom.

References

(Erhart 1991) P. Ehrhart, Properties and interactions of atomic defects in metals and alloys, edited by H. Ullmaier, Landolt-Börnstein, New Series III vol. 25 ch. 2 p. 88- (Springer, Berlin, 1991)

(Schilling 1978) Self-interstitial atoms in metals, Journal of Nuclear Materials 69&70 (1978) p. 465.

(Watkins 1997) G. D. Watkins, Native defects and their interactions with impurities in silicon, in Defects and Diffusion in Silicon Processing, edited by T. Diaz de la Rubia, S. Coffa, P. A. Stolk and C. S. Rafferty, MRS Symposium Proceedings vol. 469 p. 139 (Materials Research Society, Pittsburg 1991)