- Interstitial defect
Interstitials are a variety of
crystallographic defects , i.e. atoms which occupy a site in thecrystal 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 somecrystal s can occupy interstitial sites in an energetically favourable configuration, such as hydrogen in palladium.Self-interstitials
Self-interstitial defects are interstitial defects which contain only atoms which are the same as those already present in the lattice.
right|thumb|Structure of self-interstitial in some common metals. The left-hand side of each crystal type shows the perfect crystal and the right-hand side the one with a defect.The structure of interstitial defects has been experimentally determined in some metals and
semiconductors .Contrary to what one mightintuitively 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 theprincipal lattice directions. For instance,in several common FCC metals such as Cu, Ni and Pt theground state structure of the self-interstitial isthe split 100 interstitial structure, where two atoms aredisplaced in a positive and negative 100 direction fromthe lattice site. In BCC Fe the ground stateinterstitial structure is similarly a 110 splitinterstitial.
Curiously enough, these split interstitials are oftencalled dumbbell interstitials, because plotting the twoatoms forming the interstitial with two large spheresand a thick line joining them makes the structure resemblea
dumbbell weight-lifting device.In other BCC metals than Fe, the ground state structure is believedto be the so called 111 crowdion interstitial (although the issueis still not well established), which can beunderstood 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 anddifferent charge states may have different structures. For instance, in Si the interstitial may either have a split 110 structure or a
tetrahedral trulyinterstitial one (Watkins 1997).Impurity interstitials
Small impurity interstitials atoms are usually on trueoff-lattice sites between the lattice atoms.Such sites can be characterized by the
symmetry of theinterstitial atom position with respect to itsnearest lattice atoms. For instance, an impurity atomI with 4 nearest lattice atom A neighbours (at equal distances)in a FCC lattice is ina tetrahedral symmetry position, and thus can be called atetrahedral interstitial.Large impurity interstitials can also be insplit interstitial configurations together with alattice atom, similar to those of the self-interstitial atom.
References
(Ehrhart 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)
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