Defects in solids

Frenkel defect – Point defect where an atom / ion has been displaced into an interstital site. eg In AgCl some Ag⁺ ions occupy tetrahedral sites (fig. 16) which are normally unoccupied. Stoichiometry is unchanged.

Encountered in open structures (wurtzite, sphalerite, etc) where coordination numbers are low and open structure provides room for interstital sites to be occupied.

Frenkel defect shown opposite (fig. 17).

Extrinsic point defects:

These are inevitable because perfect purity is unattainable in crystals of any significant size.