3.091 – Introduction to Solid State Chemistry Lecture Notes No. 6 ...

LN–6k = the Boltzmann constant A = proportionality constant B. Point Defects in “Pure” Metallic SystemsPoint defects in “pure” crystalline metals are defects of atomic dimensions, such asimpurity atoms, the absence of a matrix atom and/or the presence of a matrix atom inthe wrong place. Some of these point defects are shown in fig. 2. An impurity atom thatsubstitutionaloccupies a normal latticesite is called ainterstitialsubstitutional impurityatom and an impurityatom found in thevacancyselfinterstitialinterstice between matrixatoms is called aninterstitial impurity atom.Whether a foreign atomFigure 2 Point defects in crystalline solidswill occupy a substitutionalor interstitial site depends largely on the size of the atom relative to the size of the site.Small atoms are usually interstitial impurities, while larger atoms are usuallysubstitutional impurities.A vacancy is an atom site, normally occupied in the perfect crystal, from which an atomis missing. Often the term “vacancy” is used to denote a so-called Schottky defect,which is formed when an atom or an ion leaves a normal lattice site and repositionsitself in a lattice site on the surface of the crystal. This may be the result of atomicrearrangement in an existing crystal at a high temperature when atomic mobility is highbecause of increased thermal vibrations. A vacancy may also originate in the process5