What are the imperfections of the atomic arrangements that have a significant effect on the material behaviors. Give an example of each
A crystal is a perfect geometric arrangement of motifs or basis (an atom or a group of atoms) on a three dimensional periodic arrangements of points called lattice. Crystal = Lattice + basis.
Any deviation from these perfect ideal arrangement will lead to crystal imperfections or defects.
Classification of defects or imperfections: We will Classify the defects according to their dimensionality and they are:
1. 0-dimension (Point defects): These are the lattice error at isolated points and are formed due to imperfect packing of atoms during crystallization or due to thermal vibrations of atoms at high temperatures. Point defects can be further subdivided into 4 basic sub-classes:
a. Vacancies: This is the simplest point defect in a crystal caused due to missing atom or ion (or a vacant atomic site). b. Frenkel Defect: This is also a vacancy defect caused if an atom or ion is removed from its specified lattice site to the interstitial position ( a position in the 3-D space where there is no lattice point and no atoms or ions are meant to sit there). c. Schottky defect: This also a vacancy defect caused if we remove an atom or ion from a interior lattice point to the surface of the crystal. d. Substitutional or interstitial impurity: Substitutional impurity is created when a foreign atom substitute atoms at crystal sites. Interstitial impurity occurs when foreign atom (foreign to the main crystal) occupies a interstitial position.
2. 1-dimension ( Line Defects): Line imperfection or dislocation are one dimensional defects along which some atoms are misaligned. They are basically of two types:
a. Edge dislocation: A perfect crystal can be viewed as consisting of set of vertical planes parallel to one another and and the side faces. If one of these vertical plane do not fully extend from top to bottom of the crystal and ends somewhere midway, then a Edge dislocation is produced. b. Screw dislocation: It is produced to to application of sheer stress, which leads to shifting of the upper portion as compared to the lower portion by one atomic distance.
3. 2-dimension ( Surface defects): These are two dimensional defects which arises due to change in stacking of atomic planes or at the boundary. They can be mainly divided into two types:
a. External surface imperfection: The external surface itself is a deviation from ideal 3-D crystal concept and at the surface there are bond breakings. b. Internal surface imperfection: They show up as Grain boundaries, tilt boundaries, twin boundaries and stacking faults.
4. 3-dimension (Volume defects): These are three dimensional defects or imperfections mainly caused (a) small electrostatic dissimilarities between the stacking stacking sequences of closed packed planes in metals (example: cracks) (b) when a cluster of atoms are missing or vacancies makes a cluster.
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