Point defects
- 1. MATERIAL ENGINEERING TOPIC: DEFECTS IN SOLIDS
- 2. DEFECTS/IMPERFECTION S The arrangement of atoms or molecules in most crystalline materials is not perfect. Or Crystals inherently possess imperfections, and referred to as crystalline defects.
- 3. POINT DEFECTS Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension. Strict limits for how small a point defect is are generally not defined explicitly, typically, however, these defects involve at most a few extra or missing atoms. Larger defects in an ordered structure are usually considered dislocation ( is a crystallographic defect or irregularity, within a crystal structure) loops. For historical reasons, many point defects, especially in ionic crystals, are called centers: for example a vacancy in many ionic solids is called a luminescence center, a color center, or F-center ( Farbe center or color center-from the original German Farbzentrum; Farbe means color, and zentrum center) is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons). These dislocations permit ionic transport through crystals leading to electrochemical reactions. These are frequently specified using Kröger–Vink Notation(is a set of conventions that are used to describe electric charge and lattice position for point defects species in crystals).
- 4. TYPES OF POINT DEFECTS VACANCY DEFECT Vacancy defect: It is the simplest point defect. In this system, an atom is missing from its regular atomic site. Vacancies are formed during solidification due to vibration of atoms.
- 5. INTERSTITIAL DEFECT 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.
- 6. SUBSTITUTIONAL DEFECT A Substitutional defect is introduced when one atom or ion is replaced by a different type of atom or ion.
- 7. FRENKEL DEFECT A Frenkel defect is also known as Frenkel pair or Frenkel disorder, it is a type of point defect in a crystal lattice. The defect forms when an atom or smaller ion(usually cation) leaves its place in the lattice, creating a vacancy, and becomes an interstitial by lodging (accommodation) in a nearby location.
- 8. SCHOTTKY DEFECT A Schottky defect is a type of point defect in a crystal lattice named after Walter. H.Schotty. In non-ionic crystals it means a lattice vacancy defect. In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid.