Point defects in materials.

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.