crystal physics for engineers

Unit I
Crystal Physics
Introduction
The matter is usually regarded to exist in solid state or fluid state. All the materials are composed of atoms
and molecules. A solid is an essentially an ordered array of atoms, bound together by electric forces to form a very
large molecule. There are three different types of solids. Crystalline, poly crystalline and amorphous.
In a crystal, atoms are arranged into a regular periodically repeated structure that extends throughout the
whole sample. The atoms are said to have long range order. Poly crystalline material is composed of many small
crystals or grains of somewhat irregular size. In an amorphous solid a long range order is absent. (i.e.,) they have
short range order. There is no periodicity in which atoms are arranged in space. They are also regarded as super
cooled liquids.
Examples : metallic crystals : copper, silver, aluminum etc.,
Non metallic crystals: Germanium, silicon
Amorphous or non crystalline materials: glass, rubber, plastic




Difference between crystalline and amorphous material.


S.No Crystalline Amorphous
1 Regular arrangement of atoms Random arrangement of atoms does not
gives definite and geometric shape. give definite and Geometrical shape.

2 They are anisotropic They are isotropic

3 They have sharp Melting Point They do not have sharp Melting Point
4 They possess internal symmetry They do not possess internal symmetry

,‘What is Crystal Physics?
Crystal Physics’ or ‘Crystallography’ is a branch of physics that deals with the study of all possible
types of crystals and the physical properties of crystalline solids by the determination of their actual
structure by using X-rays, neutron beams and electron beams.
CLASSIFICATION OF SOLIDS
Solids can broadly be classified into two types based on the
arrangement of units of matter.
The units of matter may be atoms, molecules or ions.
They are,
• Crystalline solids and
• Non-crystalline (or) Amorphous solids
CRYSTALLINE SOLIDS
• A substance is said to be crystalline when the arrangement of units of matter is regular and
periodic.
• A crystalline material has directional properties and therefore called as anisotropic substance.
• A crystal has a sharp melting point.
• It possesses a regular shape and if it is broken, all broken pieces have the same regular shape.
• A crystalline material can either be a single (mono) crystal or a polycrystal.
• A single crystal consists of only one crystal, whereas the polycrystalline material consists of
many crystals separated by well-defined boundaries.
o Examples
▪ Metallic crystals – Cu, Ag, Al, Mg etc,
o Non-metallic crystals – Carbon, Silicon, Germanium,
• In amorphous solids, the constituent particles t arranged in an orderly manner. They are
randomly distributed.
• They do not have directional properties and so they are called as `isotropic’ substances.
• They have wide range of melting point and do not possess a regular shape.
o Examples: Glass, Plastics, Rubber etc.,
o SPACE LATTICE
• A lattice is a regular and periodic arrangement of points in three dimensions.
• It is defined as an infinite array of points in three dimensions in which every point has
surroundings identical to that of every other point in the array.
• The Space lattice is otherwise called the Crystal lattice

, o
• Consider the points P, Q and R.
• Let us join the points P and Q by a straight line, and the point P and R by another straight line.
• The line PQ is taken as X-axis and the line PR is taken as Y-axis.
• The distance between any two successive lattice points in the X-direction is taken as `a’.
• Similarly, the distance between any two successive lattice points along the Y-direction is taken
as ‘b’.
• Here a and b are said to be lattice translational vectors. Consider a square lattice in which a=b.
• Consider two sets of points A, B, C, D, E, F and A, B, C, D, E, F.
• In these two sets, the surrounding environment looks symmetrical; i.e. the distances AB and
AB, AC and AC, AD and AD, AE and AE and AF and AF are equal.
• Therefore, in the arrangement of points, if the surrounding environment looks the same when the
arrangement is viewed from different lattice points, then that arrangement is said to be a space
lattice.
o BASIS:

• A crystal structure is formed by associating every lattice point with an unit assembly of atoms or
molecules identical in composition, arrangement and orientation. This unit assembly is
called the `basis’.
When the basis is repeated with correct periodicity in all directions, it gives the actual crystal
structure. The crystal structure is real, while the lattice is imaginary.
CRYSTAL STRUCTURE