crystal structure ppt1

8/3/2019 Crystal Structure Ppt1

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Matter

Solids Liquids Gasses


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Solids are again classified in to two types

Crystalline

Non-Crystalline (Amorphous)


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The materials, which have a regular and periodicalarrangement of atoms or molecules in a solid, are

known as crystalline materials.

Again classified into two typeso Single crystals:

Entire solid consists of only one crystal

o

Poly crystals: It has composed of many small crystals

that are separated by well defined

boundaries.


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Non-Metallic crystals:Ice, Carbon, Diamond, Nacl, Kcl etc

Metallic Crystals:Copper, Silver, Aluminium, Tungsten,Magnesium etc


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Single Crystal example


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Amorphous (Non-crystalline) Solid is

composed of randomly orientated atoms ,

ions, or molecules that do not form defined

patterns or lattice structures.

Amorphous materials have order only within a

few atomic or molecular dimensions.


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Amorphous materials do not have any

long-range order, but they have varying

degrees of short-range order.

Examples to amorphous materialsinclude amorphous silicon, plastics, and

glasses.

Amorphous silicon can be used in solar

cells and thin film transistors.


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Non-crystalline


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The science that studies geometricalform and other physical properties of

crystalline solids called Crystallography


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Lattice

Space lattice

Basis of the crystal structure

Unit cell

Primitive cell


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What is lattice? Lattice is defined as the array of points which are

imaginarily kept to represent the position of atoms in

the crystal.It is an imaginary concept.

What is lattice point? The position of atom or molecules in space denoted by

point is called lattice point or lattice sites

What is lattice lines or directions? The lines joined between the lattice points are called

lattice lines or direction.


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What is lattice planes?

A plane containing lattice points is known aslattice plane or atomic plane


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What is Space lattice ?

An infinite array of pointsin space,

Each point has identical

surroundings to all others.

Arrays are arranged

exactly in a periodic

manner.

a

bCB ED

O A

y

x

That is an infinite three dimensionalarrangement of lattice point in space inwhich the environment about the each

point in same is defined as space lattice


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The arrangement of atoms with lattice

point is called the basisor

A group of atoms which describe

crystal structure


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Crystal structure: When the basis is repeated with

correct periodicity in all directions,we can get the actual crystalstructure.

i.e. Lattice + Basis = Crystal structure


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A unit cell is a smallest unit. Which on

repetition over all the three dimension gives

the actual crystal structure.

A unit cell may be consider as brick of wall.

This unit cell may also known as elementary

pattern or building block or lattice unit.


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S

S

a

S

S


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Lattice parameter or unit cell parameter:

The unit cell and, consequently, the entire lattice, isuniquelydetermined by the six lattice constants: a, b, c,

, and . These six parameters are also called as

basic lattice parameters.


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On the basis of the lengths and directions ofthe axes of the symmetry, all the crystals

may be classified into the following seven

system.

Cubic crystal

Tetragonal crystal

Orthorhombic crystal

Monoclinic crystal Triclinic crystal

Rhombohedral crystal

Hexagonal crystal


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Crystal system Unitvector

Angles

Cubic a= b=c = ==90

Tetragonal a = b c = ==90

Orthorhombic a b c = ==90

Monoclinic a b c ==90 Triclinic a b c 90

Rhombohedral a= b=c = =90

Hexagonal a= b c ==90 =120


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There are 14 possible ways of arranging

points in space lattice from seven crystal

system. These 14 space lattices are known

as Bravais lattices.


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S.No Crystal Type Bravais lattices Symbol

123

Cubic SimpleBody centeredFace centered

PIF

45

Tetragonal SimpleBody centered

PI

6789

Orthorhombic SimpleBase centeredBody centeredFace centered

PCIF

1011

Monoclinic SimpleBase centered

PC

12 Triclinic Simple P

13 Rhombohedral Simple P

14 Hexagonal Simple P


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Number of atoms per unit cell (or) Effective

number Atomic radius

Co-ordination number

Atomic packing factor (or) Packing density(or) density of packing


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The total number of atoms present in orshared by an unit cell is known asnumber of atoms per unit cell.


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Atomic radius is defined as half of thedistance between any two nearest

neighbour atoms which have directcontact with each other

It is usually expressed in terms of cubeedge a.


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Coordination Number (CN) : The number of

nearest neighbouring atoms to a particular atom

in the lattice array.

A simple cubic has coordination number 6; a

body-centered cubic lattice, 8; and a face-

centered cubic lattice,12.


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Atomic Packing Factor (APF) is defined as the

ratio of volume of atoms within the unit cell to

the total volume of the unit cell.

cellunitofVolume

cellunitinatomsofVolume

APF

crystal structure ppt1

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