1.INTRODUCTION

Solid state is a state of matter besides liquid and gaseous state. In case of solids the inter molecular forces are very strong and empty spaces between the atoms/ions/ molecules is very less. That is why they have a fixed shape and volume.

1.1 Characteristic Properties of Solids Solids are characterised by the following properties * High density * Lowcompressibility * Rigidity * Definite shape and volume.

2. Classification of Solids

Solids are broadly classified on the basis of following parameters. * based on various properties * based on bonding present in building blocks.

2.1 On the basis of various properties Based on their various properties solids can be classified as * Crystalline solids * Amorphous solids.

Crystalline solids have a regular structure over the entire volume and sharp properties whereas amorphous solids have irregular structure over long distances and properties are not that sharp. Various differences are listed in table below.

3. STRUCTURE OF CRYSTALLINE SOLIDS
3.1 Crystal lattice and Unit Cell
The regular array of the building blocks (atoms/ions/molecules) inside the crystalline solid is called “Crystal Lattice”.
The smallest part or crystal lattice which can be repeated in all directions to generate entire crystal lattice is called “Unit Cell”. In unit cell the atoms of ions or molecules are represented by small spheres. Various lattices are formed by variation in following parameters :
* The edge length along 3 axes : a, b,c
* The interfacial angles : $ \alpha , \ \beta, \ \gamma $
* Location of atom/ions w.r.t each other in crystal lattice

3.2 Primitive Unit Cells and Bravais Lattices
In all, there are seven types of unit cells and there can be some sub types of unit cells. These seven unit cells are called Primitive Unit Cells or Crystal Habits. Which are listed in table below lattice.

For these 7 types of unit cells, 14 types of Lattices exist in nature. These 14 Lattices are called “Bravais Lattices.”

We will focus majorly on cubic unit cells and their arrangements.

3.3 Cubic Unit Cells

This is the most common unit cell. In a cubic unit cell there are following locations for the atoms or spheres
* Corners
* Body Centre
* Face Centres
Following are the contributions of a sphere kept at various locations.

3.3.1 Types of cubic Unit Cells

These unit cells differ from each other in following factors.
* Location of spheres inside the unit cell.
*  Rank of the unit cell (effective number of spheres inside a unit cell)
* Relation between edge length and radius of one sphere.
* Packing fraction (fraction of volume occupied by spheres in a unit cell)
The following parameters for all the 3 unit cells are listed is
the table below :

Valume of Z will depend upon the type of unit cell.

3.5 Close packing in Solids : Origin of unit cells
Suppose we have spheres of equal size and we have to arrange them in a single layer with the condition that spheres should come in close contact with each other. Two types of layers are possible :
1. Square Packing
2. Hexagonal Packing
In square packing spheres are placed in such a way that the rows have a horizontal as well as vertical arrangement. In this case Co-ordination Number is 4.

Hexagonal packing is more efficient. Its Co-ordination Number is 6 and voids in the packing are smaller than square packing. If we place another layer on square packing then there are following possibilities :

1. A similar layer placed just above foundation layer that is the spheres of the second layer coming just above the spheres of the first layer and layers get repeated. If first layer is termed A the packing in this case is AA... type and the unit cell is simple cubic.
2. On other hand if spheres of second layer are placed in depressions of first layer we get BCC unit cell and ABAB ..... type of packing. Arrangements based on hexagonal foundation layer are as follows :
If we put 2nd layer in depressions of first hexagonal layer A two types of voids are created. X type of voids are those which are hollow and through voids of layerA and layer B. While Y type of voids are those voids of layer B which are exactly above spheres of layer A. If we place the spheres of 2nd layer on Y voids then we are repeating layer 1 and ABABAB.... type packing is obtained. In this arrangement hexagonal unit cell is obtained and packing is called Hexagonal Close Packing (HCP). The efficiency of this packing is 74%.
If the 3rd layer is placed on X-type of voids then a new layer C is obtained and then the arrangement will berepeated. We will obtain ABCABCABC..... type of packing. The unit cell for this arrangement is FCC and the packing effeciency is 74%.

4. VOIDS

4.1 Definition

4.2 Radius Ratio

The size of void is expressed in terms of radius ratio of a sphere that can be exactly fit in the void to the radius ofsurrounding spheres. This expressed as :
Radius ratio $=\frac{r}{R}$

4.3 Types of voids

4.3.1 Trigonal void

It is a void formed of equal radii and touching each other as shown in figure.

4.3.2 Tetrahedral Void
It forms by contact of 4 spheres and is positioned at the centre of tetrahedron formed by contact of 4 spheres.

Based on these ratio ranges, ionic crystal are classified into 5 categories which are as follows

5.1 NaCl Type Structure

5.2 ZnS type Structure

5.3 Fluorite Type Structure

5.4 Anti Fluorite structure

5.5 CsCl Type Structure

6.2 Vacancies
These are defects that occur when positions that should contain atoms or ions are vacant.