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.
Characteristic Properties of Solids
Solids are characterised by the following properties
- High density
- Low compressibility
- Rigidity
- Definite shape and volume.
CLASSIFICATION OF SOLIDS
Solids are broadly classified on the basis of following parameters
- Based on various properties
- Based on bonding present in building blocks,
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.
Based on Bonding
There are various type of solids based on type of bonding present in their building blocks. Various types of solids along with their properties are given in the table below.
The different properties of the four types of solids are listed in Different Types of Solids
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-a. B. y
* 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
For these 7 types of unit cells, 14 types of Lattices exist in nature. These 14 Lattices are called "Bravais Lattices."
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
Followings are the contributions of a sphere kept at various locations.
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:
Density of cubic crystal is given by the following formula
Volume 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:
- Square Packing
- 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.
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 layer A 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 be repeated. 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
The empty spaces inside a spheres are called "voids". The size and shape of voids depends upon the type of unit cell and packing
4.2 Radius Ratio
The size of void is expressed in terms of radius ration of a sphere that can be exactly fit in the void to the radius of surrounding spheres.
This expressed as:
Radius ratio = 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.
It forms by contact of 4 spheres and is positioned at the centre of tetrahestron formed by contact of 4 spheres.
4.3.3 Octahedral Void
4.3.4 Cubic Void
This Void from by the Close Contact of 8 Sphere
Trigonal <Tetrahedral<Octahedral <Cubic
void void void void
5. CLASSIFICATION OF IONIC STRUCTURES
Ionic compounds are formed by the simultaneous arrangement of cations and anions in lattice/unit cell.
The larger of two species occupies major positions in unit cell and the smaller ones occupy voids according to their size.
Which is decided on the bases of radius ratio (r/r).
The various ratios are listed below.
5.1 NaCl Type Structure
5.4 Anti Fluorite structure
5.5 CsCl Type Structure
6. IMPERFECTIONS IN SOLIDS
Sometimes some defects or imperfections occur in crystal
6.1 Classification of defects
6.2 Vacancies
These are defects that occur when positions that should contain atom or ions are vacant.
6.3 Interstitial sites
These are sites located between regular positions sometimes atoms or ions may occupy these positions.
6.4 Stoichiometric Defects
These defects do not disturh stoichiometry of solid substance.
6.4.1 Schottky defects
It is a vacancy defect in ionic solid. No. of m missing cations and aniom is equal ao electrical neutrality is maintained. This defect decreases the density of the substance. The defect is shown by ionic substances in which ration and anson are of ant similar sizes oq. KC1, NaCl, Agr etc.
6.4.2 Frenkel defect
In ionic solids the smaller ion is dislocated from its normal position to an interstitial site. It creates vacancy defect at its original site and interstitial defect at new location.
It is also called as dislocation defect. It does not change the density of solid. This type of defect is shown by ionic substances in which there is a large difference in size of ions eq ZnS. AgCl, AgBr etc.
Note :- AgBr shows both Schottky und Frenkel defects.
6.5 Non Stoichiometric Defects
The compounds having these defects contain combining elements in a ratio different from required by their stoichiometric formulae.
6.5.1 Metal Excess Defect
Due to anionic vacancies: The anion may be missing from its lattice site leaving an e behind so that charge remains balanced. The site containing electron is called F centre. They import colour to the crystal. F stands for Farbenzenter meaning colour.
This defect is similar to schottky defect and is found is crystals having schottky defect eq. NaCl, KCI etc.
An F - Center in a Crystal
Due to the presence of extra cations in the interstitial sites. An extra cation may be present in interstitial site and an electron is present in another interstitial site so that electrical neutrality is maintained. This is similar to Frenkel defect and if found in crystal having Frenkel defect.
Metal excess defect caused by extra cation in the interstitial site.
6.5.2 Metal Deficiency Defect
This defect occurs when metal shows variable valency. eq. FeO is mostly found is varying compostions between Fe0.93O to Fe0.96O. In crystals of FeO some Fe+² cations are missing and the loss of positive charge is made up by the presence of required number of Fe+3 ions.
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