solid state 07-08-24.pdf

Solid state chemistry The branch of physical chemistry which deals about the structure and properties of the solid  NATURE OF THE SOLID STATE Solid are characterised by High density, Low compressibility, rigidity, mechanical strength and definite shape and volume. This indicates that the...

Solid state chemistry The branch of physical chemistry which deals about the structure and properties of the solid  NATURE OF THE SOLID STATE Solid are characterised by High density, Low compressibility, rigidity, mechanical strength and definite shape and volume. This indicates that the molecules, atoms or ions that make up a solid are closely packed. They are held together by strong cohesive forces and cannot move at random. Thus, In Solid well-ordered molecules, atomic or ionic arrangement. e.g. – Sodium chloride (NaCl), sulphur (S) and Sugar (C12H22O11) being in compressible, rigid, and characteristic geometrical form.  CLASSIFICATION OF SOLID Solid are classified on the basis of various properties. They are two type - 1. Crystalline Solid 2. Amorphous Solid 1. Crystalline Solid : A crystalline solid has a definite and regular geometry due to definite and orderly arrangement of molecules or atom in three dimensional Space. It has a sharp melting point i.e. it changes abruptly into liquid state. Crystalline solid are anisotropic. 2. Amorphous Solid : Amorphous solid does not have any pattern of arrangement of molecules or atoms and does not have any definite geometrical shape. Thus unlike crystalline solids, amorphous solid do not have a long-range order.  Properties of crystalline solids Arrangement They are arranged in a regular way Shape They have long range order Melting Point They have sharp melting Point Heat of fusion They have a definite heat of fusion Compressibility They are rigid and incompressible Cutting with a sharp edged tool They are given cleavage i.e. they break into two pieces with plane surface. Symmetry They possess symmetry They are anisotropic i.e. their physical Isotropy and Anisotropy properties (mechanical, optical and electrical) are different direction Volume change There is a sudden change in volume when they melt Interfacial angles They possess interfacial angles  Properties of Amorphous solids Arrangement They have not arranged in a regular way Shape They have short range order Melting Point They melt over a range of temperature Heat of fusion They do not have a definite heat of fusion Compressibility They may be compressed to some extent Cutting with a sharp edged tool They give irregular cleavage i.e. they break into two pieces with irregular surface Symmetry They do not possess symmetry They are isotropic i.e. their physical Isotropy and Anisotropy properties are same in all direction Volume change There is no sudden change in volume on melting Interfacial angles They do not possess interfacial angles  Isotropy and Anisotropy Amorphous solids differ from crystalline solids and resemble liquids in many respects. The properties of amorphous solids, such as, electrical conductivity, thermal conductivity, mechanical strength, refractive index, coefficient of thermal expansion etc. are same in all directions. Such solids are known as isotropic. Gases and liquids are also isotropic. On the other hand, crystalline solids show these physical properties different in different directions. Therefore crystalline solids are called anisotropic. The anisotropy itself is a strong evidence for the existence of orderly molecular arrangement in crystals. For example, the velocity of light passing through a crystal is different in different directions. A ray of light entering a crystal may split up into two components each following a different path and traveling with a different velocity. This phenomenon is called double refraction. In the figure two different kinds of atoms are shown in two dimensional arrangement. If the properties are measured along the direction CD, they will be different from those measured along the direction AB. This is due to the fact that in the direction AB each row is made up of one type of atoms while in the direction CD each row is made up of two types of atoms. It is important to note that in the case of amorphous solids, liquids and gases atoms or molecules are arranged disorderly therefore all directions are identical and all properties are same in all directions.  The Space Lattice & Unit Cell The 3D arrangement of points in space is called a space lattice The smallest repeat unit of a crystal structure, in 3D, which shows the full symmetry of the structure – Unit Cell. Repetition of unit cell generates entire crystal.  Space lattice and unit cell Figure: (a) Space lattice of crystalline solid (b) Unit cell  Parameters of unit cell There are six parameters of a unit cell. These are the 3 edges which are a, b, c and the angles between the edges which are α, β, γ. The edges of a unit cell may be or may not be perpendicular to each other. A unit cell can either be primitive cubic, body-centered cubic (BCC) or face- centered cubic (FCC). In this section, we will discuss the three types of the unit cell in detail. Types of Unit Cell A large number of unit cells together make a crystal lattice. Constituent particles like atoms, molecules are also present. Each lattice point is occupied by one such particle. Primitive Cubic Unit Cell Body-centered Cubic Unit Cell Face centered cubic unit cell Primitive Cubic Unit Cell In the primitive cubic unit cell, the atoms are present only at the corners. Every atom at the corner is shared among 8 adjacent unit cells. There are 4 unit cells in the same layer and 4 in the upper (or lower) layer. Therefore, a particular unit cell has the only 1/8th of an atom. Each small sphere in the following figure represents the center of a particle that occupies that particular position and not its size. This structure is known as an open structure. 1. The atoms in the primitive cubic unit cell are present only at the corners 2. Every atom at the corner is shared among eight adjacent unit cells In each cubic unit cell, there are 8 atoms at the corners. Therefore, the total number of atoms in one unit cell is 8 × 1/8 = 1 atom. Body-centered Cubic Unit Cell (BCC) A BCC unit cell has atoms at each corner of the cube and an atom at the center of the structure. The diagram shown below is an open structure. According to this structure, the atom at the body center wholly belongs to the unit cell in which it is present. 1. In BCC unit cell every corner has atoms. 2. There is one atom present at the center of the structure The total number of atoms present per unit cell = 2 atoms. Face-centered Cubic Unit Cell (FCC) An FCC unit cell contains atoms at all the corners of the crystal lattice and at the center of all the faces of the cube. The atom present at the face-center is shared between 2 adjacent unit cells and only 1/2 of each atom belongs to an individual cell. 1. In FCC unit cell atoms are present in all the corners of the crystal lattice 2. There is an atom present at the center of every face of the cube In a face-centered cubic unit cell, we have: 8 corners × 1/8 per corner atom = 8 × 1/8 = 1 atom 6 face-centered atoms × 1/2 atom per unit cell = 3 atoms Therefore, the total number of atoms in a unit cell = 4 atoms. An End-centred unit cell contains particles at the corners and one at the center of any two opposite faces.