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Solid State Physics Introduction

Updated: May 2, 2020


Crystalline nature of matter:

If the atoms or molecules in a solid are arranged in some regular periodic (array) then it is known as crystalline. When the atoms or molecules in a solid are arranged in an irregular fashion then it is known as amorphous.


A crystal is a solid composed of a periodic array of atoms i.e. a representative unit is repeated at regular intervals along with array and all directions.


The study of the geometric form and other physical properties of crystalline solids by using X-rays (by forming X-ray diffraction pattern), electron beams and neutron beams, etc., called crystallography.

Differences between crystalline and amorphous solids:

Space lattice:

Crystals made up of regular and periodic three-dimensional arrangements in space called the crystal structure. The periodic arrangement of atoms in a crystal is called the lattice. There is a group of several atoms or ions at each lattice point. This group is called a basis. So the crystal structure is formed when a basis of atoms is attached identically to each lattice point.

Lattice + basis = crystal structure.

Unit cell:

A unit cell is the smallest geometric figure the repetition of which gives the actual crystal structure.

The fundamental elementary pattern of a minimum number of atoms, molecules or group of molecules which represent fully all the characteristics of the crystal is call unit cell.

The unit cell is a parallelepiped formed by the basis vectors a,b,c as concurrent edges and including angles a, b, g between b and c, c and a and a and b respectively as shown in the figure.

Crystal lattice:

The entire lattice structure of a crystal is found to consist of identical blocks or unit cells is known as crystal lattice.

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