Question

Q2

What are the methods to take images of atomic crystal ?

Define polytypism , Inversion operation ?

Answer 1

Methods to take Images of Atomic Crystals:

                           At present there are different types of methods involved worldwide to understand the atomic structure of crystalline solids. In order to create the complete structure of the crystal, atomic or molecular structural developing methods are employed.

These methods mainly involves,

1) X- Ray crystallography:                         In this experimental method X- ray diffraction patterns are developed and by studying about these, crystal strcture is determined.                 X -ray crystallography provides very detailed atomic information, showing every atom of a crystal.

2) NMR (Nuclear Magnetic Resonance) - spectroscopy:                                   

                      NMR spectroscopy involves the information on the local conformation and distance between atoms that are close to one another .            NMR spectroscopy gives the study by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of atoms or molecules placed in a strong magnetic field.    it is a research technique that exploits the magnetic properties of certain atomic nuclei and determines the physical and chemical properties of atoms or molecules.

3) Electron microscopy:                                                                                Electron microscopy involves the developing an image of the overall shape of the atoms or molecule taking the information at the atomic level.                              There are few types of elctron microscopy, they are____

a.     TEM (Transmission Electron Microscope): This is a method which uses a high voltage electron beam to illuminate and create a magnified image of the sample.  

b.     SEM (Scanning Electron Microscope):

It uses a focused electron beam across the rectangular area of the specimen, which loses energyon passing through it. The energy is converted into other forms of energy, such as heat, light, secondary electrons, and backscattered electrons. This information can be translated to view the topography and composition of the original specimen.

c.     REM (Reflection Electron Microscope):

This method involves the detection of a beam of elastically scattered electrons that is reflected off of the specimen which is being examined.                             The reflection high-energy electron diffraction (RHEED) and reflection high-energy loss spectroscopy (RHELS) techniques are often used in this type of microscopy.

Polytypism:

Polytypism is considered to be a kind of polymorphism (ability of solid materials to exist in more than one crystal structure) in which the 2-D^l translations are preserved within the layers.

Inversion operation:

Inversion operation is the process in which all the atoms are moved by rotating through 180⁰ about the geometric center also called inversion centre of the crystal, so that the space coordinates will change from ( x, y, z) to ( - x, - y, - z).