X-Ray Diffraction Technique - Prof. Arshad Kamran, Lecture notes of Experimental Techniques

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X-RAY

DIFFRACTION

TECHNIQUE

TABLE OF CONTENTS

➢ HISTORY

➢ X-RAYS

➢ X-RAYS DIFFRACTION

➢ PRINCIPLE

➢ COMPONENTS

➢ DESCRIPTION OF COMPONENTS

➢ WORKING

➢ XRD PATTERN

➢ APPLICATIONS

➢ Advantages

➢ Limitations

WHAT ARE X-RAYS?

▪ Beams of electromagnetic radiations ,having smaller wavelength than

visible light but larger energy.

▪ Hard x-rays: Have high frequency and more energy.

▪ Soft X-rays : Have less penetration and low energy

WHAT IS XRD?

• A most useful technique for the characterization of crystalline materials, metals, ceramics, minerals and inorganic compounds.
• A rapid and non-destructive technique. ‘‘Every crystalline substance gives a pattern, the same substance always gives the same pattern ,and in a mixture of substances ,each produces its pattern independently of others.’’ Wilhelm Rontegen words
• The x-rays diffraction pattern of a pure substance is like a fingerprint of the substance based on the scattering of x-rays from crystals.
• A diffracted beam is composed of large number of scattered rays mutually reinforcing each others.

PRODUCTION OF X-RAYS

• When high energy electrons strike the anode in a sealed vacuum-rays are

generated. Anodes are often made of copper ,iron or molybdenum.

• About 1% of the total energy of electrons is converted into x-radiations and

remaining is dissipated as heat.

COLLIMATOR

• In order to get a narrow beam of x-rays ,the generated x-rays are allowed to

pass through a collimator consists of two closely packed metal plates having

gap between them.

MONOCHROMATOR

• Interference filters (^) • Crystal Monochromators
• It absorbs undesirable radiations but

allows to pass the radiations of required

wavelengths.

• These are made up of suitable crystalline

material like NaCl, lithium fluoride, quartz

etc.

DETECTORS

• Detectors are the devices used to measure flux,
• spectrum and other properties of x-rays.
• The x-rays intensities can be measured and recorded either by,
  • Photographic method
• The blackening of developed film is expressed in terms of density unit D 𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡ℎ𝑎𝑡 𝑐𝑎𝑢𝑠𝑒𝑠 𝑏𝑙𝑎𝑐𝑘𝑒𝑛𝑖𝑛𝑔 given by:
• D= 𝑙𝑜𝑔 𝐼 0 𝐼
• Where,𝐼 0 =incident beam intensity,
• I= 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 Disadvantage: It is time consuming.

WORKING PRINCIPLE: BRAGG’S LAW

• It works on the principle of Bragg’s Law.
• The interaction of incident rays with the sample produces constructive interference when the condition satisfies the Bragg’s Law : 𝒏𝝀 = 2dsin 𝜽 d = 𝒊𝒏𝒕𝒆𝒓𝒑𝒍𝒂𝒏𝒆𝒓 𝒔𝒑𝒂𝒄𝒊𝒏𝒈 𝜽 = 𝒈𝒍𝒂𝒏𝒄𝒊𝒏𝒈 𝒂𝒏𝒈𝒍𝒆 𝝀 = wavelength of x-rays n = order of plane

X-RAY DIFFRACTION METHODS

• Laue photographic (^) • Rotating crystal • Powder Method
• Different orientations
• Single crystal
• Polychromatic beam
• Fixed angle
• Disadvantage:
• Gives information about crystal shape and structure only.
• Lattice constants
• Single crystals
• Monochromatic beam
• Variable angle
• Disadvantage:
• Not every plane is able to produce diffracted spot. i.e, Bragg’s angle is limited. Because crystal rotates about single axis. ➢ Lattice parameters ➢ Polycrystalline 𝑝𝑜𝑤𝑑𝑒𝑟𝑒𝑑 ➢ Monochromatic beam ➢ Variable angle ➢ Advantage :

1. Rapid analytical technique that gives maximum information about crystals.

2. Rotating crystal method

• Single crystal is mounted with one axis normal to the monochromatic x-ray beam. And a cylindrical film is placed around the sample.
• As sample rotates, some sets of planes satisfy Bragg’s equation.

3. Powdered crystal method ➢ It is a rapid analytical technique primely used for the phase identification of a polycrystalline material and can provide information on unit cell dimensions. Fine powder is struck on a hair with gum and suspended vertically in the axis of cylindrical camera ➢ When monochromatic beam is allowed to pass different possibilities may happen:

1. There will be some particles in random orientation in the powder crystal.
2. Another fraction of grains will have a set of planes in the correct positions for the reflections to occur.
3. Reflections are possible in different orders for each set.