X-Ray Spectroscopy

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X-ray Diffractometry

INTRODUCTION

Max von Laue discovered in 1912 that crystalline substances act as diffraction gratings for X-ray wavelengths. This method is mainly used for the determination of the crystal structure and atomic spacing. X-rays are electromagnetic waves with shorter wavelengths. They are generated when the beam of light composed of high-velocity electrons from a heated filament impinge on a small target in a sealed diffraction tube. X-rays were first discovered by Roentgen within a wavelength range of 10−7 to 10−15 meters.

X-ray waves production

PRINCIPLE

The main principle involved is the diffraction of X-ray by a crystalline sample. It is a non-destructive method which provides detailed information about the composition and crystallographic structure of crystals. The principle is given by the following flowchart.

THEORY

The theory is the diffraction which is based upon the internal structure of the crystals assembled with the atoms and atomic groupings. X-ray beams hit the atom, and the electrons surrounding the atom start to oscillate with the same frequency as that of X-rays. The relationship between crystals and X-rays are given by Bragg's law, developed by the W. H. Bragg.

Steps involved in crystal lattice determination

nλ = 2d sinθ

where λ = X-ray wavelength; d = lattice space between the particles; θ = incident angle.

Bragg's law phenomenon

INSTRUMENTATION

The basic components of the X-ray diffractometer are as follows:

• Source
• Collimator
• Monochromator
• Detector

X-ray diffractometer

Source

X-rays are generated by the impingement of high-velocity electrons on a metal plate.

Collimator

The main use of the collimator is to pass the narrow beam of X-rays. That is, the broad beam of X-rays is absorbed by the collimator. This can be achieved by the placement of two sets of the closely packed metal plates. These metal plates are separated by a constant distance.

Monochromator

The main use of the monochromator is to convert the polychromatic light into monochromatic light. That is, the unwanted X-ray radiation is absorbed and the desired wavelength radiation is passed through the sample.

Two types of monochromators are commonly employed in X-ray diffractometry. They are as follows:

1. Filter monochromators (e.g. zirconium)
2. Crystal monochromators (e.g. sodium chloride and lithium chloride)

Detectors

The intensity of the X-rays diffracted is measured by detectors. There are two common types of detectors employed for the detection in X-ray diffractometer. They are as follows:

1. Photographic method of detection: In this method, the plane cylindrical method is used. This film is developed after being exposed to X-rays. The intensity of the diffracted light is expressed in terms of the density.
3. D = log Io/I
5. where D = density; I0 = intensity of the incident light; I = intensity of the diffracted light.
6. Counter-method: This can be done by using the following principles:
• Geiger Muller tube
• Proportional counter
• Scintillation counter
• Solid state semiconductor
• Semiconductor detectors

TYPES OF X-RAY DIFFRACTION METHODS

There are mainly four types of X-ray diffraction methods. They are as follows:

Laue Photographic Method

This method is mainly based on two principles:

1. Transmission: where the crystal is held stationary in a beam of X-rays which are diffracted and recorded on a photographic plate.
2. Back reflection method: this method is the same as the transmission method where the back reflection takes place instead of transmission.

Bragg's X-ray Spectrometer Method

The deflection in the electrometer is the measure of the ionization which is directly proportional to the diffraction intensity.

Bragg's spectrometer

Rotating Crystal Method

In this method, the shaft is moved to put the crystal into slow motion which produces sets of planes.

Rotating crystal diffractometer

Powder Method

In this method, the crystal is replaced with the fine powder of crystal.

ADVANTAGES

• Qualitative determinations
• Simple in sample preparation
• Rapidness
• Ability of the analysis of mixed phases

DISADVANTAGES

• Well-defined crystals are required for the analysis
• Analysis is not up to the atomic level

APPLICATIONS

• Used in phase identification
• Used in the determination of the polymer crystallinity
• Used in the determination of the residual stress which remains after the removal of external stress
• Used in soil surveys
• Used in the petroleum refinery industry
• Used in the determination of particle size
• Used in the determination of the cis-trans isomerism
• Used in the study of corrosion products
• Used in the determination of tooth enamel and dentite

REVIEW QUESTIONS

1. What is the principle involved in the X-ray diffractometry?
2. What are the requirements for X-ray diffraction?
3. Write about the theory involved in X-ray diffractometry.
4. Explain the different components of the X-ray diffractometer.
5. What are the different types of X-ray diffractometric methods?
6. What are the advantages and disadvantages of X-ray diffractometry?
7. List out the applications of X-ray diffractometry.