X-Ray Diffraction: Equipment and Methods | Clay Minerals | Soil

In this article we will discuss about:- 1. Principle of X-Ray Diffraction 2. Equipment of X-Ray Diffraction 3. Analysis 4. Methods 5. Advantages.

Principle of X-Ray Diffraction:

When a monochromatic X-ray beam with wavelength A is projected onto a crystalline material at an angle , diffraction is reinforced only along specific directions, as shown in Fig. 3.19 defined by Bragg’s law [Eq. (3.1)].

Equipment of X-Ray Diffraction:

The XRD spectrometer is shown in Fig. 3.20. X-rays generated from the X-ray tube strike the sample crystal through the collimators at angle, . The X-rays diffracted at an angle 2 are recorded by a detector in terms of number of counts per second, which is proportional to the intensity of diffracted X-rays. A motor rotates the specimen to vary the angle of incidence of X-rays continuously, and the counts per second are recorded for all angles of .

The XRD pattern is plotted with angle 2 on the x-axis and counts per second on the y-axis. The interatomic spacing (d) is computed using Bragg’s law [Eq. (3.1)] by substituting the angle 2 corresponding to the different peaks in the XRD pattern. So we get –

nλ = 2dsin …(3.1)

where n is the order of reflection, d is the spacing between planes M₁N₁and M₂N₂, and is the angle of incidence of X-rays.

The d values so obtained are the characteristics of different clay minerals. Each peak can give at least three d spacings corresponding to the first (n = 1), second (n = 2), and third (n = 3) order of diffraction using Eq. (3.1).

X-Ray Diffraction Analysis:

Following is the simple procedure used in X-ray diffraction:

1. Obtain XRD pattern.

2. Determine the d-spacings using Bragg’s law.

3. Obtain integrated intensities.

4. Compare data with known standards in the JCPDS file, which are for random orientations. There are more than 50000 JCPDS (Joint Committee on Powder Diffraction Standards that is replaced by International Centre for Diffraction Data, ICDF) cards of inorganic materials and JCPDS stands for which has the diffraction patterns of 50000 inorganic and 25000 organic single component, crystalline phases.

5. A typical XRD pattern is analyzed for (a) Peak position, (b) Peak intensity, and (c) Peak width. Peak width is the width of the peak at half of maximum intensity. The areas under the peak are related to the amount of each phase present in the sample.

Methods of X-Ray Diffraction:

Following are the various methods that are in use based on the principle of X-ray diffraction:

1. X-Ray Spectrometer:

Rotation of a perfect crystal in an X-ray beam is one method to determine the X-ray spectrum using Bragg’s law. By rotation of a single crystal with a fixed detector or using a position sensitive detector with a fixed crystal, we can perform X-ray spectroscopy experiments similar to how a diffraction grating can be used in an IR or UV spectrometer.

2. Laue Camera:

With a polychromatic incident beam, many planes will meet the Bragg condition and tracing the 2D pattern on a photographic film will reveal the planes of a zone.

3. Rotating Crystal Method:

For a monochromatic beam and a single crystal, rotate the crystal during diffraction experiment to bring Bragg planes into alignment.

4. Powder Method:

Monochromatic beam, polycrystalline sample. Usually done with a flat film in pinhole arrangement.

5. Diffractometer Method:

Similar to the powder method but uses a step-scanner and a line beam.

Advantages of X-Ray Diffraction:

X-ray diffraction is a non-destructive and rapid method of identification of minerals. Sample preparation is simple. Calculation of d-spacing is accurate. The method can also be applied in situ for single crystal, poly-crystalline, and amorphous materials. Standards are available for thousands of material systems.