XRD or X-Ray Diffraction Spectroscopy is one kind of rapid analytical technique primarily used to determine the crystallographic structure of a material. It helps to reveal the information on unit cell dimensions and the actual structure deviates from the ideal one, owing to internal stresses and defects.
This technique mainly uses by irradiating a material with the monochromatic X-rays and then measuring the intensities and scattering angles of the X-rays that leave the working material. This is a non-destructive technique that is generally used on crystalline structures owing to their regular and repeating crystal lattices and gives the opportunity for analyzing the composition and structural arrangement of the crystal, the atomic spacing between atoms in a sample, and providing insights into how the sample might behave in certain environments.
This technique is used for investigating the degree of crystallinity in polymeric materials, and in forensic labs for analyzing crystalline materials present in the textiles found at crime scenes, analyzing pharmaceutical drug samples and how they’ll behave, for analyzing the structure of minerals and different types of geological samples.
Principle of X-Ray Diffraction Spectroscopy (XRD)
A crystal is any solid material where the component atoms form a periodic arrangement and whose surface regularity reflects its internal symmetry due to its regular pattern. X-ray is considered as the invisible wave of electromagnetic radiation which is generally used to produce the diffraction pattern of crystalline materials nowadays. The X-Ray Diffraction Spectroscopy principle is based on constructive interference of a crystalline material and monochromatic X-rays. These monochromatic X-rays are produced by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the tested crystalline sample.
It produced vibration in the atomic orbitals of the tested crystalline material. It occurs with the same frequency level as that of the frequency of the incident ray and is accelerated. Then the emission of radiation of the same frequency occurs by these accelerated electrons as that of incident X-rays in all directions. This phenomenon is called elastic scattering; the electron of the atom is known as the scattered particle. The radiated monochromatic X-rays are in phase with each if it is found that the wavelength of incident radiation is large compared to the dimensions of the crystalline material.
The emission of radiation by the electrons is out of phase with each other during the atomic dimension is nearly equal to the wavelength of X-Ray. This emitted radiation by the electrons may interfere constructively or destructively to produce a diffraction pattern in certain directions. So it can be noted that a regular array of spherical waves is produced due to the regular array of scattering particles. In most of cases, these waves cancel each other out through destructive interference.
Instrumentation of X-Ray Diffraction Spectroscopy (XRD)
The X-Ray Diffraction Spectroscopy (XRD) consists of radiation sources, collimators, monochromators, and detectors. The X-rays are produced by the target material when allowed to pass through a collimator which is made of two sets of closely packed metal plates separated by a small gap. The collimator absorbs all the X-rays, but the narrow beam is not absorbed by it at the time of passing between the gaps. The monochromators help to absorb the undesirable radiation but allow the radiation of the required wavelength to pass. The monochromators are made of the materials like Sodium Chloride, Lithium Fluoride, and Quartz. Detectors help to measure the X-ray wavelengths.
The procedure of X-Ray Diffraction Spectroscopy (XRD)
At first, grind the purified sample at high concentration to a fine powder of ~10 μm(or 200-mesh) in size is preferred. Place the ground sample into a sample holder and ensure smears uniformly onto a glass slide. Then it must be ensured to create a flat upper surface and to achieve a random distribution of lattice orientations unless creating an oriented smear. The purified crystals are then exposed to an x-ray beam.
Diffraction patterns can then be processed which produce information about the crystal packing symmetry and the size of the repeating unit that forms the crystal. It requires a single orientation for the analysis of clays. So it is important to prepare the clay samples given by USGS. A small amount of a standard with known peak positions can be added and used to correct peak positions at the time of unit cell determinations.
The applications of X-Ray Diffraction Spectroscopy (XRD)
X-Ray Diffraction Spectroscopy (XRD) technique is used to identify crystalline phases and orientation of various materials. It is very much helpful to reveal the lattice parameters, grain size, strain, epitaxy, preferred orientation, and phase composition of crystalline materials. This non-destructive method is very helpful to measure the thickness of thin films and multi-layers, measurement of sample purity, determining modal amounts of minerals, determining lattice mismatch between films, determining dislocation density and quality of the film by rocking curve measurements, measuring superlattices in multilayered epitaxial structures, make textural measurements and determine the atomic arrangement.
Strengths of X-Ray Diffraction Spectroscopy (XRD)
It is a rapid and powerful and rapid technique for the identification of an unknown mineral. It helps to determine an unambiguous mineral in most cases. It requires minimal sample preparation. The units of XRD are widely available. The interpretation of data is relatively straightforward.
Limitations of X-Ray Diffraction Spectroscopy (XRD)
It requires homogeneous and single-phase material for the proper identification of the tested sample. It should have access to a standard reference file of inorganic compounds. Peak overlay problems can be arisen due to high angle ‘reflections. It needs tenths of a gram of material which must be ground into a powder. It is difficult during the indexing of patterns for non-isometric crystal systems for unit cell determination.
References
1.Sinha, S., Jeyaseelan, C., Singh, G., Munjal, T., & Paul, D. (2023). Spectroscopy—Principle, types, and applications. In Basic Biotechniques for Bioprocess and Bioentrepreneurship (pp. 145-164). Academic Press.
2.van Beek, W., Urakawa, A., & Milanesio, M. (2013). XRD–Raman and modulation excitation spectroscopy. In‐situ Characterization of Heterogeneous Catalysts, 411-439.
