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X-Ray Diffraction (XRD)By Ipshita Ranjana63120903248IntroductionWhen an incident beam of monochromatic X-rays interacts with a crystalline substance, the atomic planes of the crystal causes an incident beam of X-rays to interfere with one another as they leave the crystal. The phenomenon is called X-ray diffraction.

Braggs LawBragg Law was first proposed by William Henry BraggandWilliam Lawrence Braggin response to their discovery thatcrystallinesolids produced surprising patterns of reflected X-rays. They found that these crystals, at certain specific wavelengths and incident angles, produced intense peaks of reflected radiation (known asBragg peaks).W. L. Bragg explained this result by modeling the crystal as a set of discrete parallel planes separated by a constant parameterd. It was proposed that the incident X-ray radiation would produce a Bragg peak if their reflections off the various planes interfered constructively. The interference is constructive when the phase shift is a multiple of this condition can be expressed by Bragg's law:

n= any positive integer= wavelengthd= distance between crystal planes= scattering angle

Incident Angle Reflected angle X-ray Wavelength Total DiffractedAngle 2

XRD results AnalysisA diffraction pattern is obtained by measuring the intensity of scattered waves as a function of scattering angle. Very strong intensities known as Bragg peaks are obtained in the diffraction pattern at the points where the scattering angles satisfy the Bragg condition.The three features of a diffraction pattern:The number of peaksThe position of peaks:Intensities of the peaks:

Diffraction pattern Analysis.

A comparison of the X-ray powder diffraction patterns of NaCl (bottom) and KCl (top).

Methods Single crystal X-ray diffraction The Powder Method

Single crystal X-ray diffraction

This is a powerful technique that is commonly used to determine the structures of new materials. It has three basic steps:Obtaining a perfect crystalPlacing it under an intense beam of lightCombing data to produce a refine crystal model

LimitationsHowever, the technique is limited by the ability to grow nearly perfect crystals that are suitable for diffraction. Due to this limitation and the time and cost-intensive nature of the technique, single crystal diffraction is not used for routine structural characterization of known materials.

The Powder Method

For routine structural characterization of materials, X-ray powder diffraction is far more common. samples for powder diffraction may be large crystals, or they may be in the form of a powder composed of microcrystal's that are too small to be seen by the human eye. The underlying principles of the experiment are the same in both powder diffraction and single crystal diffraction, although the data analysis is much simpler in powder diffraction.

Applications Pharmaceutical industry used to unambiguously characterize the composition of pharmaceuticals. Forensic science XRD is used mainly in contact trace analysis. Geological applications Since the 1920s, X-ray diffraction has been the principal method for determining the arrangement of atoms in minerals andmetals. XRD is the key tool in mineral exploration. Each mineral type is defined by a characteristic crystal structure, which will give a unique x-ray diffraction pattern, allowing rapid identification of minerals present within a rock or soil sample. The XRD data can be analyzed to determine the proportion of the different minerals present.

Microelectronics industry As the microelectronics industry uses silicon and gallium arsenide single crystal substrates in integrated circuit production, there is a need to fully characterize these materials using the XRD. XRD topography can easily detect and image the presence of defects within a crystal, making it a powerful non-destructive evaluation tool for characterizing industrially important single crystal specimens.Glass industry While glasses are X-ray amorphous and do not themselves give X-ray diffraction patterns, there are still manifold uses of XRD in the glass industry. They include identification ofcrystalline particles which cause tiny faults in bulk glass, and measurements of crystalline coatings for texture and crystallite size

Refrences WikipediaX-Ray Diffraction by Hanno zur Loye http://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.htmlX-ray Diffraction, by B.E. Warren, General Publishing Company, 1969, 1990 http://www.chem.uci.edu/~lawm/263%204.pdfhttp://www.xos.com/techniques/xrd/