X-ray Powder Diffraction Analysis: Principles, Instrumentation and Applications M.S.Pandian
Department of Earth Sciences, Pondicherry University, Puducherry-605014 Introduction
X-ray powder diffraction analysis (XRD) is perhaps the most widely used X-ray based analytical techniques for characterizing materials. As the name suggests, the sample is usually in a powdery form, consisting of fine grains of crystalline material to be studied. The term 'powder' really means that the crytalline domains are randomly oriented in the sample. Therefore when the 2-D diffraction pattern is recorded, it shows concentric rings of scattering peaks corresponding to the various d spacings in the crystal lattice. The positions and the intensities of the peaks are used for identifying the underlying structure (or phase) of the material. For example, the diffraction lines of graphite would be different from diamond even though they both are made of carbon atoms. This phase identification is important because the material properties are highly dependent on structure. Theory and Methodology
The three-dimensional structure of crystalline materials, such as minerals, is defined by regular, repeating planes of atoms that form a crystal lattice. When a focused X-ray beam interacts with these planes of atoms, part of the beam is transmitted, part is absorbed by the sample, part is refracted and scattered, and part is diffracted. Diffraction of an X-ray beam by a crystalline solid is analogous to diffraction of light by droplets of water, producing the familiar rainbow. X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged. When an X-ray beam hits a sample and is diffracted, we can measure the distances between the planes of the atoms that constitute the sample by applying Bragg's Law, named after William .Lawrence Bragg, who first proposed it in 1921. Bragg's Law is:
where the integer n is the order...
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