Analysis of Materials by X-Ray Diffraction : XRay Diffraction [PDF]

X-Ray Diffraction. Quantitative analysis. As mentioned earlier, XRD can be used not only for qualitative identification

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Technology of Materials

What is X-Ray Diffraction? Analysis of Materials by XRay Diffraction (XRD) Introduction X-ray powder diffraction (XRD) is one of the most powerful technique for qualitative and quantitative analysis of crystalline compounds. The technique provides information that cannot be obtained any other way. The information obtained includes types and nature of crystalline phases present, structural make-up of phases, degree of crystallinity, amount of amorphous content, microstrain & size and orientation of crystallites. Principles of Operation When a material (sample) is irradiated with a parallel beam of monochromatic X-rays, the atomic lattice of the sample acts as a three dimensional diffraction grating causing the X-ray beam to be diffracted to specific angles. The diffraction pattern, that includes position (angles) and intensities of the diffracted beam, provides several information about the sample and are discussed below: Angles are used to calculate the interplanar atomic spacings (dspacings). Because every crystalline material will give a characteristic diffraction pattern and can act as a unique ‘fingerprint’, the position (d) and intensity (I) information are used to identify the type of material by comparing them with patterns for over 80,000 data entries in the International Powder Diffraction File (PDF) database, complied by the Joint Committee for Powder Diffraction Standards (JCPDS). By this method, identification of any crystalline compounds, even in a complex sample, can be made. The position (d) of diffracted peaks also provides information about how the atoms are arranged within the crystalline compound (unit cell size or lattice parameter). The intensity information is used to assess the type and nature of atoms. Determination of lattice parameter helps understand extent of solid solution (complete or partial substitution of one element for another, as in some alloys) in a sample. Width of the diffracted peaks is used to determine crystallite size and micro-strain in the sample. The ‘d’ and ‘I’ from a phase can also be used to quantitatively estimate the amount of that phase in a multi-component mixture.

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Quantitative analysis As mentioned earlier, XRD can be used not only for qualitative identification but also for quantitative estimation of various crystalline phases. This is one of the important advantage of X-ray diffraction technique. Several methods have been proposed and successfully used to quantify crystalline phases in mixtures. They include external standard methods, the reference-intensity-ratio (RIR) method, chemical methods and the whole pattern fitting Rietveld method. Of the available methods, the Rietveld method is probably the most accurate and reliable method. The Rietveld method is a whole-pattern fitting least squares refinement technique and has been successfully used for quantification and characterization of inorganic and organic compounds It has also been used for crystal structure refinement, to determine size and strain of crystallites.

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