X-Ray Powder Diffraction

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X-Ray Powder Diffraction : 

X-Ray Powder Diffraction Prepared By – Amit $hah M.Pharm 1st sem QA Roll NO :- 03 Guided By – Mr. Pinak R. Patel Assistant Professor Dept. P’ceutical Chem. D Dharmaj Degree Pharmacy College, Dharmaj

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2 Contents…

What is X-ray Powder Diffraction (XRD) : 

What is X-ray Powder Diffraction (XRD) X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions.  In other methods a single crystal is required whose size is much larger than microscopic dimensions. However, in the powder method as little as 1 mg of the material is sufficient for the study. The analyzed material is finely ground, homogenized, and average bulk composition is determined. 3

Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. The powder method was devised independently by Debye and Scherrer in Germany and by Hull in America at about the same time. 4 history

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X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. For every set of crystal planes , one or more crystals will be in the correct orientation to give the correct Bragg angle to satisfy Bragg's equation. Each diffraction line is made up of a large number of small spots, each from a separate crystal. Each spot is so small as to give the appearance of a continuous line. Every crystal plane is thus capable of diffraction. 5 Fundamental Principles of X-ray Powder Diffraction (XRD)

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Beam Entry Beam Exit Diffraction cones and the Debye-Scherrer geometry

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7 X-ray diffraction pattern

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The powdered sample generates the concentric cones of diffracted X-rays because of the random orientation of crystallites in the sample. Hence, instead of the sample generating only single diffraction spots, it generates cones of diffracted X-rays, with the point of all of the cones at the sample. The powder diffracts the x-rays in accordance with Bragg’s law to produce cones of diffracted beams. These cones intersect a strip of photographic film located in the cylindrical camera to produce a characteristic set of arcs on the film. When the film is removed from the camera, flattened and processed, it shows the diffraction lines and the holes for the incident and transmitted beams. 8

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The x-ray pattern of a pure crystalline substance can be considered as a “fingerprint” with each crystalline material having, within limits, a unique diffraction pattern. 9

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10 X-ray Powder Diffraction (XRD) Instrumentation - How Does It Work? Figure 1.

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The experimental arrangement of powder crystal method is shown in fig.1. its main feature are outlined as below: A is a source of X-rays which can be made monochromatic by a filter Allow the X-ray beam to fall on the powdered specimen P through the slits S1 and S2. The function of these slits is to get a narrow pencil of X-rays. Fine powder, P, struck on a hair by means of gum is suspended vertically in the axis of a cylindrical camera. This enables sharp lines to be obtained on the photographic film which is surrounding the powder crystal in the form of a circular arc. 12

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The X-rays after falling on the powder passes out of the camera through a cut in the film so as to minimize the fogging produced by the scattering of the direct beam. As the sample and detector are rotated, the intensity of the reflected X-rays is recorded.  When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs and a peak in intensity occurs. A detector records and processes this X-ray signal and converts the signal to a count rate which is then output to a device such as a printer or computer monitor. 13

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14 Figure 2. Example of an X-ray powder diffractogram produced during an X-ray scan. The peaks represent positions where the X-ray beam has been diffracted by the crystal lattice. The set of d-spacings (the distance between adjacent planes of atoms), which represent the unique "fingerprint" of the mineral, can easily be calculated from the 2-theta (2) values shown.

Applications : 

Applications Identification : The most common use of powder (polycrystalline) diffraction is chemical analysis. This can include phase identification (search/match) X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Other applications include : identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically. 15

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determination of unit cell dimensions. measurement of sample purity . Most useful for cubic crystal. Used for determining the complex structure of metals and alloys. Useful to make distinction between the allotropic modifications of the same substance. 16

Strengths and Limitations of X-ray Powder Diffraction (XRD)? : 

Strengths and Limitations of X-ray Powder Diffraction (XRD)? Strengths :- Powerful and rapid (< 20 min) technique for identification of an unknown mineral. In most cases, it provides a clear structural determination. XRD units are widely available. Data interpretation is relatively straight forward . 17

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Limitations:- Homogeneous and single phase material is best for identification of an unknown. Requires tenths of a gram of material which must be ground into a powder. For mixed materials, detection limit is ~ 2% of sample. Peak overlay may occur and worsens for high angle 'reflections’ . For unit cell determinations, indexing of patterns for non-isometric crystal systems is complicated . 18

Reference : 

Reference http://serc.carleton.edu/research_education/geochemsheets/techniques/XRD.html http://pubs.usgs.gov/info/diffraction/html/ Instrumental methods of chemical analysis by G.R.Chatwal & Sham.K.Anand … Page No. 2.324-2.326 Instrumental methods of chemical analysis by B. K Sharma… Page No.S-494-536 Practical Pharmaceutical Chemistry Beckett and Stenlake… Page No. 78-81 19 Page No.

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