X ray powder diffraction

X-RAY POWDER DIFFRACTION

PRESENTED BY SUBMITTED TOChiranjibi Adhikari Mrs. Menaka T.

M. Pharm. 1st

year Assistant professor

Mallige College of Pharmacy #71, SILVEPURA, BANGALORE: 560 090

EVALUATION SEMINAR ON

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CONTENTS…

Introduction to X-ray powder diffractionFundamental principlesX-ray powder diffractometerObtaining of XRD dataApplications Strength & Limitations

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X-RAY POWDER DIFFRACTIONDiffraction is defined as the bending of light around

or into the geometrical shadow of the obstacle.In powder X-ray diffraction, the diffraction pattern

is obtained from a powder of the material, rather than an individual crystal.

Powder diffraction is often easier and more convenient than single crystal diffraction as about 1 mg of material is sufficient for the study.

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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.

HISTORY

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X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample.

For every set of crystal planes in the fine powder, one or more crystals will be in the correct orientation to give the correct Bragg angle to satisfy Bragg's equation.

Another fraction of the grains will have another set of planes in the correct position for the reflection to occur and so on.

Also, reflections are possible in the different order for each set.

The powdered sample generates the concentric cones of diffracted X-rays because of the random orientation of crystallites in the sample.

FUNDAMENTAL PRINCIPLES OF XRD

Beam Entry Beam Exit

Diffraction cones and the Debye-Scherrer geometry

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All the like orientations of the grains due to

reflection for each set of planes and for each order will constitute a diffraction cone whose interaction with the photographic plate gives rise to a trace.

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 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.

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X-ray powder diffractometerA diffractometer is a measuring instrument for

analyzing the structure of a material from the scattering pattern, produced when a beam of

radiation or particles interacts with it.

Figure 1.

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A is a source of X-rays.These X-rays are generated by a cathode ray tube.

X-rays are filtered by monochromator to produce monochromatic radiation. It help to choose the correct wavelength.

Slits (S1 and S2) are used to adjust the shape of the beam, collimate to concentrate and direct the X-rays beam toward the sample so that the powdered specimen (P) get a narrow pencil of X-rays.

Fine powder is struck on a hair by means of gum. It 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.

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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.

When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs and the intensity of the reflected X-rays is recorded by a detector.

The detector also processes this X-ray signal and convert it into a count rate, which is then output to a device such as a printer or computer monitor.

In a more complicated apparatus, also a goniometer can be used for fine adjustment of the sample and the detector positions.

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Obtaining of XRD dataThe crystal structure can be obtained from the

arrangement of the traces and their relative intensities.

A diffraction pattern plots intensity against the angle of the detector, 2θ. The result obtained is called diffractogram.

In a diffraction pattern, the peak position depends upon the wavelength.

Absolute intensity (number of X-rays observed in an given peak) may vary by instrumental and experimental parameters.

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The peaks represent positions where the X-ray beam has been diffracted by the crystal lattice. The set of d-spacings, which represent the unique "fingerprint" of the mineral, can easily be calculated from the 2-theta values shown. 

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X-ray diffraction provides ample information about the lattice parameters. Peak represents a lattice plane and therefore can be characterized by Miller index.

If the symmetry is high as in case of cubic or hexagonal, it is not difficult to identify the peak index for an unknown phase. This is very useful in solid-state chemistry to identifying new materials. Once a pattern gets indexed, it serves as reference for new entities.

Each peak in pattern is a reflection from a different set of planes. By determining 2 for a peak, can use Braggs' law to get d for that set of planes

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APPLICATION IN POLYMORPHISM

PXRD is helpful in identification and characterization of polymorph, monitoring the stability, method development and validation for identification and quantification of drugs in Pharmaceutical Industries.

It helps in elucidation of the relevant polymorphic and pseudo-polymorphic forms in pharmaceutical development.

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The atoms in a crystal are periodically arranged, producing constructive interference at specific angles thus diffract light.

The wavelength of X-ray are similar to the distance between atoms, Powder X-ray Diffraction techniques uses this principle to elucidate the crystalline nature of materials. The scattering of X-rays from atoms produce a diffraction pattern that contains information about the atomic arrangement in crystal.

Amorphous materials like glass do not have periodic array with long-range order so; they do not produce any significant peak in diffraction pattern.

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OTHER APPLICATIONS

Powder (polycrystalline) diffraction is commonly used for chemical analysis- phase identification.

Identification of unknown crystalline materials (e.g. minerals, inorganic compounds).

Identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically.

Determination of unit cell dimensions.

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• 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.

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STRENGTHS OF X-RAY POWDER DIFFRACTION

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 .

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LIMITATIONS OF X-RAY POWDER

DIFFRACTION

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.

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X-Ray Powder Diffraction is a somewhat inefficient measurement technique…Only a small fraction of crystallites in the sample actually

contribute to the observed diffraction pattern.– Other crystallites are not oriented properly to produce

diffraction from any planes of atoms.– You can increase the number of crystallites that contribute to

the measured pattern by spinning the sample.

Only a small fraction of the scattered X-rays are observed by the detector.

– A point detector scanning in an arc around the sample only observes one point on each Debye diffraction cone.

– You can increase the amount of scattered X-rays observed by using a large area (2D) detector.

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REFERENCESInstrumental methods of chemical analysis by G R

Chatwal & Sham K Anand. Page No. 2.324-2.326

Chauhan and Chauhan. Powder XRD Technique and its Applications in Science and Technology. J Anal Bioanal Tech 2014; 5(5):1-5.

http://serc.carleton.edu/research_education/geochemsheets/techniques/XRD.html

http://pubs.usgs.gov/info/diffraction/html/

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