X ray diffraction .ppt2003

Slide 1: 

X-RAY DIFFRACTION Facilitated by, Mrs. Sudha Mallapur Assit ant Professor Presented by, Venkata krishna Y, Dept. of pharmaceutics, EAST WEST COLLEGE OF PHARMACY

CONTENTS: 

CONTENTS Bragg`s Law X-ray Powder Diffraction X-ray Powder Diffractometer

X-ray diffraction: 

X-ray diffraction When a beam of X-ray radiation is incident upon a substance, the electrons constituting the atoms of the substance become as small oscillators. These on oscillating at the same frequency as that of incident X-ray radiation emit EM radiations in all directions at the same frequency. Diffraction occurs as waves interact with a regular structure whose repeat distance is about the same as the wavelength .

Slide 4: 

When certain geometric requirements are met, X-rays scattered from a crystalline solid can constructively interfere, producing a diffracted beam.

Bragg Equation History: 

Bragg Equation History English physicists Sir W.H. Bragg and his son Sir W.L. Bragg developed a relationship in 1913 to explain why the cleavage faces of crystals appear to reflect X-ray beams at certain angles of incidence (theta, θ ).This observation is an example of X-ray wave interference . Sir William Henry  Bragg (1862-1942) , William Lawrence  Bragg (1890-1971) In 1915, the father and son were awarded the Nobel prize for physics "for their services in the analysis of crystal structure by means of X-rays".

Bragg’s law: 

Bragg’s law The conditions for diffraction are governed by Bragg’s law. When the path length in the crystal(2dsin θ ) is a multiple of the wavelength, constructive interference occurs and diffracted intensity is obtained.

Slide 7: 

Above figure shows a monochromatic beam of X-rays incident on the surface of a crystal at an angle θ .

Slide 8: 

P,Q & R represent the edges of a family of planes distance ‘d’ apart. Plane ‘P’ reflects AX in XD. Plane ‘Q’ reflects BY in YE at the same angle θ . Although the beam penetrates many more planes we need to consider only the top two.

Slide 9: 

Since ‘Q’ is lower than ‘P’, the beam path ‘BYE’ is longer than ‘AXD’ by the amount GY+YH. This is called the path difference. angle GXA=90 0 From ∆GXY, sin θ =GY/d Hence GY=d sin θ From ∆YXH, sin θ =YH/d Hence YH=d sin θ Therefore GY+YH=2d sin θ .

Slide 10: 

Now the two reflected rays, XD & YE will constructively interfere when the path difference is equal to the wavelength or a multiple of it. Thus condition for X-ray diffraction is n λ =2d sin θ where ‘n’ is an integer(1.2.3… etc) called the order of reflection.

X-RAY DIFFRACTION METHODS: 

X-RAY DIFFRACTION METHODS

The Powder Method: 

The Powder Method

The Powder Method: 

The Powder Method Experimental equipment consist of: A X-ray source Collimator Powdered Crystals A cylindrical film or camera to record the diffraction pattern

Slide 14: 

If a monochromatic x-ray beam is directed at a single crystal, then only one or two diffracted beams may result. A sample of some hundreds of crystals (i.e. a powdered sample) show that the diffracted beams form continuous cones. A circle of film is used to record the diffraction pattern as shown. Each cone intersects the film giving diffraction lines. The lines are seen as arcs on the film. If the sample consists of some tens of randomly orientated single crystals, the diffracted beams are seen to lie on the surface of several cones. The cones may emerge in all directions, forwards and backwards

Slide 15: 

A particular family of planes in a crystal will only reflect an X-ray beam when the Bragg eqn is fulfilled. If a single crystal was placed in a X-ray beam, then it would be a mere chance that a particular family of plane was in the correct position to satisfy the Bragg eqn. Suppose we take the crystal and powder it. This does not destroy the crystal structure, it simply produces millions of very small crystals pointing in all possible directions.

Slide 16: 

Thus if the powdered crystal is placed in a monochromatic X-ray beam, then for any particular family of planes, there will be at least a few having those planes satisfying the Bragg eqn.

Slide 17: 

Above figure shows a powdered crystal (A) irradiated with a monochromatic X-ray beam.

Slide 18: 

CD represents a particular family of planes in a crystal satisfying the Bragg eqn , so part of the beam is reflected along AX . In the same way a crystal having the same family of planes orient along EF it reflects part of the incident beam along AY. Now the 2 crystals have the planes oriented at θ to the beam and at right angles to the plane of the paper. The sample will also contain crystals having the same family of planes oriented at θ to the beam but not at right angles to the plane of the paper

Slide 19: 

This will give rise to reflections coming out of and going into the plane of the paper . Thus if we have all possible orientations of this family of planes w.r.t to the plane of the paper then a cone of reflected rays is produced of semi vertical angle 2 θ . The sample will contain crystals having several families of planes satisfying the Bragg eqn. Since different families have different ‘d’ values the diffraction cones will have different values of the semi vertical angle 2 θ . Thus the net result is the formation of a series of concentric cones.

POWDER DIFFRACTOMETER: 

POWDER DIFFRACTOMETER

Slide 21: 

Here a flat specimen is mounted on a turntable around which moves a detector. As the sample rotates , so the angle θ between the incident beam and the sample changes. Whenever the Bragg condition is fulfilled X-Rays are reflected to the detector . The detector is connected to the specimen table and geared in such a way that when the table rotates through θ , the detector rotates through 2 θ degrees. This results in the detector always being in the correct position to receive X-Rays from the sample.

Scintag PAD V diffractometer (θ-2θ): 

Scintag PAD V diffractometer ( θ -2 θ )

Application of XRD: 

Application of XRD Differentiation between crystalline and amorphous materials; Determination of the structure of crystalline materials Determination of electron distribution within the atoms, and throughout the unit cell; Determination of the orientation of single crystals; Determination of the texture of polygrained materials; Measurement of strain and small grain size XRD is a nondestructive technique . Some of the uses of x-ray diffraction are;

Advantages and disadvantages of X-rays: 

Advantages and disadvantages of X-rays Advantages; X-ray is the cheapest, the most convenient and widely used method. X-rays are not absorbed very much by air, so the specimen need not be in an evacuated chamber. Disadvantage; They do not interact very strongly with lighter elements.

REFERENCES: 

REFERENCES Instrumental Methods of Chemical Analysis by Gurdeep R. Chatwal and Sham K. Anand. X-Ray METHODS BY Clive Whiston. Internet source.

THANK YOU: 

THANK YOU