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X-Ray Diffraction

Presented by…J.N.V.Indira deviM.Pharmacy 1st yearPharmaceutical analysis and

quality assuranceYalamarty pharmacy collegeAndhra universityVisakhapatnam

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CONTENTS

INTRODUCTIONBRAGG EQUATION INSTRUMENTATIONX-RAY DIFFRACTION METHODSAPPLICATIONS

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ELECTROMAGNETIC SPECTRUM

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INTRODUCTION

X-Rays : X-rays are short wave length electromagnetic

radiations produced by the deceleration of high energy electrons or by electronic transitions of electrons in the inner orbital of atoms

X-ray region 0.1to100 A˚ Analytical purpose 0.7 to 2 A˚ More energetic (i.e. can penetrate deeper into a

material).  Emitted in a continuous band of white radiation as

well as a series of discrete lines that are characteristic of the target material.

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Characteristic Radiation: The characteristic lines in an atom's emission spectra are called

K, L, M, ... and correspond to the n = 1, 2, 3, ... quantum levels of the electron energy states, respectively.

α lines (n = 2 to n = 1, or n = 3 to n = 2). β lines (n = 3 to n = 1 or n = 4 to n = 2). Moseley found that :1/λ = K2 [Z - σ]2.

Electronic energy levels of an atom

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GENERATION OF X-RAYS

Generated by bombarding a metal target with an energetic beam of electrons.

Synchrotrons: Very expensive to build and maintain.

By use of radioactive source like 55Fe.

A new method of generating X rays that is not yet commercially available uses an electron-impact beam impinging on a stream of liquid gallium.

X rays may also be produced while unrolling adhesive tape from a tape dispenser.

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X-RAY TECHNIQUESX-ray absorption methods Fraction of X-ray photons absorbed is considered. Used in elemental analysis and thickness

measurements.X-ray fluorescence methods Wavelength and intensity of generated X-rays are

measured for qualitative and quantitative analysis. Non-destructive and requires little sample preparation.X-ray diffraction methods Scattering of X-rays by crystals. Determines crystalline structure.

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X-RAY DIFFRACTOMETER

The Bruker-D8 diffractometer

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d

dSin

The path difference between ray 1 and ray 2 = 2d Sin For constructive interference: n = 2d Sin

Ray 1

Ray 2

Deviation = 2

Constructive interference of the reflected beams emerging from two different planes will take place if the difference in path lengths of two rays is equal to whole number of wavelengths.

BRAGG’s EQUATION

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INSTRUMENTATION X-ray source:1.Crooke’s tube 2.Coolidge tube

Collimator

Monochromator-1.Filter type 2.Crystal type Detectors: a)Photographic methods b)Counter methods: 1.Geiger-muller counter 2.Proportional counter 3.Scintillation counter 4.Solid-state semi-conductor detector 5.Semi conductor detectors

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INSTRUMENTATION OF XRD

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X-RAY SOURCECrookes tubeCalled as cold cathode tube.

Electrons are generated by ionization

of the residual air in the tube, instead of heated filament.

An aluminum cathode plate at one end of the tube created a beam of electrons, which struck a platinum anode target at the center generating X-rays.

Advantage

Point source X-rays, which resulted in sharper images.

Disadvantage :Unreliable

Crookes tube

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Coolidge tubeCalled as hot cathode tube.Works with a very good quality vacuum (about 10-4 Pa,The electrons are produced by thermionic effect from a tungsten

filament heated by an electric current. There are two designs:

1.End-window tubes: Have thin "transmission target" to allow X-rays to pass through the target 2.side-window tubes:An Electrostatic Lens to focus the beam onto a very small spot on the anode.• A window designed for escape of the generated X-ray photons.• Power 0.1 to 18 kW.

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COLLIMATOR

Inserted in the diffracted-beam to get a narrow x-ray beam.It consists two sets of closely packed metal plates seperated by a gap.The left end of the collimator shown is mounted on the X-ray tube. The yellow-colored region at the left end determines the the size of the beam.The green region at the right end removes parasitic radiation.

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MATERIALS USED:NACL,LIF,QUARTZ ETC,.

Filter type

A window that absorbs undesirable radiation and allows required wavelength to pass.

Eg:Zr absorbs x-rays emitted by Mo.

Crystal type

Positioned in the x-ray beam so that the angle of the reflecting planes satisfied the Bragg’s equation for the required wave length.

Characteristics of a crystal:

Mechanically strong and stable

The mosaicity and resolution of the crystal, should be small.

MONOCHROMATOR

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DETECTORS

Photographic methods Contains photographic plate Blackening of developed film is expressed in terms of density ,D

D=Log Io/I.

Photostimulable phosphorsAn increasingly common method is the use of

photo stimulated luminescence Photostimulable phosphor plate (PSP plate) is used in place of the

photographic plate. After the plate is X-rayed, excited electrons in the phosphor material

remain ‘trapped' in'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface.

The light given off during laser stimulation is collected by a photomultiplier tube ADVANTAGE:The PSP plate can be reused

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Counter methods

1.Geiger-muller counter:Filled with an inert gas like argon.

Measures ionizing radiation. Detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays

Advantages:

a)Trouble free

b)Inexpensive

Disadvantages:

a)Cannot be used to measure energy of ionizing radiation.

b)Used for low counting rates

c)Efficiency falls off below 1A

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2.Proportional counter:Filled with heavier gas like xenon or krypton as it is easily ionized.

Output pulse is dependent on intensity of X-rays falling on counter.

Count the particles of ionizing radiation and measures their energy.

Advantages:

a)Count high rates with out significant error.

Disadvantages:

a)Associated electronic circuit is complex.

b)Expensive.

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3.Scintillation counter:Measures X-rays of shorter wavelengths.

The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic (usually containing anthracene), or organic liquid that fluoresces when struck by ionizing radiation.

The PMT is attached to an electronic amplifier to count and possibly quantify the amplitude of the signals.

Advantages:

a)Count high rates.

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4.Solid state semi-conductor detector: The electrons produced by X-ray beam are promoted

into conduction bands and the current which flows is directly proportional to the incident X-ray energy.

Disadvantage: Maintainted at very low Temp to minimise the noise

and prevent deterioration of the detector. 5.Semi-conductor detectors: Silicon-lithium drifted detector. The principle is similar to gas

ionization detector. Voltage of pulse=Q/C

Application: In neutron activation

analysisSemi-conductor detector

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X-RAY DIFFRACTION METHODS1.LAUE PHOTOGRAPHIC METHOD:

The Laue method is mainly used to determine the orientation of large single crystals. White radiation is reflected from, or transmitted through, a fixed crystal.

Back-reflection LaueIn the back-reflection method, the film is Placed between X-ray source and crystal. The beams which are diffracted in a backward direction are recorded.

Transmission Laue

The film is placed behind the crystal to record beams which are transmitted through the crystal.

Disadvantage: Big crystals are required

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Back-reflection Laue Transmission Laue

Crystal orientation and perfection is determined from the position of spots.

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2.BRAGG X-RAY SPECTROMETER METHOD:

Bragg analysed the structures of Nacl,Kcl and ZnS.

Method is based on Bragg’s law.

The strength of ionisation current is directly proportional to intensity of entering reflected X-rays.

SO2 or CH3I increases ionisation in the chamber.

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3.ROTATING CRYSTAL METHOD:

Shaft is moved to put the crystal into slow rotation.

This cause sets of planes coming successively into their reflecting position.

Each plane will produce a spot on the photographic plate.

Can take a photograph of the diffraction pattern in two ways1.complete rotation method 2.oscillation method

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4.POWDER CRYSTAL METHOD:

1mg material is sufficient for study. Applications: useful for

Cubic crystals.Determining complex structures of metals and alloys.Making distinction between allotropic modification of the same substance.

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APPLICATIONSStructure of crystalsPolymer characterizationState of anneal in metalsParticle size determinationSpot counting method

v=V.δθ.cosθ/2nBroadening of diffraction linesLow-angle scattering

Applications of diffraction methods to complexes Determination of cis-trans isomerism

Eg:Bis(pyridine-2-carboxamido)nickel(II) chloride Determination of linkage isomerism

Eg:Biuret+copper(II)=pottassium bis(biureto)cuprate(II) tetrahydrate

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MISCELLANEOUS APPLICATIONS

Soil classification based on crystallinity. Analysis of industrial dusts. Assessment of weathering & degradation of

minerals & polymers. Study of corrosion products. Examination of tooth enamel & dentine. Examination of bone state & tissue state. Structure of DNA&RNA. X-ray Diffraction on aerosol drugs -

Measurements on small quantities of pharmaceutical samples using the X'Celerator detector.

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REFERENCES1)Instrumental methods of chemical analysis ,B.K.sharma,17th

edition 1997-1998,GOEL publishing house.page no:329-359

2)Principles of instrumental analysis,5th edition ,by Dougles a.skoog,f.James holles,Timothy A.Niemen.page no:277-298

3)Instrumental methods of chemical analysis ,Gurudeep R.chatwal,sham k.anand,Himalaya publications page no:2.303-2.332

4) Instrumental Methods Of Chemical Analysis – H. Kaur pg.no:727-729,737

5) http://www.scienceiscool.org/solids/intro.html

6) http://en.wikipedia.org/wiki/X-ray_crystallography

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Thank you