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Presentation on X-Ray Diffraction

Submitted to Submitted byDr. Sudipta Saha Pooja Joshi M.Pharma Ist sem Pharmacology

ContentsIntroductionPrincipleBraggs law & diffractionInstrumentationX-Ray diffraction methodsApplicationsConclusionReference

INTRODUCTION

Every crystalline substance gives a pattern; the same substance always gives the same pattern; & in a mixture of substances each produces its pattern independently of the others

The X-ray diffraction pattern of a pure substance is, therefore, like a fingerprint of the substance. It is based on the scattering of x-rays by crystals.

Definition :-The atomic planes of a crystal cause an incident beam of X-rays to interfere with one another as they leave the crystal. The phenomenon is called X-ray diffraction

Principle

Diffraction occurs only when Braggs Law is satisfied Condition for constructive interference (X-rays 1 & 2) from planes with spacing dX Ray 1X Ray 2Braggs Law and Diffraction

d

dSin

Ray 1Ray 2

Deviation = 2

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

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

for constructive interference, n=2dsin this is called as BRAGGS LAW

INSTRUMENTATIONProduction of x-raysCollimatorMonochromator a. Filter b. Crystal monochromatorDetectors a. Photographic methods b. Counter methods

PRODUCTION OF X-RAYS:

X-rays are generated when high velocity electrons impinge on a metal target.

Approximately 1% of the total energy of the electron beam is converted into x-radiation.

The remainder being dissipated as heat.

Many types of x-ray tubes are available which are used for producing x-rays.

A . Positive voltage in the form of anode having a target. B . Battery to emit thermo ionic electrons.C . Cathode filament of tungsten metal.The electrons are accelerated towards the target AOn striking the target the electrons transfer their energy to its metallic surface which gives off x-ray radiation

Coolidge tubeACB

COLLIMATOR

In order to get a narrow beam of x-rays, the x-rays generated by the target material are allowed to pass through a collimator which consists of two sets of closely packed metal plates separated by a small gap.

The collimator absorbs all the x-rays except the narrow beam that passes between the gap.

TYPES OF MONOCHROMATORS

In order to do monochromatization,2 methods are available 1.Filter 2.Crystal monochromator a)Flat crystal monochromator b)Curved crystal monochromator

Materials used- NaCl, quartz etc,.

1.FILTER:

X-ray beam may be partly monochromatized by insertion of a suitable filter

A filter is a window of material that absorbs undesirable radiation but allows the radiation of required wavelength to pass

2)CRYSTAL MONOCHROMATOR Crystal monochromators is made up of suitable crystalline material positioned in the x-ray beam so that the angle of reflecting planes satisfied the Braggs equation for the required wavelength.The beam is split up into component wavelengths crystals used in monochromators are made up of materials like Nacl, lithium fluoride , quartz etc.

DETECTORSThe x-ray intensities can be measured and recorded either by 1)Photographic methods 2)Counter methods a) Geiger - Muller tube counter b) Proportional counter c) Scintillation detector d) Solid state semi conductor detector e) Semi conductor detectors

Both these types of methods depends upon ability of x-rays to ionize matter and differ only in the subsequent fate of electrons produced by the ionizing process.

Photographic method: To record the position and intensity of x-ray beam ,a plane or cylindrical film is used The film after exposing to x-ray is developedThe blackening of the developed film is expressed in terms of density units D given by D=log I/I I- incident intensities I- transmitted intensities D-Total energy that causes blackening of the film D is measured by densitometerThe photographic method is mainly used in diffraction studies since it reveals the entire diffraction pattern on a single film.Dis advg: time consuming and uses exposure of several hours.

COUNTER METHODS: a) Geiger - Muller tube counterGeiger tube is filled with inert gas like argonCentral wire anode is maintained at a positive potential of 800 to 2500V .

The electron is accelerated by the potential gradient and causes the ionisation of large number of argon atoms ,resulting in the production of avalanche of electrons that are travelling towards central anode

X-RAY

Collision with filling gas

Production of an ion pairElecton-central anodePositive ion-moves to outer electrode

b)Proportional counter Construction is similar to Geiger tube counter.Proportional counter is filled with heavier gas like xenon and krypton.Heavier gas is preferred because it is easily ionized.Operated at a voltage below the Geiger plateau. The dead time is very short (~0.2s), it can be used to count high rates without significant error.

c) Scintillation DetectorIn a scintillation detector there is large sodium iodide crystal activated with a small amount of thallium.When x-ray is incident upon crystal , the pulses of visible light are emitted which can be detected by a photo multiplier tube.Useful for measuring x-ray of short wavelength.Crystals used in scintillation detectors include sodium iodide , anthracene ,napthalene and p-terphenol.

d) Solid state semi-conductor detectorIn this type of detector ,the electrons produced by x-ray beam are promoted into conduction bands and the current which flows is directly proportional to incident x-ray energy

Disadvantage: Semi conductor device should be maintained at low temperatures to minimize noise and prevent deterioration

When x-ray falls on silicon lithium drifted detector an electron (-e) and a hole (+e)Pure silicon made up with thin film of lithium metal plated onto one endUnder the influence of voltage electrons moves towards +ve charge and holes towards veVoltage generated is measure of the x-ray intensity falling on crystalUpon arriving at lithium pulse is generatedVoltage of pulse=q/c; q-tot charge collected on electrode, c-detector capacity.

e)semi-conductor detectors

X-RAY DIFFRACTION METHODS These are generally used for investigating the internal structures and crystal structures of various solid compounds. They are:- 1.Laues photographic method a)Transmission method b)Back reflection method

2.Braggs X-ray spectrometer method

3.Rotating crystal method

4.Powder method

1) Laue photographic method

a)Transmission Laue method

In the transmission Laue method, the film is placedbehindthe crystal to record beams which are transmitted through the crystal.One side of the cone of Laue reflections is defined by the transmitted beam. The film intersects the cone, with the diffraction spots generally lying on an ellipse.Can be used to orient crystals for solid state experiments.Most suitable for the investigation of preferred orientation sheet particularly confined to lower diffraction angles.Also used in determination of symmetry of single crystals.

b)Back-reflection method

In the back-reflection method, the film is placedbetweenthe x-ray source and the crystal. The beams which are diffracted in a backward direction are recorded.One side of the cone of Laue reflections is defined by the transmitted beam. The film intersects the cone, with the diffraction spots generally lying on an hyperbola.This method is similar to Transmission method however, black-reflection is the only method for the study of large and thick specimens.Disadvantage: Big crystals are required

Laue-beam of x-ray-crystal-emitted x-ray obtained on photographic plate-using photograph-brag analyzed structures of crystals of Nacl, Kcl,and ZnS.

Bragg devised a spectrometer to measure the intensity of X-Ray beam.This method is based on Braggs law .

2) The Braggs x-ray spectrometer method

The Braggs x-ray spectrometer method Various components of Braggs X-ray spectrometers are as follows:-X-rays from the anticathode A are allowed to pass through two adjustable slits B & B to get a thin beam of X-rays. Then , these rays are made to fall upon the crystal C.The position of crystal can be adjusted by the vernier V capable of motion along the circular scale C.

iii) The reflected X-rays from the crystal after passing through the slit F enter the ionization chamber E through a narrow aluminium window W. iv) One plate of ionization chamber is connected to the positive terminal of a H.T Battery , while negative terminal is connected to quadrant electrometer Q(measures the strength of ionization current)

Working:Crystal is mounted such that =0 and ionization chamber is adjusted to receive x-rays.Crystal and ionization chamber are allowed to move in small steps, so that the angle through which the chamber is moved is twice the angle through which the crystal is rotated.X-ray spectrum is obtained by plotting a graph between ionization current and the glancing angle Peaks are obtained, peaks corresponds to Braggs reflection corresponding to different order glancing angles 1, 2, 3 are obtained, with known values of d and n and from the observed value of , can be measured.

3)ROTATING CRYSTAL METHODPhotographs can be taken by :1.Complete rotation method: in this method series of complete revolutions occur.Each set of a plane in a crystal diffracts four times during rotationFour diffracted beams are distributed into a rectangular pattern in the central point of photograph2.Oscillation method: the crystal is oscillated at an angle of 15 or 20The photographic plate is also moved back and forth with the crystal The position of the spot on the plate indicates the orientation of the crystal at which the spot was formed

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. The analyzed material is finely ground, homogenized, and average bulk composition is determined.4)POWDER CRYSTAL METHOD

When monochromatic beam is allowed to pass different possibilities may happen :-

There will be some particles out of random orientation of small crystals in the fine powder.Another fraction of grains will have another set of planes in the correct positions for the reflections to occur.Reflections are possible in different orders for each set.

If the angle of incidence is then the angle of reflection will be 2If the radius is r the circumference 2r corresponds to a scattering angle of 360

From the above equation the value of can be calculated and substituted in Bragg's equation to get the value of d ApplicationsUseful for determining the complex structures of metals and alloys.characterization of crystalline materials.identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically.determination of unit cell dimensions.measurement of sample purity.

=360*1/r

Applications Of XRD

Structure of crystals.Polymer characterization.State of anneal in metals.Particle size determination.Spot counting method.Broadening of diffraction lines.Low-angle scattering.5 Applications of diffraction methods to complexes.Determination of cis-trans isomerism.Determination of linkage isomerism.6.Miscellaneous applications.

1)Structure of Crystals

a-x-ray pattern of salt Naclb-x-ray pattern of salt Kclc-x-ray pattern of mixture of Nacl &Kcld-x-ray pattern of a powder mixed crystal of Nacl & Kcl

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2) Polymer Characterization

Determine degree of crystanillityNon-crystalline portion scatters x-ray beam to give a continuous background(amorphous materials)Crystalline portion causes diffraction lines that are not continuous.(crystalline materials

3)State of anneal in metals: XRD is used to test the metals without removing the part from its position and without weakening it.

4)Particle size determination

Spot counting method: v=V..cos/2n

V=volume of individual crystallite V=total volume irradiated n=no. of spots in diffraction ring =divergence of x-ray beam

5)Miscellaneous ApplicationSoil classification based on crystallinityAnalysis of industrial dustsAssessment of weathering & degradation of minerals & polymersStudy of corrosion productsExamination of tooth enamel & dentineExamination of bone state & tissue stateStructure of DNA&RNA

CONCLUSIONS

For materials including metals, minerals, plastics, pharmaceuticals and semiconductors XRD apparatus provide highly accurate tools for non-destructive analysis. The diffraction systems are also supported by an extensive range of application software

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Refrencences

1) Chatwal R Gurdeep, Anand k Sham, Instrumental methods of chemical analysis, Himalayan publications. page no:2.303-2.332

Sagar vidya G, Instrumental methods of drug analysis, Pharma med press. Page no:- 491-510

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

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