x-ray-diffraction-technique

$Page 1: x-ray-diffraction-technique$
X- Ray Diffraction

PresentationBy

ArchanaM.Pharmacy (Pharmaceutics)

GPRCP

1

Ch.Archana,M.Pharmacy(Pharmaceutics),Roll no:15

CONTENTSINTRODUCTIONGENERATION OF X-RAYSPRINCIPLEINSTRUMENTATIONMETHODSAPPLICATIONSCONCLUSIONSREFERENCES

2Ch.Archana,M.Pharmacy(Pharmaceutics),Roll no:15

INTRODUCTION:

X-rays were discovered by Wilhelm Roentgen who called them x-rays because the nature at first was unknown so, x-rays are also called Roentgen rays. X-ray diffraction in

crystals was discovered by Max von Laue. The wavelength range is 10-7 to about 10-15 m.

The penetrating power of x-rays depends on energy also, there are two types of x-rays.

i) Hard x-rays: which have high frequency and have more energy.

ii) soft x-rays: which have less penetrating and have low energy

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Max Von Laue


X-RAYS

1.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

2.X-ray region 0.1to100 A˚3.Analytical purpose 0.7 to 2 A˚


PRINCIPLE 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 towards the sample. The interaction of incident rays with the sample produces constructive interference when conditions satisfy Bragg’s law.


BRAGG’s EQUATION

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 path lengths of two rays is equal to whole number of wavelengths”.

for constructive interference, nλ=2dsin this is called as BRAGG’S LAW


INSTRUMENTATION Production of x-rays Collimator Monochromator a.Filter b.Crystal monochromator Detectors a.Photographic methods b.Counter methods


Instrumentation of XRD


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.


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a . Positive voltage in the form of anode having a target a

• b . Battery to emit thermoionic electrons• C. Cathode –filament of tungsten metal• The electrons are accelerated towards the target

a• On striking the target the electrons transfer their

energy to its metallic surface which gives off x-ray radiation

b c a


Coolidge tube

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

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In order to do monochromatization,2 methods are available

1.Filter2.Crystal monochromatora)Flat crystal monochromatorb)Curved crystal monochromator

Materials used-Nacl,quartz etc,.


A.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

Ch.Archana,M.Pharmacy(Pharmaceutics),Roll no:15 14

• 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 Bragg’s equation for the required wavelengththe 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 I₀/II₀- incident intensitiesI- transmitted intensitiesD-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 gasProduction of an ion

pair

Electon-central anodePositive

ion-moves to

outer electrod

e

b)PROPORTIONAL COUNTER:Construction is similar to Geiger tube

counterProportional counter is filled with heavier gas

like xenon and kryptonHeavier gas is preferred because it is easily

ionizedOperated at a voltage below the geiger

plateau The dead time is very short (~0.2μs), it can

be used to count high high rates without significant error.


C)SCINTILLATION DETECTOR:In 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 ixylene.

The dead time is shortCh.Archana,M.Pharmacy(Pharmaceutics),Roll no:15 20

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

Dis advantage:Semi – conductor device should be

maintained at low temperatures to minimize noise and prevent deterioration


e)semi-conductor detectors:

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 end Under the influence of voltage electrons moves towards +ve charge and holes

towards –ve Voltage generated is measure of the x-ray intensity falling on crystal Upon arriving at lithium pulse is generated Voltage of pulse=q/c; q-tot charge collected on electrode, c-detector capacity.


X-RAY DIFFRACTION METHODS These are generally used for investigating the

internal structures and crystal structures of various solid compounds.

They are

1.Laue’s photographic method

a)Transmission method

b)Back reflection method

2.Bragg’s X-ray spectrometer method

3.Rotating crystal method

4.Powder method



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a)Transmission Laue method

In the transmission Laue method, the film is placed behind the 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.Ch.Archana,M.Pharmacy(Pharmaceutics),Roll no:15

b)Back-reflection method

In the back-reflection method, the film is placed between the 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



Crystal orientation is determined from the position of the spots. Each spot can be indexed, i.e. attributed to a particular plane, using special charts.

The Greninger chart is used for back-reflection patterns and the Leonhardt chart for transmission patterns.

The Laue technique can also be used to assess crystal perfection from the size and shape

The Bragg’s x-ray spectrometer method:Laue-beam of x-ray-crystal-emitted x-ray obtained on

photographic plate-using photograph-brag analysed structures of crystals of Nacl,Kcl,and Zns-brags equation

Single plane generates several diffraction lines-sum tot of diffraction lines gives diffraction patterns-from the pattern we can deduce different distances between planes-angle between planes in each of three dimensions


source

The Bragg’s x-ray spectrometer method:

A-anti cathodeB-B’ – Adjustable slitsC-crystalE-ionization chamberOne plate of ionization chamber is connected to

the positive terminal of a H.T Battery , while negative terminal is connected to quadrant electrometer(measures the strength of ionization current) Ch.Archana,M.Pharmacy(Pharmaceutics),

Roll no:15 29

The Bragg’s x-ray spectrometer method

Working:Crystal is mounted such that ѳ=0° and ionization

chamber is adjusted to receive x-raysCrystal and ionization chamber are allowed to move

in small stepsThe angle through which the chamber is moved is

twice the angle through which the crystal is rotatedX-ray spectrum is obtained by plotting a graph

between ionization current and the glancing angleѳPeaks are obtained.peaks corresponds to Bragg’s

reflectionDifferent order glancing angles are obtained with

known values of d and n and from the observed value of ѳ , λ can be measured.


DETERMINATION OF CRYSTAL STRUCTURE BY BRAGG,S LAW

X-Rays falls on crystal surfaceThe crystal is rotated and x-rays are made to

reflect from various lattice planes The intense reflections are measured by

bragg’s spectrometer and the glancing angles for each reflection is recorded

Then on applying bragg’s equation ratio of lattice spacing for various groups of planes can be obtained.

Ratio’s will be different for different crystalsExperimentally observed ratio’s are

compared with the calculated ratio’s ,particular structure may be identified.


ROTATING CRYSTAL METHOD:

Photographs 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 rotation Four diffracted beams are distributed into a rectangular pattern in the

central point of photograph 2.Oscillation method:the crystal is oscillated at an angle of 15° or 20° The photographic plate is also moved vack and forth with the crystal The position of the spot on the plate indicates the orientation of the

crystal at which the spot wasformedCh.Archana,M.Pharmacy(Pharmaceutics),Roll no:15 32

POWDER CRYSTAL METHOD:

Fine powder is struck on a hair with a gum ,it is suspended vertically in the axis of a cylindrical camera

When monochromatic beam is allowed to pass different possibilities may happen 1. There will be some particles out of random orientation of small crystals in the

fine powder2. Another fraction of grains will have another set of planes in the correct

positions for the reflections to occur3. Reflections are possible in different orders for each set


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.

If the angle of incidence is ѳ then the angle of reflection will be 2ѳ

If the radius is r the circumference 2πr corresponds to a scattering angle of 360°

From the above equation the value of ѳ 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 materialsidentification of fine-grained minerals such as clays and

mixed layer clays that are difficult to determine opticallydetermination of unit cell dimensionsmeasurement of sample purity


Ѳ=360*1/πr

APPLICATIONS OF XRD1. Structure of crystals2. Polymer

characterisation3. State of anneal in

metals4. Particle size

determinationa) Spot counting

methodb) Broadening of

diffraction linesc) Low-angle

scattering

5.Applications of diffraction methods to complexes

a) Determination of cis-trans isomerism

b) Determination of linkage isomerism

6.Miscellaneous applications


1.STRUCTURE OF CRYSTALS

a-x-ray pattern of salt Nacl

b-x-ray pattern of salt Kcl

c-x-ray pattern of mixture of Nacl &Kcl

d-x-ray pattern of a powder mixed crystal of Nacl & Kcl


2.POLYMER CHARACTERISATIONDetermine 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)


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

4.PARTICLE SIZE DETERMINATION

Spot counting method: v=V.δθ.cosθ/2n

V=volume of individual crystallite V=total volume irradiated n=no. of spots in diffraction ring δθ =divergence of x-ray beam Ch.Archana,M.Pharmacy(Pharmaceutics),

Roll no:15

MISCELLANEOUS APPLICATIONS

Soil 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


X-ray diffraction pattern for a single alum crystal.

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X-ray diffraction image of a crystal of lysozyme




Bruker's X-ray Diffraction D8-Discover instrument

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

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



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