81347482 x-ray-diffraction-technique

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X- Ray Diffraction

Presentation

By

Archana

M.Pharmacy (Pharmaceutics)

GPRCP

1Ch.Archana,M.Pharmacy(Pharmaceutics),Roll no:15

CONTENTS INTRODUCTION GENERATION OF X-RAYS PRINCIPLE INSTRUMENTATION METHODS APPLICATIONS CONCLUSIONS REFERENCES


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

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

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˚


PRINCIPLEX-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

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.Filterb.Crystal monochromator

Detectorsa.Photographic methodsb.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.


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

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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.Filter

2.Crystal monochromator

a)Flat crystal monochromator

b)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.


DETECTORS The 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 developed

The blackening of the developed film is expressed in terms of density units D given by

D=log I₀/I

I₀- incident intensities

I- transmitted intensities

D-Total energy that causes blackening of the film

D is measured by densitometer

The 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 counter

Geiger tube is filled with inert gas like argon

Central 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 pair

Electon-central anode

Positive 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 (~0.2μs), 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-terphenolixylene.

The dead time is short


d)Solid state semi-conductor detector

In 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



X-Ray Diffraction Method

Laue Rotating Crystal Powder

Orientation

Single Crystal

Polychromatic Beam

Fixed Angle

Lattice constant

Single Crystal

Monochromatic Beam

Variable Angle

Lattice Parameters

Polycrystal (powdered)

Monochromatic Beam

Variable Angle

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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.


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 cathode

B-B’ – Adjustable slits

C-crystal

E-ionization chamber

One 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)


The Bragg’s x-ray spectrometer method

Working:

Crystal is mounted such that ѳ=0° and ionization chamber is adjusted to receive x-rays

Crystal and ionization chamber are allowed to move in small steps

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 angleѳ

Peaks are obtained.peaks corresponds to Bragg’s reflection

Different 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 surface

The crystal is rotated and x-rays are made to reflect from various lattice planes

The intense reflections are measured by bragg’sspectrometer 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 crystals

Experimentally 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 wasformed


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

Applications Useful 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


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 CHARACTERISATION Determine degree of crystanillity

Non-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 crystalliteV=total volume irradiatedn=no. of spots in diffraction ring

δθ =divergence of x-ray beam


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


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 Douglesa.skoog,f.James holles,Timothy A.Niemen.page no:277-298

3)Instrumental methods of chemical analysis ,GurudeepR.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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81347482 x-ray-diffraction-technique