X-ray diffraction is a technique used to analyze the crystal structure of materials. When x-rays interact with the electrons in a crystal, they produce a unique diffraction pattern. Bragg's law describes the angles and wavelengths that produce constructive interference and diffraction peaks. X-ray diffraction instruments contain an x-ray source, sample holder, and detector. The document discusses several methods for collecting and analyzing x-ray diffraction data, including Laue, Bragg, rotating crystal, and powder methods. Applications include determining crystal structures, analyzing polymers and metals, and characterizing particles.
2. Contents
o Introduction
o Principle
o Bragg’s law & diffraction
o Instrumentation
o X-Ray diffraction methods
o Applications
o Conclusion
o Reference
3. 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”
5. Diffraction occurs only when Bragg’s Law is satisfied Condition for
constructive interference (X-rays 1 & 2) from planes with spacing d
X Ray 1
X Ray 2
Bragg’s Law and Diffraction
6. d
Ray 1
Ray 2
Deviation = 2
The path difference between ray 1 and ray 2 = 2d Sin
For constructive interference: n = 2d Sin
BRAGG’s
EQUATION
7. “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
8. INSTRUMENTATION
Production of x-rays
Collimator
Monochromator
a. Filter
b. Crystal monochromator
Detectors
a. Photographic methods
b. Counter methods
9.
10. 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.
11. 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 A
On striking the target the electrons transfer their energy to
its metallic surface which gives off x-ray radiation
Coolidge tube
A
CB
12. 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.
13. 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,.
14. 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
15. 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 wavelength.
The beam is split up into component wavelengths
crystals used in monochromators are made up of materials
like Nacl, lithium fluoride , quartz etc.
16. 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.
17. 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.
18. 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
19. 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 rates without significant error.
20. 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.
21. 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
Disadvantage: Semi – conductor device should be maintained
at low temperatures to minimize noise and prevent
deterioration
22. 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.
e)semi-conductor detectors
23. 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
25. 1) Laue photographic method
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.
26. 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
27. 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 Bragg’s law .
2) The Bragg’s x-ray spectrometer method
28. The Bragg’s x-ray spectrometer method
Various components of Bragg’s X-ray spectrometers are as
follows:-
i) 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.
ii) The position of crystal can be adjusted by the vernier V
capable of motion along the circular scale C.
29. 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)
30. 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, 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 ѳ
Peaks are obtained, peaks corresponds to Bragg’s reflection
corresponding to different order glancing angles ѳ1, ѳ2, ѳ3 are
obtained, with known values of d and n and from the observed
value of ѳ , λ can be measured.
31. 3)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 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
32. 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
33. 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 powder.
2. Another fraction of grains will have another set of planes in
the correct positions for the reflections to occur.
3. Reflections are possible in different orders for each set.
34. 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
35. Applications Of XRD
1. Structure of crystals.
2. Polymer characterization.
3. State of anneal in metals.
4. Particle size determination.
a) Spot counting method.
b) Broadening of diffraction lines.
c) Low-angle scattering.
5 Applications of diffraction methods to complexes.
a) Determination of cis-trans isomerism.
b) Determination of linkage isomerism.
6.Miscellaneous applications.
36. 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
37. 2) Polymer Characterization
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
38. 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.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
39. 5)Miscellaneous Application
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
40. 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
41. 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