4. How to Determine the Atomic And Molecular
Structure of Crystal
In this technique a beam of X-rays strikes a crystal and causes the
beam of light to ( diffract ( ~spread ) into many specific directions.
From the angles and intensities of these diffracted beams, a
crystallographer can produce a three dimensional picture of the
density of electrons within the crystal.
From this Electron Density, the mean position of atom in the crystal
can be determined, as well as their chemical bonds, their
crystallographic disorder and various other information.
6. A X-ray Source
01. A Wavelength Selector
or Mono-Chromator
02.
A Sample Holder
03.
A Detector
04.
Components Of
Instrumentation
7. A X-ray Source
01.
The cathode in the form of heated tungsten filaments
emit electrons, which are accelerated towards anode
due to its positive potential. Accelerated electrons hit
the metallic target, used in the form of an anode. When
the electrons hit the target material, X-rays are emitted
out. The voltage applied for heating the cathode decides
the number of electrons emitted by the cathode.
X ray Tube or Coolidge Tube
Certain radioactive substances produce X-rays as a result
of their radioactive decay process and can acts as a
source for X-rays. The elements such as 26Fe5, 27C07,
+Cd109, and 125produce X-rays by electron capture or K-
capture. Tritium (1H³), lead (82Pb210) produce X- rays by
B-emission process..
8. 02.
A Crystal Monochromator
A Wavelength Selector
1.
The isolation of a narrow range of wavelength
(monochromatic X-Rays) from the full range can be
achieved by using following devices.
1. Filter
2. Monochromator
9. A Sample Holder
03.
The sample holder is nothing but a rotating
table called as a crystal mount. A sample
crystal is placed at the centre of a crystal
mount, which is kept rotating at a particular
speed
11. Bragg’s Law
The X-ray diffracted from atom in the crystal plane obeys
the law of reflection.
Law of reflection- is state that when the ray of light is reflect to the
surface of atom in crystal the angle of incidence is equal to the
angle of reflection.
Consider the crystal that has one crystal plane 1 and the second
crystal plane 11. The space between the two crystal plane is
dominated by ‘d’.
When x-rays are scattered from a crystal lattice, peaks of scattered
intensity are observed which correspond to the following conditions
1. The angle of incidence = angle of scattering.
2. The path length difference is equal to an integer
number of wavelengths.
nλ = 2d sinΘ
12. X-ray diffraction studies have provided very large wealth
of information about the arrangement and the spacing of
atom in crystalline substances both organic and
inorganic.
Types of X-ray Diffraction
1. Single-Crystal x-ray Diffraction
2. Powder Diffraction Method *
3. Rotating Crystal Technique *
X-ray Diffraction Method
13. The oldest and most precise method of X-ray crystallography is single-crystal X-
ray diffraction.
In which a beam of X-rays strikes a single crystal, producing scattered beams.
When they land on a piece of film or other detector, these beams make a
diffraction pattern spots: the strengths and angles of these beams are recorded.
Each spot is called a reflection, since it corresponds to the reflection of the X-rays
from one set of evenly spaced planes within the crystal.
The atoms in a crystal are not static, but oscillate about their mean positions,
usually by less than a few tenths of an angstrom.
X-ray crystallography allows measuring the size of these oscillation
1. Single Crystal X-ray Diffraction
15. Procedure
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15
The first step and difficult
step are to obtain an
adequate crystal material
of the material under the
study
STEP 1
The Crystal is placed in an intense
beam of X-ray, usually of a single
wavelength ( Monochromatic X-ray )
producing the regular pattern of
reflection
The intensity of every spot is
recorded at every orientation of the
crystal. Multiple data sets may have
to be collected
STEP 2
These data are combined
computationally with
complementory chemical
information to produce and
refine a model of the
arrangement of atom within
the crystal and now its
called crystal structure
STEP 3
16. Rotating Crystal Technique
The rotating crystal method was developed by Schiebold.
The X-rays are generated in the X-ray tube and then the beam is made monochromatic by a filter.
From the filter, the beam is then allowed to pass through collimating system which permits a fine pencil of
parallel X-rays.
From the collimator, the X-ray beam is made to fall on a crystal mounted on a shaft which can be rotated at
a uniform angular rate by a small motor
Now the shaft is moved to put the crystal into slow rotation about a fixed axis. This causes the sets of
planes coming
successively into their reflecting positions i.e. The value of satisfies the Bragg’s relation. Each plane will
produce a spot on the photographic plate.
we can take a photograph of a diffraction pattern upon a photographic plate perpendicular to X-ray beam or
upon a film in a cylindrical camera.
17.
18. The powder diffraction method is only analytical method which is capable of
furnishing both qualitative and quantitative information about the compound
present in a solid sample.
The rotating crystal method could only be used if a single undistorted crystal is
available. To overcome this limitation, the powder method was devised.
In this method the crystalline material contained in capillary tube placed in
camera containing a film strip as shown in figure.
The powdered sample contains small crystals arranged in all orientation some of
these will reflect X-ray from each lattice plane at the same time.
The reflected X-rays will make an angle 2Θ with the original direction.
Powder Diffraction Method or Debye
Scherrer Method
19.
20. Applications
1. X-ray powder diffraction is most widely used for the identification of
unknown crystalline materials ( e.g. minerals, inorganic compounds )
2. Determination of unknown solids is critical to studies in geology,
environmental science, material science, engineering and biology.
3. Used to identify fine grained minerals such as clays
4. Measurement of sample Purity
21. Other Applications
1. Structure of Crystals
Used to Determine molecular structure of samples. It also used to measure the size of crystal
planes
2. Polymer Characterization
The amorphous material in polymer will scatter at all wavelength and give a scattered pattern
however crystalline material include crystal structure
3. Particle size Determination
Used to Determine the size of particles or crystallites
22. Other Applications
4. Miscellaneous Application
Soil classification based on crystallinity. Different types of soils, such as various types of clays
and sands, exhibit different types and degrees of crystallinity.
Tooth enamel and dentine have been examined by X-ray diffraction.
Corrosion products can be studied by this method.
X-ray diffraction can also be used to assess the weathering and degradation of natural and
synthetic minerals
