X-RAY DIFFRACTIONby-Ms. Deepika Pandit, Mr. K.S. Rathore, Dr. Anju GoyalB.N. Girls College of Pharmacy, Udaipur-313001 (Raj.) : X-RAY DIFFRACTIONby-Ms. Deepika Pandit, Mr. K.S. Rathore, Dr. Anju GoyalB.N. Girls College of Pharmacy, Udaipur-313001 (Raj.) INTRODUCTION
APPLICATIONS INTRODUCTION : INTRODUCTION X-Ray spectroscopy is gradually getting importance because it belongs to a category of non destructive method of analysis. A variety of x-ray techniques and methods are in use. But we shall classify all method into 4 main category. These are-
X-ray diffraction TYPES : TYPES X-ray absorption: In these method, a beam of x-rays is allowed to pass through the sample and the attenuation or fraction of x-ray photons absorbed is considered to be a measure of the concentration of the absorbing substance.
X-ray emission: X-ray are obtained by employment of radioactive source whose decay process results in x-ray emission.
X-ray fluorescence: In these methods, X-rays are generated within the sample and by measuring the wavelength and intensity of the generated x-rays, one can perform quantitative and qualitative analysis.
X-ray diffraction method: These methods are based on the scattering of x-ray by crystals. By these methods, one can identify the crystal structures of are extremely important as compared with x-ray absorption& x-ray fluorescence methods BASIC PRINCIPLE : BASIC PRINCIPLE In 1901 roentgen received a noble prize for the discovery of x- ray these rays exist in the region of 0.01-10nm but 0.08-0.2nm is most useful region for analytical purpose.
In an atom the electrons are arranged in layers or shell’s
The electrons migrate from the vacant outer orbital to inside vacant orbit to fill up the vacant slot. The kind of energy generated leads to origin of the x- rays, time scale is approximately 10-12 -10-4 sec
The x-ray are generated by –
Bombardment of metal target with beam of high energy electrons.
Exposure of matter to primary x-rays beam to generate secondary x-ray showing fluorescence.
Use radioactive element which on disintegration leads to x-ray formation.
From synchronization of radiation source but the last is most expensive process X-RAY SOURCE : X-RAY SOURCE In x-ray instruments, sources are-
Secondary fluarescencent sources.
The most common source is a highly evacuated tube.
The anode is heavy, hollow, water cooled block of copper with a metal target plated. The metal having high melting point, good thermal conductivity and large atomic number (N).
Such metal are silver, iron, copper, chromium, tungsten, rhodium, cobalt, molybdenum. INSTRUMENTATION : INSTRUMENTATION X-ray generating equipment:
A series of closely spaced, parallel metal plates or by a bundle of tubes, 0.5mm or smaller in diameter.
When the wavelength of two spectral lines is nearly the same there is an element may be used as a filter to reduce the intensity of the line with the shorter wavelength. Table of filters: : Table of filters: Slide 10: Analyzing crystals
The relationship between the wavelength of the x-ray beam, the angle of diffraction, θ, and the distance between each set of atomic planes of the crystal lattice, d, is given by the Bragg condition:
mλ = 2d sin θ
Where, m= order of diffraction. The geometrical relationship are shown in fig4.
For the ray diffracted by the second plane of the crystal, the distance CBD represents the additional distance of travel in comparison with a ray reflected from the surface plane. Angles CAB and BAD are both equal to θ.Therefore,
CB= BD =AB sin θ
CBD = 2 AB sin θ
Where AB is the interplanar spacing, d. Slide 11: . The d-value should be small enough to make the angle 2θgreater than approximately 8° even at the shortest wavelength used
A small d-spacing is also favorable for producing a larger dispersion δθ/δλ, of the spectrum, as seen by differentiating the Bragg equation:
On the other hand, a small d value imposes an upper limit to the range of wavelengths that can be analyzed because at
λ = 2d
the angle 2θ becomes 180°. Slide 12: Detector:
Semiconductor detector Reciprocal lattice concept : Reciprocal lattice concept Diffraction phenomena are interpreted most conveniently with the aid of the reciprocal lattice concept.
A plane can be represented by a line drawn normal to the plane.
When a normal is drawn to each plane in a crystal with a length inversely proportional to the interplanar spacing and the normal's are drawn from a common origin, the terminal points of these normal constitute a lattice array.
This is called the reciprocal lattice because the distance of each point from the lattice is reciprocal to the interplanar spacing of the planes that it represents. Diffraction pattern : Diffraction pattern Laue photographic method
Bragg x-ray spectrometer method
Rotating crystal method
Laue photographic method:
In this method the crystal is held stationary in a beam of x-rays , after passing through the crystal is diffracted and is recorded on a photographic plate. Laue pattern can be used to orient crystals for solid state experiments.
Back reflection method:
This method provides similar information as the transmission method. Slide 15: Bragg’s x-ray spectrometer method:
Using the Laue’s photograph, Bragg analyzed the structures of crystals of sodium chloride, KCl and ZnS.
Bragg devised a spectrometer to measure the intensity of x-ray beam.
The spectra obtained in this way can be employed for crystallographic analyses.
This is based on the Bragg’s equation:
nλ =2d sin θ
This equation gives the condition which must be satisfied for the reflection of x-rays from a set of atomic planes. Slide 16: Rotating crystal method:
In this method monochromatic x-radiation is incident on a single crystal that is rotated about one of its axes.
The reflected beams lie as spots on the surface of cones that are coaxial with the rotation axis.
The diffracted beam directions are determined by intersection of the reciprocal lattice points with the sphere of reflection.
By remounting the crystal successively about different axes, one can determine the complete distribution of reciprocal lattice points.
One mounting is sufficient if the crystal is cubic but two or more may be needed if the crystal has lower symmetry. Slide 17: Powder method:
In these method the crystal is replaced by a large collection of very small crystals, randomly oriented, and a continuous cone of diffracted rays is produced.
There are some important differences, with respect to rotating crystal method.
The cone obtained with a single crystal are not continuous because the diffracted beams occur only at certain points along the cone, whereas the cones with the powder method are continuous.
The cone produced in the powder method is determined by the spacing of prominent planes and are not uniformly spaced. Application: : Application: Determination of crystal structure by bragg’s law
Determination of cis-trans isomerism:
Particle size determination:
Spot counting method:
Broadening of diffraction lines:
Determination of linkage isomers THANK YOU : THANK YOU