X-Ray Diffraction


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X-Ray Diffraction






INTRODUCTION X-Rays : 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 X-ray region 0.1to100 A ˚ Analytical purpose 0.7 to 2 A ˚ More energetic (i.e. can penetrate deeper into a material ). Emitted in a continuous band of white radiation as well as a series of discrete lines that are characteristic of the target material.

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Characteristic Radiation: The characteristic lines in an atom's emission spectra are called K, L, M, ... and correspond to the n = 1, 2, 3, ... quantum levels of the electron energy states, respectively. α lines (n = 2 to n = 1, or n = 3 to n = 2). β lines (n = 3 to n = 1 or n = 4 to n = 2). Moseley found that :1/λ = K 2 [Z - σ] 2. Electronic energy levels of an atom

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GENERATION OF X-RAYS Generated by bombarding a metal target with an energetic beam of electrons. Synchrotrons: Very expensive to build and maintain. By use of radioactive source like 55 Fe. A new method of generating X rays that is not yet commercially available uses an electron-impact beam impinging on a stream of liquid gallium. X rays may also be produced while unrolling adhesive tape from a tape dispenser.


X-RAY TECHNIQUES X-ray absorption methods Fraction of X-ray photons absorbed is considered. Used in elemental analysis and thickness measurements. X-ray fluorescence methods Wavelength and intensity of generated X-rays are measured for qualitative and quantitative analysis. Non-destructive and requires little sample preparation. X-ray diffraction methods Scattering of X-rays by crystals. Determines crystalline structure.

X-Ray Diffractometer: 

X-Ray Diffractometer The Bruker-D8 diffractometer

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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 difference in path lengths of two rays is equal to whole number of wavelengths. BRAGG’s EQUATION


INSTRUMENTATION X-ray source:1.Crooke’s tube 2.Coolidge tube Collimator Monochromator-1.Filter type 2.Crystal type Detectors: a)Photographic methods b)Counter methods: 1.Geiger-muller counter 2.Proportional counter 3.Scintillation counter 4.Solid-state semi-conductor detector 5.Semi conductor detectors



X-ray source: 

X-ray source Crookes tube Called as cold cathode tube. Electrons are generated by ionization of the residual air in the tube, instead of heated filament . An aluminum cathode plate at one end of the tube created a beam of electrons, which struck a platinum anode target at the center generating X-rays. Advantage Point source X-rays, which resulted in sharper images. Disadvantage :Unreliable Crookes tube

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Coolidge tube Called as hot cathode tube. Works with a very good quality vacuum (about 10 -4 Pa, The electrons are produced by thermionic effect from a tungsten filament heated by an electric current. There are two designs: 1.End-window tubes : Have thin "transmission target" to allow X-rays to pass through the target 2.side-window tubes: An Electrostatic Lens to focus the beam onto a very small spot on the anode. A window designed for escape of the generated X-ray photons. Power 0.1 to 18 kW .


COLLIMATOR Inserted in the diffracted-beam to get a narrow x-ray beam. It consists two sets of closely packed metal plates seperated by a gap. The left end of the collimator shown is mounted on the X-ray tube. The yellow-colored region at the left end determines the the size of the beam. The green region at the right end removes parasitic radiation.

Materials used:Nacl,LiF,quartz etc,. : 

Materials used: Nacl,LiF,quartz etc,. Filter type A window that absorbs undesirable radiation and allows required wavelength to pass. Eg:Zr absorbs x-rays emitted by Mo. Crystal type Positioned in the x-ray beam so that the angle of the reflecting planes satisfied the Bragg’s equation for the required wave length. Characteristics of a crystal: Mechanically strong and stable The mosaicity and resolution of the crystal, should be small. MONOCHROMATOR


DETECTORS Photographic methods Contains photographic plate Blackening of developed film is expressed in terms of density ,D D=Log Io/I. Photostimulable phosphors An increasingly common method is the use of photo stimulated luminescence P hotostimulable phosphor plate (PSP plate) is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain ‘trapped' in'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube ADVANTAGE:The PSP plate can be reused

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Counter methods 1. Geiger-muller counter: Filled with an inert gas like argon. Measures ionizing radiation . Detect the emission of nuclear radiation: alpha particles , beta particles or gamma rays Advantages: a)Trouble free b)Inexpensive Disadvantages: a)Cannot be used to measure energy of ionizing radiation. b)Used for low counting rates c)Efficiency falls off below 1A

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2. Proportional counter: Filled with heavier gas like xenon or krypton as it is easily ionized. Output pulse is dependent on intensity of X-rays falling on counter. Count the particles of ionizing radiation and measures their energy. Advantages: a)Count high rates with out significant error. Disadvantages: a)Associated electronic circuit is complex. b)Expensive.

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3.Scintillation counter: Measures X-rays of shorter wavelengths. The sensor , called a scintillator , consists of a transparent crystal , usually phosphor, plastic (usually containing anthracene ), or organic liquid that fluoresces when struck by ionizing radiation . The PMT is attached to an electronic amplifier to count and possibly quantify the amplitude of the signals. Advantages: a)Count high rates.

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4.Solid state semi-conductor detector: The electrons produced by X-ray beam are promoted into conduction bands and the current which flows is directly proportional to the incident X-ray energy. Disadvantage: Maintainted at very low Temp to minimise the noise and prevent deterioration of the detector. 5.Semi-conductor detectors: Silicon-lithium drifted detector. The principle is similar to gas ionization detector. Voltage of pulse=Q/C Application : In neutron activation analysis Semi-conductor detector

X-RAY DIFFRACTION METHODS 1.Laue photographic method: The Laue method is mainly used to determine the orientation of large single crystals. White radiation is reflected from, or transmitted through, a fixed crystal. : 

X-RAY DIFFRACTION METHODS 1. Laue photographic method: The Laue method is mainly used to determine the orientation of large single crystals. White radiation is reflected from, or transmitted through, a fixed crystal . Back-reflection Laue In the back-reflection method, the film is Placed between X-ray source and crystal. The beams which are diffracted in a backward direction are recorded. Transmission Laue The film is placed behind the crystal to record beams which are transmitted through the crystal. Disadvantage: Big crystals are required

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Back-reflection Laue Transmission Laue Crystal orientation and perfection is determined from the position of spots.

2.Bragg X-ray spectrometer method:: 

2.Bragg X-ray spectrometer method: Bragg analysed the structures of Nacl,Kcl and ZnS. Method is based on Bragg’s law. The strength of ionisation current is directly proportional to intensity of entering reflected X-rays. SO2 or CH3I increases ionisation in the chamber.


3.ROTATING CRYSTAL M ethod: Shaft is moved to put the crystal into slow rotation. This cause sets of planes coming successively into their reflecting position. Each plane will produce a spot on the photographic plate. Can take a photograph of the diffraction pattern in two ways 1.complete rotation method 2.oscillation method


4.POWDER CRYSTAL METHOD: 1mg material is sufficient for study. Applications : useful for Cubic crystals. Determining complex structures of metals and alloys. Making distinction between allotropic modification of the same substance.


APPLICATIONS Structure of crystals Polymer characterization State of anneal in metals Particle size determination Spot counting method v=V. δθ .cos θ /2n Broadening of diffraction lines Low-angle scattering Applications of diffraction methods to complexes Determination of cis-trans isomerism Eg:Bis(pyridine-2-carboxamido)nickel(II) chloride Determination of linkage isomerism Eg:Biuret+copper(II)=pottassium bis(biureto)cuprate(II) tetrahydrate

Miscellaneous applications: 

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. X-ray Diffraction on aerosol drugs - Measurements on small quantities of pharmaceutical samples using the X'Celerator detector.


REFERENCES 1)Instrumental methods of chemical analysis ,B.K.sharma,17 th edition 1997-1998,GOEL publishing house.page no:329-359 2)Principles of instrumental analysis,5 th 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) Instrumental Methods Of Chemical Analysis – H. Kaur pg.no:727-729,737 5) http://www.scienceiscool.org/solids/intro.html 6) http://en.wikipedia.org/wiki/X-ray_crystallography

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