X ray diffraction ppt

X-ray Diffraction --> x-ray source, types of x-rays, principle, Bragg's law, instrumentation (x-ray tube, collimator, monochromator, sample preparation, detectors and methods), advantages, disadvantages and applications.

1. X-RAY DIFFRACTION Presentor , Vanitha . N B.Pharmacy KMCH COP Modern Methods of Pharmaceutical Analysis 1

2. X-RAYS 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 as Roentgen rays . The penetrating power of X-rays depends on energy also, there are two types of X-rays. They are, Hard X-rays: which have high frequency and have more energy Soft X-rays: which have less penetrating and have low e nergy X-rays are short wavelength 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 – 0.1 to 100 A° Analytical purpose – 0.7 to 2 A° 2

3. X-RAY TECHNIQUES X-ray absorption methods : A beam of x-ray is passed through a sample , a fraction of x-ray photon absorbed is a measure of concentration of absorbing substance. X-ray diffraction methods : This is based on scattering of x-ray by crystals. X-ray fluorescence methods : It is the emission of characteristic “secondary X-rays” from a material that has been excited by bombarding with high energy X-rays. It is used for qualitative and quantitative analysis of larger fractions of geological materials like rock, minerals, sand, etc.., 3

4. ORIGIN OF X-RAYS X-rays are produced by bombarding a metal target with high speed of electrons . A heated cathode emits electrons by thermionic emission. These are accelerated to the anode and the target. Target material (or) metals used are Molybdenum ,Copper, Cobalt, Iron, Chromium . The electrons lose about 99% of their energy in collision and about 1% reappears as X-rays . 4

5. X-RAY DIFFRACTION Every crystalline substance gives a pattern, the same substance always gives same pattern and in a mixture of substances each produces its pattern independently of the others. 5 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 as X-ray diffraction .

6. When a beam of x-ray is incident upon a substance ,the electrons constituting the atoms of the substance become as small oscillators . These on oscillating at the same frequency as that of incident x-radiation emit EMR in all directions at the same frequency as incident X -radiation. The scattered waves are coming from electrons which are arranged in a regular manner in a crystal lattice & then travelling in certain directions. If these waves undergo constructive interference they are said to be diffracted in the plane. The diffracted beams are referred as reflections . Constructive interference of the reflected beams emerging from two different beams emerging from the two different planes will take place if the path length of two rays is equal to whole number of wavelengths . 6

7. PRINCIPLE X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. Those X-rays are generated by a cathode ray tube , filter ed 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 . 7

8. BRAGG’S LAW The conditions for diffraction are governed by Bragg’s law . Constructive interference of the reflected beams emerging from two different beams emerging from the two different planes will take place if the path length of two rays is equal to whole number of wavelengths . If one x-ray is striking from the top crystal plane at A &the other x-ray is striking the second crystal lane at B ,the path difference between two parallel x-ray is equal to CB+BD from the planes with spacing d at an angle Ө . The path difference between the ray 1 and ray 2 = 2d sin Ө 8

9. BRAGG’S LAW DERIVATION : For constructive interference, n  = CB+BD CB=BD n  = 2CB  eq . (1) sin = CB/d CB = d sin  Substitute CB = dsin  in eq.(1) n  = 2dsin Where, n = order of diffraction(integer)  = X-ray wavelength = X-ray incident angle d = lattice spacing of atomic planes. 9

10. INSTRUMENTATION 10 X-ray source Collimator Monochromator Filter Crystal monochromator 4) Sample and sample holder 5 ) Detector Photographic methods Counter methods

11. X-RAY SOURCE X rays are generated when a high velocity electrons impinge on a metal target. X-rays are produced by an X-ray tube . The various types of X-ray tube includes, Gas discharge/Crookes tube Regulator tube Vacuum tube Coolidge tube Shockproof dental X-ray unit Coolidge tube is mostly used for XRD. 11

12. COOLIDGE TUBE (Hot Cathode Tube) It consists of: Source of electrons: Cathode assembly ie . Filament and focusing cup A means to high voltage across anode and cathode A means to slow down electrons suddenly: Anode assembly with target (tungsten) Glass envelope (borosilicate) to enclose two electrodes with vacuum level – 10 -7 to 10 -8 mmHg 12

13. WORKING The electrons are produced by thermionic effect from a tungsten filament (cathode) heated by an electric current. The high voltage potential is between cathode an anode, the electrons are thus accelerated and then hit the anode (made of tungsten or molybdenum). The anode is specially designed to dissipate the heat and this intense focused barrage of electrons area heated is cooled by circulating coolant (or) cold water . The anode is precisely angled at 1-20° perpendicular to electron current so as to escape some of X-rays. Power of C oolidge tube – 1 to 4kW. 13

14. The choice of target material depend upon the sample to be examined . Atomic number of target material should be greater than that of elements being examined in the sample . Energy of emitted x-ray should be greater than that required to excite the elements being irradiated. DISADVANTAGE : Lack of focusing of electrons , so the whole surface become a source of x- ray. 14

15. COLLIMATOR It is an X-ray beam restrictor device. In order to get a narrow beam of 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. Collimator makes random directional X-rays to be narrow and parallel. Usually made up of lead, tungsten, stainless steel and ceramics . ADVANTAGES : Provides an infinite variety of X-ray fields. Light beam shows the centre and the exact configuration of the X-ray field. 15

16. STRUCTURE Two sets of shutters to control the dimension. Each shutter contains 4 or more lead plates , which move in independent pairs. When the shutters closed they meet at the centre. Light beam from a light bulb in the collimator. The light beam is deflected by a mirror mounted in the path of the X-ray beam at an angle of 45 . The target of the X-ray tube and the light bulb should be the exactly same distance form the centre of the mirror.   16

17. MONOCHROMATOR Monochromators are device used to convert polychromatic radiation into monochromatic radiation of desired wavelength. Inorder to get monochromatization of x-rays, two methods are available: Filter Crystal Monochromator a)Flat crystal monochromator b ) Curved crystal monochromator 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 allow radiation of required wavelength to pass . Eg : Zirconium filter used for molybdenum radiation 17

18. 2) CRYSTAL MONOCHROMATOR : It 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. Beam is split by the crystalline material into component wavelengths. Crystals used in monochromators are made up of materials like NaCl , LiF , Quartz, Graphite , etc.., There are two types, a)Flat crystal monochromator b ) Curved crystal monochromator CHARACTERISTICS OF A CRYSTAL : Mechanically strong and stable The resolution of the crystal should be small 18

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20. SAMPLE PREPARATION 20

21. DETECTOR Detection is based on the ability of X-rays to ionize matter and differ only in the subsequent fate of electrons produced by the ionizing process. The two methods are, Photographic methods (or) imaging detectors Counter methods Geiger– M uller tube counter Proportional counter Scintillation counter Solid state semiconductor d etector Semiconductor d etectors 21

22. PHOTOGRAPHIC DETECTOR 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 Io/I Where, Io – incident intensities I – transmitted intensities D – total energy that causes blackening of the film D is measured by densitometer . Photographic methods is mainly used in diffraction studies since it reveals the entire diffraction pattern on a single film. 22

23. Special films used as flat or cylindrical detectors → beam appears as dark spots or line . Darkening of spot is proportional to beam intensity. Useful when entire diffraction pattern is desirable. ADVANTAGE : Easy to interpret position of beam DISADVANTAGES : Time consuming Exposure of several hours Difficult to use for quantitative intensity data 23

24. COUNTER METHODS The counter detector converts the incoming X-rays into surges (or) pulsed of electric current which are fed into various electronic components for processing. The electronics counts the number of current pulses per unit time and this number is directly proportional to the intensity of the X-ray beam entering the detector. 24

25. GEIGER MULLER TUBE COUNTER Geiger tube is filled with inert gas like argon . Central wire anode is maintained at a positive potential of 800-2500V. X-ray entering GM tube → collision with filling gas → production of an ion pair . The electron produced moves towards the central anode & positive ion move towards outer electrode . 25

26. The electron is accelerated by the potential gradient and causes the ionisation of large number of argon atoms, resulting in the production of electrons that are travelling towards the central anode. This results in an output pulse of 1-10V which can be measured very easily . ADVANTAGES : Inexpensive Relatively trouble free DISADVANTAGES : Used for low counting rates Efficiency falls off below 1A° Cannot be used to measure the energy of ionizing radiation(no spectrographic information) 26

27. 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 ( 0.2 µs), it can be used to count high rates without significant error. ADVANTAGE : Counts high rates DISADVANTAGE : Circuit is complex and expensive   27

28. 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 pulse of visible light are emitted is proportion to X-ray intensity which can be detected by a photomultiplier tube (PMT) . The size of pulse is proportional to energy of X-ray photon absorbed. Useful for measuring X-ray of short wavelength . Crystals used in scintillation detectors include sodium iodide, anthracene , naphthalene and p- terpenol . 28

29. SOLID STATE SEMICONDUCTOR DETECTOR Electrons produced by x-ray beam are promoted into conduction bands . Current which flows is directly proportional to the incident x-ray energy . It consists of a thin layer of n-type material on the surface of a large piece of p-type material. DISADVANTAGE : Semiconductor device should be maintained at low temperature to minimize the noise & prevent deterioration. 29

30. SEMICONDUCTOR DETECTOR Si (Li) and Ge ( Li ) are used as semiconductors. When x-ray falls on a silicon lithium drifted semiconductor, it generates an electron(-e) and a hole(+e) . In this a very pure silicon block is set up with a thin film of lithium metal plated onto one end . Operates with a combination of p-type and n-type semiconductor . Under the influence of voltage electrons moves towards + ve charge and holes towards – ve charge. 30

31. The voltage generated is a measure of x-ray intensity falling on the crystal . Upon arriving at the lithium coating ,a pulse is generated. voltage of pulse=q/c Where , q - total charge collected on electrode c - detector capacity No. of pulse = No. of x-ray photons falling on detector 31

32. X-RAY DIFFRACTION METHODS These are generally used for investigating the internal structures and crystal structures of various solid compounds. They are, Laue’s photographic method Transmission method Back reflection method Bragg’s X-ray spectrometer method Rotating crystal method Powder diffraction method 32

33. LAUE’S PHOTOGRAPHIC METHOD TRANSMISSION METHOD BACK REFLECTION 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 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 reflected beam . 33

34. TRANSMISSION METHOD BACK REFLECTION METHOD The film intersects the cone with the diffraction spots generally lying on an ellipse . The film intersects the cone with the diffraction spots generally lying on an hyperbola . USES: Can be used to orient crystals for solid state experiments. Most suitable for investigation of preferred orientation sheet particularly confirmed to lower diffraction angles . Also used in determination of symmetry of single crystals. USES: This method is similar to transmission method, however back reflection is the only method for the study of large and thick specimens . DISADVANTAGE : There is uncertainity in the significance due to unhomogenous nature of x-rays. DISADVANTAGE : Big crystals are required. 34

35. BRAGG’S X-RAY SPECTROMETER METHOD Using Laue’s photograph, Bragg analysed structures of crystals of NaCl , KCl and ZnS . Bragg devised a spectrometer to measure the intensity of X-ray beam. The spectra obtained can be employed for crystallographic analyses . This method is based on Bragg’s law . 35

36. WORKING 36 Crystal mounted such that Ө = 0° and ionization is adjusted to receive X-rays. X-rays from source → pass from slits (S1,S2) → falls on crystal ‘C’ (vertically rotating about its axis) → reflected beam pass to ionization chamber → ionization of gas → current measured by galvanometer → intensity of reflected X-rays is measured. The ionization current is measured for different values of glancing angle Ө . A graph is drawn between the glancing angle and ionizing current.

37. Peaks are obtained, peaks corresponds to Bragg’s reflection corresponding to different order glancing angles Ө 1 , Ө 2 , Ө 3 are obtained. With known values of d and n and from observed value of Ө , λ can be measured using, Bragg’s law, n λ = 2dsin Ө λ = 2dsin Ө /n The strength of ionization current is proportional to the intensity of entering reflecting x-rays. 37

38. ROTATING CRYSTAL METHOD The x-rays are generated in the x-ray tube and the beam is made monochromatic by filter . Beam is then passed through collimator to the crystal mounted on a shaft which is rotated by a motor . 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 . Each plane will produce a spot on photographic plate. 38

39. Photograph can be taken by, 1) Complete rotation method : A series of complete revolutions occur. Each set of a plane in a crystal diffracts 4 times during rotation. 4 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. 39

40. POWDER DIFFRACTION METHOD It is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analysed material is finely grounded, homogenised and average bulk composition is determined. 1mg of the sample material is sufficient. Unknown crystalline substances can be identified by comparing the diffraction data with the data of International Centre for Diffraction Data. 40

41. WORKING : X-ray is made monochromatic by filter & x-ray beam fall on powdered specimen by passing through slit . Fine powder diffraction is suspended vertically in the axis of cylindrical camera . Sharp lines are obtained on photographic film which is surrounding the powder crystal in the form of a circular arc . X-rays after falling on the powder passes out of camera through a cut in the window. The diffracted X-rays are detected. 41

42. When monochromatic beam is allowed to pass different possibilities may happen, There will be some particles out of random orientation of small crystals in the fine powder. Another fraction of grains will have another set of planes in the correct positions for the reflections to occur. Reflections are possible in different orders for each set. 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 cells that are difficult to determine optically. Determination of unit cell dimensions. Measurement of sample purity. 42

43. METHOD LAUE’S METHOD ROTATING CRYSTAL POWDER METHOD DETECT Orientation Lattice constant Lattice parameters CRYSTAL TYPE Single crystal Single crystal Poly crystal (powdered) BEAM Polychromatic beam Monochromatic beam Monochromatic beam ANGLE Fixed angle Variable angle Variable angle ORIENTATION Single Ө Ө varied by orientation Many Ө s (orientations) 43

44. X-RAY DIFFRACTION ADVANTAGES : X-rays are the least expensive, most convenient and the most widely used method to determine crystal structures. XRD is a non-destructive technique. X-rays are not absorbed very much by air, so the sample need not be in an evacuated chamber. DISADVANTAGES : X-rays do not interact very strongly with lighter elements. X-rays are hazardous to use. XRD has size limitations. It is much more accurate for measuring large crystalline structures rather than small ones. (smaller ones that are present only in trace amounts will often go undetected by XRD readings) 44

45. APPLICATIONS Structure of crystals It is a non-destructive method. Gives information on molecular structure and size of crystal planes. The patterns obtained are characteristic of a particular compound from which crystal is formed. 45

46. 2) Polymer characterization Determine the degree of crystallinity . A polymer can be considered partly crystalline and partly amorphous. The crystalline parts give sharp narrow diffraction peak s and the amorphous component gives a very broad peak. The ratio between these intensities can be used to calculate the amount of crystallinity in the material. 46

47. 3) State of anneal in metals. XRD is used to test metals without removing the part from its position and without weakening it. ( Annealing is the heat treatment process that softens a metal that has been hardened by cold working. At  this stage, any defects caused by deformation of the metal are repaired . U sed to reduce hardness, increase ductility and help eliminate internal stresses. ) Well annealed metals are in well ordered form & gives sharp diffraction lines. If the metal is subjected to drilling, hammering or bending, it becomes fatigued, i.e. its crystals become broken & the x-ray pattern more diffuse . Employed to occasionally check moving parts for metal fatigue (airplane wings, engine parts ) . 47

48. 4) Particle size determination. Spot counting methods Broadening of diffraction lines Low-angle scattering methods are used. Spot counting method : For determining particle size >5 microns. Powder diffraction pattern consists of a series of lines or rings having a spotty appearance. From that we can determine particle size using, v=V. δ Ө . cos Ө /2n Where, v = volume of individual crystalline V = total volume irradiated n =no. of spots in diffraction ring δ Ө = divergence of X-ray beam Broadening of diffraction line: Used for particles in range of 30-1000 Å Low angle scattering: Used to determine distribution of particle size. 48

49. 5) Applications of diffraction methods to complexes. D etermination of cis -trans isomerism Determination of linkage isomerism 6) Miscellaneous applications: Soil classification based on crystallinity Analysis of industrial dusts Assessment of weathering and degradation of minerals and polymers Study of corrosion products Examination of bone state and tissue state Examination of tooth enamel and dentine 49

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