Anzeige

# X-ray Crystallography

9. Oct 2018
Anzeige

### X-ray Crystallography

1. X-RAY CRYSTALLOGRAPHY Presented by:- Akansh Goel M’Pharm (Sem 1)
2. CONTENTS • Introduction • History • Principles • Methods • Instrumentation • Applications • References
3. Introduction • X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X rays strikes a crystal and causes the beam of light to 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.
4. Continuation.. • Because X-rays have wavelengths similar to the size ofatoms , they are useful to explore within crystals. • It is a tool used for identifying the atomic and molecular structure of a crystal.
5. History • The English physicist Sir William Henry Bragg pioneered the determination of crystal structure by X- ray diffraction methods. • X-ray crystallography is a complex field that has been associated with several of science’s major breakthroughs in the 20th century.
6. Continuation.. • Using X-ray crystal data, Dr. James Watson and Dr. Francis Crick were able to determine the helix structure of DNA in 1953. • In 1998 Dr. Peter Kim, a scientist, was able to determine the structure of a key protein responsible for the HIV infection process.
7. Principles • Ray diffraction by crystals is a reflection of the periodicity of crystal architecture, so that imperfection in the crystal lattice usually results in poor diffraction properties. • A crystal can be described with the aid of grid or lattice, defined by three axis and angles between them.
8. Continuation.. • Along each axis a point will be repeated as distances referred to as the unit cell constants, labeled a, b, c. • Within the crystalline lattice, infinite sets of regularly spaced planes can be drawn through lattice points. • These pinlanes can be considered as the source of diffraction and are designated by a set of three numbers called the Miller indices(hkl).
9. DIFFERENT X-RAY METHODS • X-ray Absorption • Auger Emission Spectroscopy • X-ray Flourescence • X-ray Diffraction
10. X-RAY ABSORPTION • The intensity of X-ray is diminished as they pass through material. • Wavelength at which a sudden change in absorption occurs is used to identify an element present in a sample, and the magnitude of the change determines the amount of particular element present. • Used in elemental analysis of barium and iodine in body.
11. Fig:- X-ray absorption Fig:-Auger Emission Spectroscopy
12. AUGER EMISSION SPECTROSCOPY • The primary X-rays eject electrons from inner energy levels. • Just outer level electrons fall into vacant inner levels by non radiative processes. • Excess energy ejects electrons from outer levels.
13. X-ray fluorescence • The primary X-ray ejects electron from inner energy levels where the wavelength is equal to absorption edge. • But when the wavelength is shorter than absorption edge it emits secondary X-ray when electrons fall into inner vacant levels.
14. Fig:- X-ray fluorescence
15. X-RAY DIFFRACTION METHODS • When a beam of monochromatic X radiation is directed at a crystalline material, one observes reflection or diffraction of the X-rays at a various angle with respect to the primary beam. • The relationship between the X-radiation, angle of diffraction and distance between each set of atomic planes of crystal lattice is given by Braggs condition. nλ= 2dsinθ
16. METHODS 1. Laue Photographic Method 2. Bragg X-ray Spectrometer Method 3. Rotating Crystal Method 4. Powder Crystal Method
17. 1. LAUE PHOTOGRAPHIC METHOD • Laue has studied the phenomenon of diffraction of crystal by two methods:- o Transmission method o Back reflection method
18. Transmission Method • A is a source of X-rays. This emits beams of continuous wavelength, known as white radiation which is obtained from a tungsten target at about 60,000 volts. • B is a pinhole collimator. When X-rays obtained from A are allowed to pass through this pinhole collimator, a fine pencil of x-rays is obtained.
19. Continuation.. • This diameter of pinhole is of importance from the stand point of detail in diffraction pattern. The smaller is the diameter, the sharper is the interference. • C is a crystal whose internal structure is to be investigated. The crystal is set on a holder to adjust its orientation.
20. Continuation.. • D is a fine arranged on a rigid base. This film is provided with beam stop to prevent direct beam from causing excessive fogging of the film. • The x-rays are recorded on photographic plate and study of diffraction patterns helps to know the structure of crystal.
21. Laue transmission method Fig:- Laue diagram of crystal Fig:-
22. 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. • This method is similar to Transmission method however, black-reflection is the only method for the study of large and thick Specimens.
23. Continuation.. • It is very simple and rapid and does not involve the calculations in solving the patterns obtained. • The main disadvantage of Laue’s method is that a big crystal is required and furthermore there is uncertainty in significance due to unhomogenous nature of X-rays.
24. Fig:- Back reflection method
25. 2. Bragg X-ray Spectrometer method • 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.
26. Continuation.. • The condition for maximum intensity contained in Bragg's law above allow us to calculate details about the crystal structure, or if the crystal structure is known, to determine the wavelength of the x-rays incident upon the crystal.
27. Continuation.. • The Braggs equation is nλ= 2dsinθ • where n is a positive integer • λ is the wavelength of incident wave • d is the path length • Θ is the incident angle
28. 3. Rotating Crystal Method • The rotating crystal method was developed by Schielbold in 1919. • The X-ray beam passed to the crystal through collimating system. • The rotating shaft hold the crystal and it also rotates. • This causes the sets of planes coming successively into their reflecting positions.
29. Continuation.. • Each plane will produce a spot on photographic plate. • One can take photographs in two ways; • Complete rotation method • Oscillation method Rotating Crystal Method Fig:-
30. Continuation.. • Complete rotation method: In this method there occurs a series of complete revolutions. It is observed that each set of planes in crystal diffracts four times during the rotation. These four diffracted beams are distributed into a rectangular pattern about the central point of photograph.
31. Continuation.. • Oscillation method: In this method, the crystal is oscillated through an angle of 15º or 20°. The photographic plate is also moved back and forth with a same period as that of rotation of the crystal. The position of spot on the plate indicates the orientation of crystal at which the spot was formed.
32. 4. Powder Crystal Method • 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 and homogenized.
33. • A is a source of X-rays which can be made monochromatic by a filter. • Allow the X-ray beam to fall on the powdered specimen P through the slits S1 and S2. • Fine powder, p, struck on a hair by means of gum is suspended vertically in the axis of a cylindrical camera. This enables sharp lines to be obtained on the photographic film which is surrounding the powder crystal in the form of a circular arc.
34. Fig:-Powder Crystal Method
35. Continuation.. Theory: When a monochromatic beam of X-rays is allowed to fall on the powder of a crystal, then following possibilities may happen:- • There will be some particles out of the random orientation of small crystal in the fine powder, which lie within a given set of lattice planes for reflection to occur.
36. Continuation.. • While another fraction of the grains will have another set of planes in the correct position for the reflections to occur and so on. • Also, reflections are also possible in the different order for each set.
37. Instrumentation • This includes, 1. Production of X-rays 2. Collimator 3. Monochromator 4. Detectors
38. 1. Production of X-ray • X-rays are produced inside the x-ray tube when high energy projectile electrons from the filament interact with the atoms of the anode . • Conditions necessary:  Source of electrons  Target (anode)  High potential difference  Cooling facility
39. Production of X-ray Fig :- Coolidge X-ray Tube
40. Continuation.. • There is a cathode which is a filament of tungsten metal heated by a battery to emit the thermionic electrons. • This beam of electrons moves towards anode and attain the kinetic energy and 99% of energy is converted into heat via collision and remaining 0.5- 1% is converted to X-rays via strong coulomb interactions ( Bremsstrahlung process).
41. Continuation.. • Generally the target gets very hot in use. This problem has been solved to some extent by cooling the tube with water.
42. 2. Collimator
43. Continuation.. • The X-rays produced by the target material are randomly directed. • They form a hemisphere with a target at the centre. 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 gaps.
44. 3. Monochromator They are two types; A. Filter monochromator: • A filter is a window of material that absorbs undesirable radiation but allows the radiation of required wavelength to pass. • An interesting example is use of zirconium filter which is used for molybdenum radiation.
45. Continuation.. • When X-rays emitted from molybdenum are allowed to pass through a Zirconium filter, the Zirconium strongly absorbs the radiation of molybdenum at short wavelengths but weakly absorbs the K alpha lines of molybdenum. • Thus, zirconium allows the K beta lines to pass.
46. Continuation.. B. Crystal monochromator: • A crystal monochromator is made up of a suitable crystalline material positioned in the X-ray beam so that the angle of reflecting planes satisfied Bragg’s equation for the required wavelength.
47. Continuation.. • The beam is split up by the crystalline material into the component wavelengths in the same way as a prism splits up the white light into rainbow. • Such a crystalline substance is called an analysing crystal.
48. Continuation..
49. 4. Detectors A. Photographic methods: • In order to record the position and intensity of X-ray beam a plane or cylindrical film is used. • The film after exposing to X-rays is developed. The blackening of the developed film is expressed in terms of density units D given by D = log I˳/I
50. Continuation.. • Where, I˳ and I refer to the incident and transmittance intensities of X-rays. • The quantity D is related to the total X-ray energy that causes the blackening of photographic film. • The value of D is measured by densitometer. • This is used in diffraction studies since it reveals the entire diffraction pattern on single film but this method is time consuming and uses several hours
51. Continuation.. B. Counter methods: • The Geiger tube is filled with an inert gas like argon and the central wire anode is maintained at a positive potential of 800 to 2500V. • When an X-ray is entering the Geiger tube, this ray undergoes collision with the filling gas, resulting in the production of an ion pair: the electron produced moves towards the central anode while the positive ions move towards outer electrode.
52. Continuation.. • The electron is accelerated by the potential gradient and causes the ionisation of large number of argon atoms, resulting production of an avalanche of electrons that are travelling towards the central anode.
53. Continuation.. Fig:- Gieger Tube
54. Continuation.. • The Geiger tube is in expensive and is relatively trouble free detector. This tube gives the highest signal for given X-ray intensity. • The disadvantages are:- The efficiency of Geiger tube falls rapidly at wavelength below 1 Aº.
55. Continuation..  As the magnitude of the output pulse does not depend upon the energy of the X-ray which causes ionisation, a Geiger tube cannot be used to measure the energy of ionising radiation.
56. Application of X-ray Diffraction • 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. • Determination of Cis- trans isomerism.
57. Continuation.. • Differentiation of sugar. • X-ray analysis of milk powder.
58. REFERENCES • Instrumental method of analysis-Willards, 7th edition, CBS Publishers • Instrumental method of chemical analysis- Chatwal , Himalayan Publishing House
Anzeige