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X-ray Diffraction

X-ray Diffraction. Frequency (Hz). 10 12. 10 9. 10 24. 10 21. 10 18. 10 15. 10 6. 10 3. 10 -12. 10 -9. 10 -6. 10 -3. 1.0. 10 3. 10 -15. 10 6. Wavelength (m). Electromagnetic Spectrum. Gamma Rays. UV. IR. Long Radio Waves. X-rays. Micro. TV. FM. AM. What are X-rays?.

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X-ray Diffraction

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  1. X-ray Diffraction

  2. Frequency (Hz) 1012 109 1024 1021 1018 1015 106 103 10-12 10-9 10-6 10-3 1.0 103 10-15 106 Wavelength (m) Electromagnetic Spectrum Gamma Rays UV IR Long Radio Waves X-rays Micro TV FM AM

  3. What are X-rays? • X-Rays (Roentgen rays) were dicovered by Roentgen in 1895. They are a kind of electromagnetic radiation of very short wavelengthand very high energyproduced when high-speed electrons strike a solid target. The wavelength range lyes between 0.0001 and 10 nm i.e., therange between ‘gamma rays’ and ‘ultraviolet rays’

  4. Wilhelm Conrad Röntgen (1845-1923) Bertha Röntgen’s Hand 8 Nov, 1895 A modern radiograph of a hand History of X-ray and XRD • Wilhelm Conrad Röntgen discovered X-Rays in 1895. • 1901 Nobel prize in Physics

  5. Risks • X-Rays penetrat matter but not all of them come out. Some get lost. The missing X-rays having very high energy get absorbed in the body and the X-ray's energy is released. This energy is transferred to an electron, which does a lot of damage in a small area. • X-Rays affect the DNA of cells. • For the levels of radiation one gets at the dentist and the doctor, the body is able to repair most all of the damage done.

  6. Origin of X-Rays • A vacuum tube consists of a cathode, an anode, an a heating filament. • The heated cathode emits electrons (e-). • A field of ~50kV accelerates the electrons onto the anode. • The elctrons bump on atoms and slow down, emitting radiation of a continuous distribution of wavelengths (``Bremsstrahlung''). • Some electrons cause sharp atomic transitions, resulting in X-rays with definite wavelengths (0.1~50A).

  7. X-rays The X-ray generator

  8. The X-ray generator

  9. Wavelengths for X-Radiation are Sometimes Updated • Often quoted values from Cullity (1956) and Bearden, Rev. Mod. Phys. 39 (1967) are incorrect. • Values from Bearden (1967) are reprinted in international Tables for X-Ray Crystallography and most XRD textbooks. • Most recent values are from Hölzer et al. Phys. Rev. A56 (1997) • Has your XRD analysis software been updated?

  10. What are the atomic processes that can produce X-rays? (1) White radiation • electrons from an external source are deflected around the nucleus of a target atom and X-rays of multiple wavelengths are emitted, depending on the kinetic energy lost by the electron on deflection

  11. What are the atomic processes that can produce X-rays? (2) Characteristic emission An incident e- bumps onto an inner level e-. Both electrons fly out and energy is emitted as an electromagnetic wave (white). The “hole” is filled with a higher level electron and this is accompanied by X-ray emission (characteristic). These peaks are labeled K, K, L, L, etc, depending on the specific energy levels involved.

  12. X-Ray interactions with an object • X-Ray elastic scattering is the basis for crystallographic analysis WXRD and SAXS • X-Ray inelastic scattering is the basis for analytical methods such as XPS (X-ray Photoelectron Spectroscopy), XRF, AES (Auger Electron Spectroscopy)and EDAX (Energy Dispersive Analysis by x-ray)

  13. Max von Laue (1897-1960) History of X-ray and XRD • The first kind of scatter process to be recognised was discovered by Max von Laue who was awarded the Nobel prize for physics in 1914 "for his discovery of the diffraction of X-rays by crystals". His collaborators Walter Friedrich and Paul Knipping took the picture on the bottom left in 1912. It shows how a beam of X-rays is scattered into a characteristic pattern by a crystal. In this case it is copper sulphate. • The X-ray diffraction pattern of a pure substance is like a fingerprint of the substance. The powder diffraction method is thus ideally suited for characterization and identification of polycrystalline phases.

  14. Sir William Henry Bragg (1862-1942) William Lawrence Bragg (1890-1971) Bragg’s Law • The father and son team of Sir William Henry and William Lawrence Bragg were awarded the Nobel prize for physics "for their services in the analysis of crystal structure by means of Xrays“ in 1915. • Bragg's law was an extremely important discovery and formed the basis for the whole of what is now known as crystallography. This technique is one of the most widely used structural analysis techniques and plays a major role in fields as diverse as structural biology and materials science.

  15. Bragg’s Law • Correlates X-ray wave length, l, interplanar spacing, d, and reflection angle, q. • Scattering atoms (circled in red) behave like slits in Young’s experiment.

  16. Crystal planes and Miller’s indices • Each plane in a crystal is defined by Miller’s indices z=1 z y y=1 x=1 x

  17. Crystal Planes, Miller indices and diffraction Since a crystal has an ordered 3-D periodic arrangement of atoms (ions or molecules) the atomic planes in any crystal can be related to the unit cell. One can label each set of planes uniquely by considering their (fractional) intersection with the unit cell axes a,b,c and converting these to INTEGERS h, k, and l. e.g. the planes that intersect the b-axis at ½ and are parallel to a and c. ( a/ , b/2, c/) are defined by the MILLER INDiCES (0 2 0)

  18. A reminder: finding Miller Indices of a plane • Extend the plane to make it cut the crystal axis system at points (a1, b1, c1) • Note the reciprocals of the intercepts, i.e.: • Multiply or divide by the highest common factor to obtain the smallest integer numbers. • Replace negative integers with bar over the number, i.e. we replace -h by • Note: • If the plane is parallel to an axis, we say it cuts at and • If the plane passes through the origin, we translate the unit cell in a suitable direction. • We use round brackets ( ) to describe a single plane and curly brackets { } to describe a family of planes.

  19. Examples:

  20. The Interplanar Spacing, d(hkl) • Can be found for each set of planes provided we know the unit cell parameters. e.g. for a CUBIC unit cell: d(100) = a d(010) = b (= a) d(030) = ( )a d(110) = ( )a For unit cells with  =  =  = 90º Which for a CUBIC unit cell simplifies to

  21. Bragg Example d = λ / (2 Sin θB) λ= 1.54 Ǻ = 1.54 Ǻ / ( 2 * Sin ( 38.3 / 2 ) ) = 2.35 Ǻ

  22. Different Types of Unit cells in Materials • Bravis Lattices

  23. Bravais Lattices

  24. Let’s go to another class

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