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X-ray crystallography

X-ray crystallography. What tools do we have to ID minerals so far?. How do we know atomic structure of minerals?. Atoms are 1-5Å apart so how do we see this?. X-rays. Between 0.1 and 10Å (1Å = 0.1 nm). X-rays. Amplitude Wavelength. X-ray interference. Constructive is in phase.

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X-ray crystallography

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  1. X-ray crystallography

  2. What tools do we have to ID minerals so far?

  3. How do we know atomic structure of minerals? • Atoms are 1-5Å apart so how do we see this?

  4. X-rays • Between 0.1 and 10Å (1Å = 0.1 nm)

  5. X-rays • Amplitude • Wavelength

  6. X-ray interference Constructive is in phase Destructive if out of phase

  7. Wave behavior Interference and diffraction If a wave encounters a barrier with several holes, the propagating wave will have sharper peaks of intensity at certain angles.

  8. X-rays interact with atoms • Atoms cause scattering • Diffraction: coherent scattering of waves off a periodic arrangement of matter

  9. Generation of X-rays • Need • Vacuum environment • Source of electrons • Large accelerating voltage • Target metal

  10. Generation of X-rays • Tungsten filament as source of e- • E- sent to target metal (usually Cu) • X-rays generated off Cu • Higher voltage, faster e-, more energy

  11. X-ray Crystallography X-ray generation W C a t h o d e C u A n o d e ( - ) ( + ) electrons X - r a y s

  12. Two types of X-rays generated • Continuous spectra (white radiation) • Electrons hit target surface, loose energy, stop • NO change of target electron configuration • Removed by filtering

  13. Two types of X-rays generated • Characteristic radiation • Electrons interact with target electron configuration • Fingerprint of target metal

  14. Two types of X-rays generated • Bombarding electrons dislodge electron from target • K-shell or L-shell • If vacancy in K-shell filled from L-shell, Kradiation • If vacancy if K-shell filled from M-shell, K radiation

  15. Two types of X-rays generated • K radiation has more energy than K radiation

  16. Two types of X-rays generated • K radiation has more energy than K radiation • Quantized energy and characteristic of metal target

  17. Table 2-1: Wavelengths in Nanometer Units of Characteristic Emission Lines for Selected Elements. Element K[alpha] K[alpha]1 K[alpha]2 K[beta]1 L[alpha]1 Cr 0.229092 0.228962 0.229351 0.228480 **** Cu 0.154178 0.154051 0.1544330 0.139217 1.3357 Mo 0.071069 0.070926 0.071354 0.063225 0.54062 W 0.021060 0.020899 0.021381 0.018436 0.14763

  18. Characteristic radiation more intense than continuous • Want Ka radiation to go to sample so need to filter everything else

  19. Sample sees monochromatic radiation

  20. X-ray detection • Photographically • Electronic detectors • Result: see planes of atoms and what orientation planes are in • Don’t get image of individual atoms

  21. X-ray diffraction • Works thanks to Bragg Equation • X-rays diffract at specific angles based on spacing of atomic planes

  22. X-ray Crystallography Destructive and constructive interference of waves Bragg Equation: in phase in phase Y x q q q d

  23. X-ray Crystallography nl=2dsinq n is the “order” As soon as the crystal is rotated, the beam ceases (This is diffraction, not reflection) Only get diffraction at certain angles! Relation between l and d and q Y x q q d

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