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X-ray diffraction

X-ray diffraction. Diffraction. Coherent scatter of waves off a periodic arrangement of matter Bragg equation n  = 2dsin . XRD pattern of halite. XRD pattern of beryl. X-ray Crystallography. Methods: 1) Single-Crystal:.

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X-ray diffraction

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  1. X-ray diffraction

  2. Diffraction • Coherent scatter of waves off a periodic arrangement of matter • Bragg equation n = 2dsin

  3. XRD pattern of halite

  4. XRD pattern of beryl

  5. X-ray Crystallography Methods: 1) Single-Crystal: Fig 7.39 of Klein (2002) Manual of Mineral Science, John Wiley and Sons

  6. X-ray Crystallography Methods: 1) Single-Crystal: Fig 7.40 of Klein (2002) Manual of Mineral Science, John Wiley and Sons

  7. X-ray Crystallography • Methods: • 2) Powder- • Easiest • Infinite orientations at once, so only need to vary q • Cameras and diffractometers

  8. Sample prep for powders

  9. Powder sample is randomly oriented • Sample rotates through angles, detector is at 2

  10. What a powder XRD gives you • Spectra of detected X-rays that were reflected • Peaks mean a plane of atoms whose d-spacing was such that x-rays are reflected

  11. What a powder XRD gives you • Know  • Machine tells you 2 • Can calculate d-spacing • Calculate by decreasing peak intensity

  12. Practice calculating d-spacing • Fig 7.13 of text • Calculate 4 most intense peaks d-spacing

  13. Practice calculating d-spacing • Å •  = 15.86, 22.74, 28.26, 13.68° • Once have 4 most intense peaks, start searching • From d-spacing can calculate {HKL} values

  14. Isometric structures easiest • 1/d2hkl = (h2+k2+l2)/ a2 • Orthorhombic minerals: need to solve 3 d-spacings, have a,b,c and 3 simultaneous equations • Monoclinic and triclinic: need trig on top of that

  15. What to do with n-value in Bragg equation? • May be {100} or {200} or {400} • Mainly assume n=1

  16. Variations in chemical composition • Element substitutions change unit cell parameters, d-spacing, location of x-ray peaks

  17. Sources of error • Instrumental error • Sample with preferred orientation • Sample prep

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