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Convergent-beam electron diffraction

Convergent-beam electron diffraction

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Convergent-beam electron diffraction

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  1. Convergent-beam electron diffraction Applications

  2. Bragg’s Law

  3. Applications - in common with spot patterns • 1 Lattice spacings • 2 Unit cell • 3 Orientation

  4. Applications - special to CBEDEstablished • 1 Crystal symmetry • 2 Local strain • 3 Direct phase identification • 4 Thickness

  5. Applications - special to CBEDAdvanced • 1 Crystal structure determination • 2 Bonding measurement • 3 Phase determination • 4 Improved defect analysis

  6. Advanced Techniques • The Tanaka methods • The techniques • LACBED • Other variations (CBIM, SA-CBED) • Applications • Spatial variation • Defect analysis • Other Techniques • Coherent CBED • Energy filtering

  7. Lattice Spacings The lattice spacing is determined from the distance between the diffracted beams. In spot patterns it is the distance between spots. In convergent-beam patterns it is the distance between discs. These are generally equally accurate.

  8. FeS2 [110] K-C Hsieh

  9. Unit Cell Determination If a very short camera length is used, the unit cell can be determined, in principle, from a single diffraction pattern. In practice this may be tricky. The centering of the Bravais lattice can be easily obtained at a suitable zone axis.

  10. Orientation If the diffraction pattern is indexed, the orientation of the sample is determined. A selected area pattern can determine the orientation to within a few degrees. In convergent-beam diffraction additional information, from details in the discs or from Kikuchi lines, gives the result to a fraction of a degree.

  11. Symmetry The determination of the symmetry of a crystalline specimen is one of the most powerful applications of convergent-beam diffraction. It is valuable both to identify known phases and to determine the symmetry of new phases.

  12. Pyrite [001] K-C Hsieh

  13. Strain from HOLZ lines • Limitations • The strain must be uniform through the thickness of the specimen. • The result is for the strain in the thin foil - not the strain in the original sample. • Results are relative not absolute without dynamical calculation.

  14. Phase Identification • All convergent-beam zone axis patterns are unique and serve to identify phases. • You must educate your eye. • Limitations • The patterns do change with thickness • The uniqueness is not absolute.

  15. V3Si Doug Konitzer

  16. InP [100] G. Rackham

  17. M23C6 [110]

  18. Ni3Al [110] S. Court

  19. Ni3Al [110] S. Court

  20. Thickness • The method uses two-beam conditions. • Some care must be taken in the analysis. • The thickness is for the crystalline part of the sample only.

  21. Crystal Structure • The phase problem • Crystal structure determination • Bonding measurement

  22. Crystal Potential

  23. Because of the complex interference between diffracted beams in dynamical electron diffraction, electron diffraction intensities are very sensitive to small changes in Vg. • Electron diffraction can thus determine bonding electron densities - but the calculations are complicated.

  24. Midgley, Saunders, Vincent and Steeds Ultramicroscopy 59 (1995) 1-13

  25. Midgley, Saunders, Vincent and Steeds Ultramicroscopy 59 (1995) 1-13

  26. Tanaka, Terauchi, Tsuda and Saitoh CBED IV 2002

  27. Tanaka, Terauchi and Tsuda CBED III 1994

  28. The Tanaka Methods • Traditional microscopy taught that the microscope should be focussed on the specimen or on the diffraction pattern in the back focal plane. • Tanaka liberated us and gave rise to a family of new techniques by telling us to look in other places.

  29. GaAs [100] K. Christenson

  30. Ni3Mo

  31. Ni3Mo BF Tanaka pattern

  32. Al layer on GaAs Tanaka Group

  33. Defect Analysis • Large-Angle Convergent-Beam patterns provide an improved method of determining the Burgers vectors of dislocations. (And characterizing other defects.) • The dislocations have to be well separated.

  34. Fe,30Ni,19Cr [114] Cherns and Preston

  35. Fe,30Ni,19Cr [114] Cherns and Preston

  36. Fe,30Ni,19Cr [114] Cherns and Preston

  37. Si TanakaGroup