CH3 - Diffraction and Imaging Flashcards
(15 cards)
What does hkl represent?
A plane that intercepts the three points a/h, b/k, c/l (Miller indices)
How are electrons accelerated?
In an electric field.
TEM compared to X-Ray?
Cannot compete with X-ray diffraction in terms of defining precise atomic positions. Except for when sample is too small for X-ray -> Accelerated electrons have shorter wavelengths than X-rays, thus they can be used to probe finer structures (atomic columns)
What is resolution of TEM image defined by?
The wavelength of radiation and ability of objective lens
Limitations of TEM?
Unlike optical lenses, there are unavoidable aberrations which affect the relative phases of the diffracted beams. Limits effective resolution to 1.5 A
1. Objective lens - phase shifts
2. Low resolution
3. Strong scatter
What do apertures control?
How much of the diffraction pattern is included in the image
Why is lanthanum hexaboride used to generate electrons?
As it has a low work function thus yield of electrons is high and have narrow spread of energies
Brightness depends on?
Intensity of beam
Can displacements be resolved by high resolution imaging?
No, small displacements which occur during structural transformations around defects cannot be seen, however they result in small strains which considerably impact diffraction contrast
Locally compresses lattice spacing which alters Bragg angle
what is the Abbe theory?
- Object scatters incident light (diffraction pattern contains all info about object)
- Diffracted beams propagate beyond focal plane to image frame. Overlap to form inverted image of object
Electrostatic potential?
The amount of work done to move a unit charge inside the field without acceleration. i.e. strength of charges and nuclei
formation of lattice fringe images?
form due to interference between transmitted and diffracted electrons (multiple beams). Appear as dark and light stripes corresponding to d-spacing (atomic plane spacing)
Can displacements be resolved by high resolution imaging?
No, small displacements which occur during structural transformations around defects cannot be seen, however they result in small strains which considerably impact diffraction contrast
Locally compresses lattice spacing which alters Bragg angle
Sub-grain boundaries
boundaries within a mineral defined by a wall of defects where there is a low angle difference between the orientation of the grain structure on either side
Grain surface
areas where crystal structure is not the same as the bulk. Considered to be a defect and present along cracks or edges of grain
