Electron Diffraction
description
Transcript of Electron Diffraction
Electron Diffraction Analyses and the Powder Diffraction File
Cyrus E. Crowder, David Crane, Justin Blanton, Charles Weth, Joe Sunzeri
ICDD, Newtown Square, PA
Electrons and Electron Diffraction X-rays and X-ray Diffraction
Particles Electromagnetic waves
λ = 2 – 10 pcm (0.02 – 0.10 Å) λ = 30 – 250 pcm (0.30 – 2.50 Å)
2θ = 0 - 2˚ 2θ = 0 - 180˚
Coulombic interactions with structure Wave/particle interactions with structure
Small beam size (few nanometers) Larger beam size (typically microns to millimeters)
High absorption coefficients - sample thickness limited to < 10 nm
Lower absorption coefficients
Significant dynamic diffraction - affects intensities, forbidden reflections may be observed
A Comparison of Electrons and X-rays
TEM Set-up for Selected Area Electron Diffraction (SAED) Pattern Collection
Incident Electron Beam
Specimen
Diffraction Pattern
Image Screen
Scattered Electrons
Electromagnetic Objective Lens
Incoming electron beam
Specimen Detector
Phosphor Screen
SEM Set-up for Electron Back-Scatter Diffraction (EBSD) Pattern* Collection
70˚ tilt
* Often called Kikuchi patterns
SAED Simulation EBSD Simulation
The PDF-4+ database software can be used to simulate kinematic SAED and EBSD patterns for any entry in the database that contains a space group and cell parameters.
Basis of electron diffraction simulated intensities:
1. PDF entries with atomic coordinates . . .
Intensities are calculated using electron diffraction scattering
factors, the user specified voltage, and the given crystal structure
2. Calculated PDF entries without atomic coordinates . . .
Intensities are calculated using pre-calculated structure factors
based on electron diffraction scattering factors for 200 keV
electrons
3. Only experimental d’s and I’s, and known space group . . .
Intensities are estimated using X-ray powder diffraction
intensities with the multiplicity factors divided out
All simulated intensities are based on kinematic scattering
only. No attempt has been made to simulate the dynamic
scattering resulting from multiple electron scattering
SAED Simulation The user interface and kinematic electron diffraction spot pattern simulation - YAlO3 0 0 1 zone
SAED Simulation Kinematic electron diffraction spot pattern simulation - YAlO3 1 1 0 zone
EBSD Simulation The user interface for kinematic electron back-scatter pattern simulation - YAlO3 2 1 1 zone
EBSD Simulation The user interface for kinematic electron back-scatter pattern simulation - YAlO3 211 zone with shortened camera length
SAED Pattern obtained for Ca3(PO4)2
The interactive menu can be used to determine the Zone, adjust the Center of View, Rotation Angle, and Camera Length to match the imported pattern.
For clarity, the Spot color is set to ‘Red’ and the Display Font color to ‘Blue’.
The 0 0 1 zone is displayed by default, but does not appear to be a good match with the experimental pattern.
Another Zone Axis, the 0 1 6, appears to have a better ratio of spacing distances.
Rotating the simulated pattern angle by 99˚ gives a better row alignment direction.
The scale of the simulated pattern matches that of the experimental pattern when a camera length of 64 cm is entered.
Finally, adjusting the center of view to 0.09, 0.08 provides a superimposed simulated pattern.
400
200
Note dynamical spots that violate extinction rules for P21/a
Phase Identification From SAED Patterns
Mn3O4 Powder Pattern
Mn3O4 Ring Pattern
Mn3O4 Ring Pattern
2 0 0
0 2 0
2 2 0
0 4 0
4 0 0 2 -2 0
-2 2 0
0 -2 0
-2 0 0
0 -4 0
-2 -2 0
-4 0 0
0 0 1 Zone
Mn3O4 Ring Pattern
0 0 1 Zone
1 0 0 Zone
Mn3O4 Ring Pattern
0 0 1 Zone
1 0 0 Zone
1 1 2
2 1 1
3 2 1
2 2 4
Mn3O4 Ring Pattern
not observed in 001 or 100 zones
3 1 2
0 0 1 Zone
1 0 0 Zone
1 1 2
2 1 1
3 2 1
2 2 4
Mn3O4 Ring Pattern
All are observed when 111 zone is added
3 1 2
1 1 1 Zone
Another Set of hree ‘observed’ zones
Mn3O4 Ring Pattern
1 1 1 Zone
2 1 0 Zone
1 0 1 Zone
Another set of three ‘observed’ zones
Mn3O4 Ring Pattern
1 1 1 Zone
2 1 0 Zone
1 0 1 Zone
1 0 3
3 1 2
Two Bragg peaks not observed among these 3 zones
Using a known camera constant, or a standard material, one can determine the diffraction angle for each of the observed spots in an SAED pattern and thus obtain a list of d-space values. (For the previous example, this list will have two significant d-spacings missing as they were not observed in the three observed zones – 1 0 1, 2 1 0, and 1 1 1)
To perform an phase identification search with a PDF-4+ database, this list of observed SAED spot d-spacings is required, along with an estimate of their relative intensities. Additionally, to restrict the search subset to a reasonable number of entries, one element present in the phase is required.
Electron Diffraction Phase ID Search
Search Results
. . .
. . .
Search Results List Sorted by GOM
Best match entry – 04-001-7621 (Mn3O4) showing line match and pattern match.
Summary
Simulations of kinematic SAED and EBSD patterns can be created for PDF-4 entries having unit cell and space group information. The user specifies the zone direction desired, the electron voltage, camera length (or constant), and plot size. Phase identification for the PDF-4 add-on software, Sieve+, has been modified to work with d-spacing lists from SAED patterns that may not include all strong lines or may include symmetry-forbidden peaks. Currently one element must be specified to make the search practical.