Electron Diffraction

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Electron Diffraction

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  • 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

    = 2 10 pcm (0.02 0.10 ) = 30 250 pcm (0.30 2.50 )

    2 = 0 - 2 2 = 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 ds and Is, 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.