edx
TRANSCRIPT
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Phase identification by combining local composition from EDX with information
from diffraction database
János L. Lábár
•Introduction to EDX analysis
•Usage of the XRD database
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Composition by EDX• Ionization by fast electrons in the TEM• Alternative ways of de-excitation• Photons leaving the sample• Detection / detectors• Qualitative vs. quantitative analysis• Precision, accuracy, detection limits, spatial
resolution• Artifacts and their elimination• Effect of crystal structure: ALCHEMI
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Excitation and de-excitation• Primary process:
ionization EELS• Competing secondary
processes: XR / AE• Single-electron process:
X-ray photon emission• Two-electron process:
Auger electron emission• Connection:
fluorescence yield =NX/(NX+NA)
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Fluorescence yield
First problem with light element detection
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Cascading of X-ray lines
• Naming convention• Quantitative analysis uses one analytical line weight of lines is needed
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Qualitative analysis is based on Moseley’s law
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Self-absorption in the sample
• Absorption path length vs. thickness, ideal geometry Lt*cosec()• Thin-film approximation No thickness is needed• Methods to determine thickness (EELS, CBED, …)• Accuracy problems with light elements, irregular samples
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Detection in EDS
• , Fano factor• Escape peak• Dead-layer• Detector
thickness
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From detector to X-ray analyzer
• Detector + preamplifier• Main amplifier, MCA,
pile-up rejection• Spectral resolution, • Si Ge
FWHM2 =N + FE• Temperature
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From detector to X-ray analyzer
• Temperature Window
• Detection of light elements
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Artifacts: ice
Can be identified and removed
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Quantitative analysis
• kAB is dependent on the detector
• Significant differences in „sensitivity”
• Standards vs. standardless
Cliff-Lorimer: thin film appr.
cA/cB=kAB*(IA/IB)
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Quantitative analysis: standardless
• Intensity:
– For high energy electrons: NQ(E0)
• Atomic data, Detector parameters
• Sample thickness: absorption• Secondary fluorescence• Artifacts: escape, contamination, spectral, channelling
liliiiliA EPRVI ,
3,22,13,112,1233 fffNfNNVL
ddj
j
j
ttEP
exp1exp
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Thin sample criterion
• Different condition for EDS and imaging• Thickness not needed for many samples • Depends on detector position for EDS• Depends on combination of elements• Determination of thickness: CBED, …
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Artifacts: spectral contamination
•Stray radiation from thick parts
•Can be identified
•Frequently can be corrected for
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Structure from „artifact”: ALCHEMI
• Bloch-waves in crystals• Orientation-dependent
excitation• Inhomogeneous within unit
cell syst. error• Main components at known
sites = inner standards• Location of minority c.
(additional information)
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ALCHEMI example: garnet• Calculations predicted
distinct variation of all three crystallographic sites (in a rest. range)
• Experiment proved it for main components
• Location of minority Ca and Mn is unambiguously determined
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Summary: EDS analysis in the TEM
• Multi-elemental, parallel• 5 Z (with ATW)• Elemental compositions (not sensitive to the
chemical state)• Detection limit 0.1 wt%• Accuracy 2-10% (standardless vs. standards, stray
radiation)• Spatial resolution: 1 nm (FEG), 10 nm (LaB6),
(sample thickness)
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The XRD powder database
• Evolution of the ICDD database– JCPDS cards– Pdf-2 database– Pdf-4 relational database, time-lock, atomic p.
• Usage of the database– ICDD software– Manufacturer’s software– Other programs (ProcessDiffraction)
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The JCPDS cards in the Pdf-2 databaseAs shown by the PCPDFWIN program
Name & reference
d-spacing, Intensity, Miller-indices
Space group, cell parameters
Radiation, wavelength, filter
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Searching for known structures in the XRD database
ICDD softwares
• PcPdfWin• PcsiWin
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Searching for known structures in the XRD database: ProcessDiffraction
Filtering for elements
Filtering for d-values
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Usage of XRD database information in ProcessDiffraction
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Why XRD database can only be used for qualitative phase analysis in electron
diffraction?• X-rays are scattered on the electrons of the sample • Fast electrons of the TEM are scattered on total
charge (electrons + nuclei)• Intensities of the diffracted lines are different• Quantitative phase analysis needs a calculation of
intensities from a structural model and nanocrystalline samples
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Conclusion
• Unambiguous phase identification needs both compositional and structural information.
• Composition from EDS (or EELS)• XRD database is a useful collection of known
structures easiest first source of information during assessment of SAED patterns
• Quantitative phase analysis needs a calculation of intensities from a structural model