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Transcript of EDX
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
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
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)
Fluorescence yield
First problem with light element detection
Cascading of X-ray lines
• Naming convention• Quantitative analysis uses one analytical line weight of lines is needed
Qualitative analysis is based on Moseley’s law
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
Detection in EDS
• , Fano factor• Escape peak• Dead-layer• Detector
thickness
From detector to X-ray analyzer
• Detector + preamplifier• Main amplifier, MCA,
pile-up rejection• Spectral resolution, • Si Ge
FWHM2 =N + FE• Temperature
From detector to X-ray analyzer
• Temperature Window
• Detection of light elements
Artifacts: ice
Can be identified and removed
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)
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
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, …
Artifacts: spectral contamination
•Stray radiation from thick parts
•Can be identified
•Frequently can be corrected for
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)
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
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)
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)
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
Searching for known structures in the XRD database
ICDD softwares
• PcPdfWin• PcsiWin
Searching for known structures in the XRD database: ProcessDiffraction
Filtering for elements
Filtering for d-values
Usage of XRD database information in ProcessDiffraction
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
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