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