The single crystal electron diffraction pattern is a series of spots equivalent to a magnified view of a planar section through the reciprocal lattice normal to the incident beam .
specimen Ewald sphere (1/ >>g) 1/ Camera Length (L) r L r = 1 g ; rd hkl =L , L - camera constant r
Types of electron diffraction patterns:
Ring pattern – from polysrystalline specimen. Major use:
Identification of the phases;
Analysis of texture;
Determination of the camera constant L .
Spot pattern – from single-crystal region of the specimen. Major use:
The foil orientation can be determined;
Identification of phases;
The orientation relationship between structures can be determined.
The reciprocal lattice becomes a series of sphere concentric with the origin of the reciprocal lattice.
The main steps of indexing ring patterns:
Measuring ring diameters D 1 , D 2 , D 3 …….
Calculation of the d hkl (using the expression: rd hkl =L )
Use some structure database to index each ring.
beam O hkl sphere D
All diffraction spots are obtained from planes belonging to one zone .
O g 1 g 2 g 3 Crystal beam Ewald sphere Reciprocal lattice plane h 1 k 1 l 1 h 2 k 2 l 2 beam Zone of reflecting planes B – is a zone axis B Schematic representation of diffraction pattern: Real diffraction pattern: h 1 k 1 l 1 h 2 k 2 l 2
Indexing the SAED pattern (spot pattern):
Choose a parallelogram with smallest R 1 , R 2 , R 3 .
Measure distances R 1 , R 2 , R 3 and angles 1 , 2 .
Calculate d 1 ,d 2 ,d 3 (using the rule rd=L ).
Correlate the measured d-values with d hkl taken from the list of standard interplanar distances for the given structure and ascribe h 1 k 1 l 1 and h 2 k 2 l 2 and h 3 k 3 l 3 indices for the chosen three spots.
Check the condition that h 1 +h 2 =h 3 ; k 1 +k 2 =k 3 ; l 1 +l 2 =l 3 .
Compare the measured angles (both 1 and 2 ) with the calculated angles.
h 1 k 1 l 1 h 2 k 2 l 2 h 3 k 3 l 3 1 2 R 3 R 1 R 2 Zone axis of the ED pattern = (h 1 k 1 l 1 ) (h 2 k 2 l 2 )
In the tutorial of the school you will find three electron diffraction patterns.
These patterns are taken from Cu and Al. (Crystallographic data and L of the microscope - are given).
Index the SAED patterns and calculate the Zone Axis (ZA).