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Thuc tap ve phan tich TEM

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Information about Thuc tap ve phan tich TEM

Published on November 6, 2007

Author: luyenkimnet

Source: slideshare.net

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Thuc tap ve phan tich TEM
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Exercise: Indexing of the electron diffraction patterns Louisa Meshi

Formation of electron diffraction and HRTEM image

Ewald sphere construction: Bragg’s conditions are satisfied when the Ewald sphere cuts a reciprocal lattice point specified by the indices of the reflecting plane. sin  = = = g/2 1/  1/  1/d hkl * 1/2 =  /2d hkl Bragg’s law g hkl O P hkl Origin of the reciprocal lattice 2  specimen 1/  Points of reciprocal lattice (hkl) plane

Bragg’s conditions are satisfied when the Ewald sphere cuts a reciprocal lattice point specified by the indices of the reflecting plane.

For diffraction in electron microscope: 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

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 .

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.

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.

Ring pattern: 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

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.

Spot pattern 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

All diffraction spots are obtained from planes belonging to one zone .

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 )

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.

Practice time: 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).

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).

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