Two-beam Diffraction Conditions/First Bethe
Approximation in TEM Measurements
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In two-beam conditions in TEM, low index reflections are normally excited instead of high index reflections. In the two-beam condition, the two beams have the same intensity in the form of Bloch waves:
         i) one is peaked at the specimen atoms,
         ii) the other one is peaked at half way between the atoms.

It is difficult to obtain diffraction disks in CBED and true 'two-beam' kinematic conditions from the small reciprocal lattice dimensions in some materials with the large lattice parameters.

Defocus CBED using high-index reflections provides a more accurate determination method of displacement vectors (R) of stacking faults than two-beam method with exciting low-index reflections, selected area electron diffraction method with low index reflections, and HRTEM technique.

Table 3769. Applications of two-beam diffraction conditions.

Applications Page
Sample thickness determination using CBED page4622
Determination of Burgers vectors of dislocations page3463