Diffraction contrast stacking faults

The key here was the theory. The pioneers familiarity with both the kinematic and the dynamic theory of diffraction and with the real structure of real crystals (the subject-matter of Lai s review cited in Section 4.2.4) enabled them to work out, by degrees, how to get good contrast for dislocations of various kinds and, later, other defects such as stacking-faults. Several other physicists who have since become well known, such as A. Kelly and J. Menter, were also involved Hirsch goes to considerable pains in his 1986 paper to attribute credit to all those who played a major part. 

The reflections include a particular g in which the dislocation is invisible (i.e., g b = 0 when b is normal to the reflecting plane). With these criteria in diffraction contrast, one can determine the character of the defect, e.g., screw (where b is parallel to the screw dislocation line or axis), edge (with b normal to the line), or partial (incomplete) dislocations. The dislocations are termed screw or edge, because in the former the displacement vector forms a helix and in the latter the circuit around the dislocation exhibits its most characteristic feature, the half-plane edge. By definition, a partial dislocation has a stacking fault on one side of it, and the fault is terminated by the dislocation (23-25). The nature of dislocations is important in understanding how defects form and grow at a catalyst surface, as well as their critical role in catalysis (3,4). 

Stacking faults are characterised by a fault plane and a fault displacement vector. On one side of the fault plane, the atoms that are located fer from the fault are displaced by a vector R in relation to the positions they would occupy in the absence of the fault. Strain fields emanating from any reconstructive bonding that is present near the fault plane will lead to additional displacements for atoms near the fault plane. Thus, the specification of R determines the positions of the atoms that are sufficiently distant so that the strain field generated by the fault is below some specified tolerance. For a planar fault, R may be determined experimentally by analysis of the diffraction contrast obtained with different diffraction vectors g. The positions of atoms near the fault may be determined theoretically by total energy minimisation calculations. Knowledge of these positions is essential to determine the electronic structure of the fault. 

Fig. 5.4 Diffraction contrast images of (a) stacking faults, (b) dislocations, and (c) a dislocation loop...

The characteristics of stacking faults and other defects can also be found using both diffraction contrast [6] and convergent beam techniques [160]. Simple stacking... 

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