Diffraction power

In this chapter we discuss double-crystal topography, in which we obtain a map of the diffracting power of a crystal compared to that of a reference. We first treat the principles and geometries, the mechanisms of image contrast and resolution and the ttse of laboratory and synchrotron radiation. We then discuss applicatiorrs wafer inspection, strain contour mapping, topography of curved crystals. 

In practice the compounding of waves from the different atoms is done by calculation. The expression for compounding waves from different atoms (diffracting power /) situated at different points in the unit cell (coordinates x, y, z in fractions of the unit cell edges) is, quite generally, for any reflecting plane hkl,... 

The following discussion relates chiefly to complex crystals in w hich all the atoms have much the same diffracting powers—crystals such as those of many organic compounds for it is in these circumstances chat the indirect method of trial and error must often be used. For crystals containing a minority of heavy atoms together with a larger number of lighter atoms in the unit cell, the direct or semi-direct methods described in Chapter X are more appropriate. 

These verdicts are fairly consistent in particular, it appears that a movement of C2 upwards or Cl to the right would have fhe desired effect. Since tjjp diffracting power of Cl is much greater than that of C, a movement of Cl would be much more effective than a similar movement of C. But if Cl is moved, it is found that the intensities of some of the other reflections are adversely affected in particular, 410 becomes too strong. The movements now required to weaken 410 are as follows ... 

The second condition is that there must be, at the centre of symmetry, an atom whose diffracting power is very much greater than that of any of the other atoms in the cell. The third is that there must be only one such atom per lattice point in the projected unit cell j this atom is conveniently taken as the origin of the projected cell. In these circumstances, it is certain that the phase angles of all the reflections with respect to the origin are 0°, since the wave from the heavy atom at... 

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