Charts structure amplitude

It should be remembered that when a reference atom is moved, all the atoms related to it by symmetry elements move also in a manner determined by the symmetry elements and the problem is to know, for any particular reflection, the direction in which to move the reference atom so that the contribution of the whole group of related atoms either increases or decreases. This problem is best solved by the use of charts which show at a glance the magnitude of the structure amplitude for such a group of atoms for all coordinates of the reference atom. 

For the 230 reflection a chart showing cos 2tt(2x- - Sy) pgg for all values of x and y is required such a chart is shown in Fig. 162. It is made square for convenience the real shape of the projected cell does not matter, since structure amplitudes do not depend on the m shape of the cell but only on atomic coordinates expressed as fractions of the unit cell edges. 

The value of these charts, in showing what adjustments of atomic parameters will increase or decrease the structure amplitude, has already been mentioned. To this little need be added except a suggestion on procedure in dealing with several independent reference atoms simultaneously. This is best explained by an actual example. In attempting to find the x and y coordinates of the atoms in the hkO projection of polychloroprene (Bunn, 1942 a) a particular set of postulated coordinates... 

Three-dimensional charts are not practicable, but for many space-groups the plane charts just described can be utilized for general (hkl) reflections in the following way. For space-group P212121 the structure amplitude is (A2jr B2), where... 

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