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The third and last case of special reflections is where hR = h and ht = 0 (modulo 1). The 2 (or 3 or 4 or 6 or more) contributions are equally large and have the same phase and thus enlarge each other. The expected intensity () of such a reflection is 2 (or 3 or 4 or 6 or more) times as large as that of a general reflection. The factor of enhancement is called epsilon () and is easily derived as the number of rotation matrices Ri that can be applied on h and give back h.
In P2 there are two symmetry operations:
All reflections of the type (0 k 0) will fulfil the criteria hR = h and ht = 0 for both the symmetry operations, and thus these reflections have = 2. It is clear that is not the effect of systematic absences. See Fig. 5.
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