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Figure 2
(a), (b) Two-beam calculations of the wavefield modulation for the centrosymmetric model, space group [Fm{\overline 3}m] (No. 225), with the Bragg condition fulfilled for (a) the [{\overline 1}{\overline 1}{\overline 1}] reflection and (b) the 111 reflection. (c), (d) Similarly for the non-centrosymmetric model, space group [F{\overline 4}3m] (No. 216). In the analytical calculations for both the centrosymmetric and non-centrosymmetric models, the origin is chosen at the same position within the figure (vertical dotted line). The origin for both models is at the position resulting in a real and positive structure factor for the 111 reflection (and thus also for the [{\overline 1}{\overline 1}{\overline 1}] reflection). This origin is in the Sn plane in the centrosymmetric model and the crystal had to be shifted by 25°/360° parts on the repetition unit for the non-centrosymmetric model.

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