Abstract
PHASE determination by direct methods is more difficult for non-centrosymmetric than for centrosymmetric crystals. The phase, α(h,k,l), can take any value for the former crystal between 0 and 2π while the choice for the latter crystal is limited to two values, 0 or π. If the structure is non-centrosymmetric and contains a small number of atoms which scatter anomalously, then α(h,k,l) can be determined from the Bijvoet difference, for example, ΔI= |F(h,k,l)|2−|F(h̄,k̄,l̄)|2, and the known phase αp′ of the anomalous scatterers1,2. From where αN′ is α(h,k,l), the phase of the reflexion, FN′, if there were no anomalous scattering. F″p is the contribution from the absorption term Δf″P. The indeterminacy in equation (1) arises because the cosine is an even function. In practice, the ambiguity has been resolved by choosing αNl′ to be the phase closest to αP′ (ref. 3) or by calculating a double phased synthesis2. An alternative, indeed the original method, is to calculate the Patterson sine function4.
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MacDonald, A. C., and Sikka, S. K., Acta Cryst., B, 25, 1804 (1969).
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HAZELL, A. Structure Determination by the Combination of Anomalous Scattering and Direct Methods. Nature 227, 269 (1970). https://doi.org/10.1038/227269a0
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DOI: https://doi.org/10.1038/227269a0
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