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In crystallography, the Sayre equation, named after David Sayre who introduced it in 1952, is a mathematical relationship that allows one to calculate probable values for the phases of some diffracted beams. It is used when employing direct methods to solve a structure. Its formulation is the following:

$${\displaystyle F_{hkl}=\sum _{h'k'l'}F_{h'k'l'}F_{h-h',k-k',l-l'}}$$

which states how the structure factor for a beam can be calculated as the sum of the products of pairs of structure factors whose indices sum to the desired values of ${\displaystyle h,k,l}$. Since weak diffracted beams will contribute a little to the sum, this method can be a powerful way of finding the phase of related beams, if some of the initial phases are already known by other methods.

In particular, for three such related beams in a centrosymmetric structure, the phases can only be 0 or ${\displaystyle \pi }$ and the Sayre equation reduces to the triplet relationship:

$${\displaystyle S_{h}\approx S_{h'}S_{h-h'}}$$

where the ${\displaystyle S}$ indicates the sign of the structure factor (positive if the phase is 0 and negative if it is ${\displaystyle \pi }$) and the ${\displaystyle \approx }$ sign indicates that there is a certain degree of probability that the relationship is true, which becomes higher the stronger the beams are.

References

1. Sayre, D. (1952). "The squaring method: A new method for phase determination". Acta Crystallographica. 5: 60–65. doi:10.1107/S0365110X52000137.
2. Werner, Massa (2004). Crystal Structure Determination. Springer. p. 102. doi:10.1007/978-3-662-06431-3. ISBN 3540206442.

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