Sayre equation

{{No footnotes|date=November 2019}}

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:

$$F\_\{hkl\}\; =\; \backslash sum\_\{h\text{'}k\text{'}l\text{'}\}\; F\_\{h\text{'}k\text{'}l\text{'}\}F\_\{h-h\text{'},k-k\text{'},l-l\text{'}\}$$

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 $h,k,l$.{{r|sayre|werner}} 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 $\backslash pi$ and the Sayre equation reduces to the triplet relationship:

$$S\_\{h\}\; \backslash approx\; S\_\{h\text{'}\}\; S\_\{h-h\text{'}\}$$

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