Determination of Atomic Occupancies

Abstract

The most important and most reliable method of determining the ordering of different atomic species over different structural sites is undoubtedly that based on crystal structure refinement using X-ray or neutron diffraction. The reason for its pre-eminence is essentially due to the adequacy of the models that we have available for relating the observational data to structural features, and also to the fact that we are able to make a very large number of independent measurements, much larger in number than the number of unknowns in the model. The disadvantage of the method is that for accurate work it requires elaborate equipment and a very large expenditure of time and effort, and even if it is desired to determine only atomic ordering parameters it is impossible to derive these on their own — a large proportion of the effort goes into determining other parameters which, although of interest in other ways, may not be of much interest to the thermodynamicist.