The determination of site occupancies using a new strategy in Rietveld refinements

Using X-ray data, the determination of the cation distribution in crystals becomes problematic as a result of parameter correlations in the structure refinements. To circumvent the problem, a new refinement strategy (LOT analysis) was proposed by Kroll et al. [Eur. J. Mineral. (1997), pp. 705–734] and successfully applied to X-ray single-crystal data. The purpose of the study reported here was to determine if the LOT method can also be used in Rietveld refinements. Model calculations in the form of refinements on calculated powder diffraction data were carried out to check the conditions required for a successful application of the LOT method. As a further test, the Fe²⁺, Mg²⁺ distributions obtained from refinements on single-crystal and powder data of a natural olivine were compared. Model calculations and refinements show an improvement in the determination of the site occupancy factors when the LOT strategy is used. Their standard uncertainties are reduced from about σ(X) ≃ ±0.02 to σ(X) = ±0.004 for powder data.