Powder and single-crystal X-ray structure refinements on a natural chromite: dependence of site occupancies on experimental strategies

a natural chromium-rich spinel, characterized by a high absorption coefficient, was used to evaluate site occupancies obtained by Rietveld structural refinement and single-crystal X-ray diffraction. the aim of this qork is to test the influence of powder thickness, counting times and refinement strategies on variations in atomic parameters determined by the Rietveld method, and then to compare them with the results of single-crystal structural refinement of the same material. Results showed that the atomic coordinates do not change, while T and M site occupancies are very sensitive to sample thickness and, for the same sample to the counting statistic. Powder results match those from single-crystalsamples with decreasing thickness. Best-fit factors in single-crystal refinement were obtained by using fully ionized scattering curves for cations and partially ionizedones for oxygen, giving total electrons from T and M sites closer to those calculated from the chemical composition. The best results in Rietveld refinements were obtained using the same scattering curves as those in the single-crystal experiment. In conclusion, powder thickness is crucisl when the material has a hifìgh absorption coefficient, and the best results are obtained only with a high rate of statistical counting.