Contribution of the Rietveld method to non-stoichiometric phase modeling. Part I: generalities

While being the most efficient way of analyzing a powder diffraction diagram for evidencing an additional phase, refining lattice parameters and performing quantitative phase analysis, the Rietveld method is also able to perform structure refinement including accurate determination of atomic positions and atom occupancies at the different sites. Owing to technological improvements of both powder diffractometers, and Rietveld analysis software, the method is now able to compete with single crystal analysis. It is actually the only alternative when single crystals do not exist or when no single crystal diffractometer is available. The Rietveld method makes possible the determination of vacancy ratio or relative occupancies when atoms of different kinds can occupy the same sites. It allows the measurement of the order parameter in partially ordered structures and the determination of the mechanism for accommodating the excess or deficient atoms in non-stoichiometric phases. This makes the Rietveld method the ideal tool to use in conjunction with the sublattice model especially if experimental site fractions can be taken into account in phase diagram assessment software. Powder diffraction associated with Rietveld analysis should therefore be widely used as an efficient experimental tool for thermodynamic optimization.