Rare earth oxyfluorosulfides : a new class of compounds with modulated structure
Résumé
New rare earth oxyfluorosulfides with La4-xYxO2F2S3 (x = 0, 1, 2) composition have been prepared by solid-state reactions. The structures of these compounds vary according to the x value. La4O2F2S3 exhibits an incommensurately modulated structure that has been investigated by means of powder X-ray and electron diffraction. The modulated structure has an orthorhombic symmetry, space group Amam(00)ss0 in (3 + 1) formalism with the following parameters a = 7.0250(4) Å, b = 7.2171(4) Å, c = 4.2696(3) Å, and q* = 0.26070(5) c*. A three-dimensional approximation of the structure can be proposed by considering a superstructure of the basic cell with c1 = 23 c. La4-xYxO2F2S3 (x = 1, 2) compounds exhibit commensurate networks related to a Ce4O4S3 type structure and have been solved from X-ray powder diffraction data. These new phases exhibit ribbons and are closely related to the La2O2S type phases. The formation of these phases and the structural arrangements are explained by shearing mechanisms from the La2O2S structure. The origin of the modulation is discussed in terms of sheet arrangements and the local environment of the rare earth compound. Thus, it has been shown for the first time that the rare earth compound size, i.e., the nature, the number of neighbors, and the cation-anion bond distances, strongly influence the setting up of such modulations that occur to support constraints around the rare earth compound.