| Abstract
| - New rare earth oxyfluorosulfides with La4-xYxO2F2S3 (x = 0, 1, 2) composition have been preparedby solid-state reactions. The structures of these compounds vary according to the x value. La4O2F2S3exhibits an incommensurately modulated structure that has been investigated by means of powder X-rayand electron diffraction. The modulated structure has an orthorhombic symmetry, space group Amam(00γ)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 proposedby considering a superstructure of the basic cell with c1 = 23 c. La4-xYxO2F2S3 (x = 1, 2) compoundsexhibit commensurate networks related to a Ce4O4S3 type structure and have been solved from X-raypowder diffraction data. These new phases exhibit ribbons and are closely related to the La2O2S typephases. The formation of these phases and the structural arrangements are explained by shearingmechanisms from the La2O2S structure. The origin of the modulation is discussed in terms of sheetarrangements and the local environment of the rare earth compound. Thus, it has been shown for thefirst 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 constraintsaround the rare earth compound.
- New rare earth oxyfluorosulfides have been prepared by the solid state route and exhibit commensurate and modulated structures identified by TEM as well as poweder XRD analysis. The formation of these phases and the structural arrangements are explained by a shearing mechanism. The rare earth size and the nature and number of neighbors influence the setting up of such modulations.
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