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X-ray powder diffraction study of the stability of solid solutions in LaO(Cl1−xBrx)

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Abstract The formation of solid solutions in the LaO(Cl 1− x Br x ) series was studied by X-ray powder diffraction (XPD), Rietveld profile refinement and bond valence calculations. The LaO(Cl 1− x Br x ) (0 ≤ x ≤ 1, step 0.2, and x = 0.5) powder samples were prepared by the solid state reaction between La 2 O 3 and a mixture of ammonium chloride and bromide. The X-ray powder diffraction patterns were collected at room temperature between 5 and 125° in 2Θ using Cu K α 1 radiation (λ = 1.5406 A). The XPD data between 20 and 90° were analyzed with the DBWS-9006PC Rietveld profile refinement program. All the LaO(Cl 1− x Br x ) phases studied crystallize in the tetragonal PbFCl-type structure with the space group P4/ nmm (No. 129). The cell parameters a and c , the z -coordinate of the halide, the La-ligand distances as well as the R -values increased, and the z -coordinate of the lanthanum atom decreased rather smoothly, even if not completely linearly, with increasing bromine content. The global instability index (GII) yielded values below 0.2, indicating complete solid solubility in the system, but showed a local maximum in the middle of the series.