Preparation and characterization of fluorite-related superstructure phases Sr2RF7 with R ≡ Sm - Lu and Y

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Abstract

The binary systems SrF2-RF3 with R ≡ La — Nd, Sm — Lu, and Y were investigated systematically for the appearance of an intermediate phase with composition Sr2RF7. As a result of this study, we have discovered that this type of compound exists only for R ≡ Sm — Lu and Y. For R ≡ La — Nd, either the corresponding disordered solid solution with anion-excess fluorite (cα) was found, or a diphasic mixture of cα and other intermediate phases with compositions richer in RF3. Guinier X-ray powder diffraction was used for the structural characterization of all phases. Additionally, electron diffraction from single crystals was carried out for Sr2RF7 phases with R ≡ Sm, Gd, Tb, Er, Yb, Y. For all Sr2RF7 phases, a tetragonal fluorite-related superstructure was established with as = 0.5(10)12aF, cs = 3aF, Z = 30, space group I4/m. This superstructure type t is the same as in the case of Ca2RF7 phases, but different from Ba2RF7 (t') where only partial ordering occurs one-dimensionally along the z axis.

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