The Crystal Structures of SrAlF5 and Ba0.43(1)Sr0.57(1)AlF5

Two members belonging to the pseudobinary phase diagram of Ba/SrF₂ and AlF₃ were synthesized. Single domain crystals of SrAlF₅ and Ba_0.43(1)Sr_0.57(1)AlF₅ were prepared from the corresponding metal fluorides. The mixed compound Ba_0.43(1)Sr_0.57(1)AlF₅ has been synthesized and characterized in detail for the first time. The structure of SrAlF₅ was reinvestigated; a superstructure was found which is absent in Ba_0.43(1)Sr_0.57(1)AlF₅. The compounds crystallize at room temperature in the centrosymmetric tetragonal space groups I4₁/a and I4/m. Lattice parameters are a = b= 1988.22(14), c = 1432.24(19) and a = b = 1431.32(14), c = 722.83(7) pm. Both structures differ mainly in the AlF₆ arrangement situated in the cavities of the matrix structure. In SrAlF₅, ordered dimer [Al₂F₁₀]^4– units are found, whereas in the Ba²⁺ mixed sample a linear chain of AlF₆ octahedra is observed. For both compounds no partial occupation was observed. Sm²⁺ doped samples are further characterized by luminescence measurements. Four independent sites of alkaline earth atoms were found in the Sm²⁺ luminescence of the host SrAlF₅.