PHASE RELATIONS IN Y_1-xSr _2+1.5xF₇ ( -1.0 x 0.5) SYSTEM

Avesh K. Tyagi , Sadiqua J. Patwe and Nagabhusan S. Achary

Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
email: sichem@magnum.barc.ernet.in

Keywords: Fluorides, X-ray diffraction, Phase relations

A series of mixed fluorides of general formula Y_1-xSr_2+1.5xF₇ (x = -1.0< x< 0.5) was synthesized. The change in the crystal symmetry was systematically followed by powder XRD. It was found that up to Y_0.7Sr_2.45F₇ the parent fluorite lattice was retained but after this composition and up to Y_0.9Sr_2.15F₇ a mild tetragonal distortion was observed whereas stoichiometric YSr₂F₇ showed splitting of all the major peaks resulting into a large tetragonal unit cell. After the composition YSr₂F_7, further addition of YF₃ to give compositions such as Y_1.1Sr_1.85F₇, Y_1.3Sr_1.55F₇, Y_1.4Sr_1.40F_7, Y_1.5Sr_1.25F₇, Y_1.8Sr_0.80F₇, Y₂Sr_0.5F₇ was also attempted_. It was found that the doublets of the stoichiometric phase (YSr₂F₇) start merging. This feature was persistent in Y_1.1Sr_1.85F₇ and Y_1.2Sr_1.70F₇ which were found to have a cubic unit cell with 'a' = 11.4110±003 and 11.4350±005 A respectively. Futher addtion of YF₃ to Y_1.2Sr_1.70F₇ led to phase separation into two phases. These two phases could be observed at and after the nominal composition Y_1.3Sr_1.55F₇. It was found that one set of new lines which started appearing at and beyond this nominal composition match very well with compound Y₂Sr_0.5F₇, which was synthesized independently and indexed on a hexagonal LaF₃ (tysonite) type cell with parameters 'a' = 6.882±0.002 A, 'c' = 7.032±0.003 A. Another phase could be indexed on a rhombohedral cell with lattice parameter 'a' = 8.9940±002 A and 'a' = 107.3° which is similar (1) to a rhombohedral supercell observed for Ca_7+dY_6-dF_32-d (d = 0.475). It was found by us that the dissolution of YF₃ up to 22 m% does not cause any noticeable distortion of the fluorite lattice unlike the obsevation of Sobolev et al (2) who found that about 41 m% of YF₃ can be retained in the SrF₂ lattice without causing any distortion. This difference between solubility limit of YF₃ in SrF₂ could be due to the difference in heat treatment as Sobolev et al quenched the samples from high temperature whereas ours were slow cooled samples. This implies that more and more YF₃ can be retained in SrF₂ lattice at higher temperature. The existence of YSr₂F₇ in this system was observed unequivocally which was shown to have a fluorite type superlattice somewhat similar to that in the compounds Ca₂HoF₇, Ca₂TmF₇ and Ca₂YbF₇ (3).

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