PHASE RELATIONS IN Y1-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 Y1-xSr2+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 Y0.7Sr2.45F7
the parent fluorite lattice was retained but after this
composition and up to Y0.9Sr2.15F7
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 YSr2F7, further addition of YF3
to give compositions such as Y1.1Sr1.85F7,
Y1.3Sr1.55F7, Y1.4Sr1.40F7,
Y1.5Sr1.25F7, Y1.8Sr0.80F7,
Y2Sr0.5F7 was also attempted.
It was found that the doublets of the stoichiometric phase (YSr2F7)
start merging. This feature was persistent in Y1.1Sr1.85F7
and Y1.2Sr1.70F7 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 Y1.2Sr1.70F7
led to phase separation into two phases. These two phases
could be observed at and after the nominal composition Y1.3Sr1.55F7.
It was found that one set of new lines which started appearing at
and beyond this nominal composition match very well with compound
Y2Sr0.5F7, 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 Ca7+dY6-dF32-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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2. B. P. Sobolev, K. B. Seiranian, L. S. Garashina. and P.P
Fedorov J. Solid. State. Chem., 28, (1979) 51
3. J. P. Laval. A. Abaouz., B. Frit. and A. Le.Bail., J.Solid.
State.Chem., 85, (1990) 133