Synthesis and characterization of mixed fluoride Y1-xSr2+1.5xF7 (-1.0 < x < 0.5)

Article abstract of DOI:10.1016/S0025-5408(99)00070-7

A series of mixed fluorides with the general formula Y_1-xSr_2+1.5xF₇ (x = -1.0 < x < 0.5) was synthesized and characterized by powder X-ray diffraction (XRD). It was found that the compound with the stoichiometry Y_0.5Sr_2.75F₇ shows a cubic symmetry (fluorite-type structure) with a = 5.776 angstrom. On decreasing the Sr²⁺ content or increasing the Y³⁺ content in Y_1-xSr_2+1.5xF₇, a phase with tetragonal symmetry with a = 11.416 angstrom and c = 13.291 angstrom was observed in the composition YSr₂F₇, whereas in the composition Y₂Sr_0.5F₇, a phase with hexagonal symmetry with a = 6.882 angstrom and c = 7.032 angstrom was observed. The existence of phases such as YSr₂F₇ and Y₂Sr_0.5F₇ was confirmed. We observed a coexistence of rhombohedral and hexagonal phases beyond a particular nominal composition in this series of compounds.

Full text of DOI:10.1016/S0025-5408(99)00070-7

Materials Research Bulletin, Vol. 34, No. 5, pp. 761–769, 1999
Copyright © 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/99/$–see front matter
PII S0025-5408(99)00070-7
SYNTHESIS AND CHARACTERIZATION OF MIXED
FLUORIDE Y1؊xSr2؉1.5xF7 (؊1.0 < x < 0.5)
S.J. Patwe, S.N. Achary, A.K. Tyagi*, and P.N. Moorthy
Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay,
Mumbai 400 085, India
(Refereed)
(Received April 6, 1998; Accepted July 22, 1998)
ABSTRACT
A series of mixed fluorides with the general formula Y1ϪxSr2ϩ1.5xF7 (x ϭ
Ϫ1.0 Ͻ x Ͻ 0.5) was synthesized and characterized by powder X-ray
diffraction (XRD). It was found that the compound with the stoichiometry
Y0.5Sr2.75F7 shows a cubic symmetry (fluorite-type structure) with a ϭ 5.776
Å. On decreasing the Sr2ϩ content or increasing the Y3ϩ content in
Y1ϪxSr2ϩ1.5xF7, a phase with tetragonal symmetry with a ϭ 11.416 Å and c ϭ
13.291 Å was observed in the composition YSr2F7, whereas in the composi-
tion Y2Sr0.5F7, a phase with hexagonal symmetry with a ϭ 6.882 Å and c ϭ
7.032 Å was observed. The existence of phases such as YSr2F7 and Y2Sr0.5F7
was confirmed. We observed a coexistence of rhombohedral and hexagonal
phases beyond a particular nominal composition in this series of compounds.
© 1999 Elsevier Science Ltd
KEYWORDS: A. fluorides, C. X-ray diffraction, D. phase equilibria
INTRODUCTION
In recent years, the study of solid inorganic fluorides has become an emerging field because
of their interesting properties, such as luminescence, high chemical stability, wide spectral
transmission, and high ionic conductivity. Several fluorides act as a host lattice for active ions
such as Ti3ϩ, Cr3ϩ, and Ln3ϩ (lanthanides) and exhibit luminescence [1]. High quantum
efficiency of phosphorescence is observed in (Ba,Ca)F2–LnF3 (Ln ϭ La–Lu, Y) doped with
*To whom all correspondence should be addressed. E-mail: sichem@magnum.barc.ernet.in.
761

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S.J. PATWE et al.
Vol. 34, No. 5
Eu2ϩ [2,3]. Fluoride glasses have also been used as light guides or as cladding for light
guides and in other optical applications. In addition, fluorides with ions containing unpaired
electrons exhibit interesting magnetic properties [4–6].
During the course of thermal decomposition in vacuum of YBa2Cu3O3F7, a maximum
fluorinated derivative of YBa2Cu3O6.5ϩ␦, we accidentally encountered a new double fluoride
phase, which was characterized as YBa2F7 [7]. To confirm the formation of this compound,
its different analogs containing other rare-earths were synthesized and characterized [8].
Recent studies [9] have shown that there are many compounds of the AB2F7 (A ϭ Sr, B ϭ
Sc, Rh) type, which are promising host lattices for active ions. The fluoride ion conducting
properties of many underdoped alkaline-earth fluorides MЈF2–MF3, (MЈ ϭ Ba, Sr; M ϭ La
to Lu, Y) have also been well studied [10,11]. Sobolev and his group [12] have studied the
solid solution of SrF2–LnF3 (Ln ϭ La to Lu, Y) and explained their thermal behavior and
phase equilibria. The liquidus phases in the SrF2–YF3 system were studied by Nafziger et al.
[13], while Sobolev et al. [12] studied sub-solidus phases.
In the continuation of our earlier work on REBa2F7, we extended similar studies to other
alkaline-earth materials. It is noteworthy that Sobolev and coworkers [12] found ordered
phases in the the temperature range above 850°C in the heavier lanthanides, i.e., from Gd to
Lu and also Y. Herein we report certain ordered phases, which were retained even after slow
cooling to room temperature, and their lattice parameters. The solubility limit of YF3 in SrF2
under normal slow cooling condition was also investigated. To prepare different compounds
in this system, several reactions with different molar ratios of SrF2 and YF3 were performed.
The change in the crystal symmetry was systematically followed by powder XRD.
EXPERIMENTAL
The chemicals used were ammonium hydrogen fluoride (Riedel De Haen), Y2O3 (IRE),
SrCO3 (Fluka AG), and 48% HF. YF3 was synthesized by repeatedly heating Y2O3 with
excess of NH4HF2 at 450°C. SrF2 was synthesized by treating SrCO3 with 48% HF in a hot
water bath. It was further heated at 200°C to remove any trace of HF present in the product.
To prepare Y1ϪxSr2ϩ1.5xF7, homogeneous mixtures of SrF2 and YF3 in proper ratios were
prepared by grinding the reactants together and pressing each mixture into a pellet 6 mm in
diameter.
The general equation for the reaction carried out could be written as follows:
(2 ϩ 1.5x)SrF2 ϩ (1 Ϫ x)YF3 3 Y1ϪxSr2ϩ1.5xF7
Each pellet was wrapped in a platinum foil, sealed in an evacuated quartz tube, heated to
900°C for 8 h, and cooled back to room temperature at a rate of 2°C/min. The XRD patterns
were recorded on a Philips X-ray diffractometer (PW1710) using Ni-filtered Cu K␣ radiation,
with silicon as an external standard. The course of reaction was monitored by powder XRD.
Absence of the lines attributable to the reactants was considered as an indication for
completion of the reaction.
RESULTS
The powder XRD patterns of the compounds with different stoichiometries are given in
Figure 1(A)–(L). We consider the compositions toward the left-hand side of stoichiometric

Vol. 34, No. 5
YTTRIUM STRONTIUM FLUORIDES
763
FIG. 1
Powder XRD patterns of different compositions. (A) Y0.5Sr2.75F7, (B) Y0.6Sr2.60F7,
(C) Y0.7Sr2.45F7, (D) Y0.9Sr2.15F7, (E) YSr2F7, (F) Y1.1Sr1.85F7, (G) Y1.2Sr1.70F7,
(H) Y1.3Sr1.55F7, (I) Y1.4Sr1.40F7, (J) Y1.5Sr1.25F7, (K) Y1.8Sr0.8F7, and (L) Y2Sr0.5F7.
YSr2F7 to be SrF2-rich and those toward its right-hand side to be YF3-rich in the SrF2–YF3
system. The XRD patterns in Figure 1 show that the parent fluorite lattice was retained up to
Y0.7Sr2.45F7 (A–C), but after this the lines start to split. The powder XRD patterns of the
compounds Y0.9Sr2.15F7 and YSr2F7 are given in Figure 1(D) and (E), respectively. The XRD
patterns of the Y0.7Sr2.45F7 (C) and Y0.9Sr2.15F7 (D) show the presence of doublets that are
not well separated. But for YSr2F7 (E), it is observed that all the major peaks are split and
well separated; in addition, some weak lines are observed. The XRD pattern of YSr2F7 could
be indexed on a tetragonal symmetry with a ϭ 11.416 Ϯ 0.004 Å and c ϭ 13.291 Ϯ 0.007
Å (Table 1). The splitting possibly is due to a small distortion created by Y3ϩ substitution at
Sr2ϩ site beyond a threshold limit, to retain the fluorite-type structure. This is in contrast to
the CaF2–YF3 system, where the Caϩ2-rich compositions are reported to exhibit cubic phase
[14], i.e., a significant solubility of YF3 in CaF2 exists. Further, Sobolev et al. [12] found that

764
S.J. PATWE et al.
Vol. 34, No. 5

Vol. 34, No. 5
YTTRIUM STRONTIUM FLUORIDES
765
TABLE 2
Typical Fit for the Powder XRD Data of
Y0.5Sr2.75F7
dobs (Å)
dcal (Å)
I/I0 ϫ 100
h k l
3.335
2.889
2.043
1.742
1.667
1.444
3.335
2.888
2.042
1.741
1.667
1.444
100
18
40
21
3
1 1 1
2 0 0
2 2 0
3 1 1
2 2 2
4 0 0
3
Cubic, a ϭ 5.776 Ϯ 0.001 Å.
about 41 mol% YF3 could be retained in the SrF2 lattice. In contrast, we observed a retention
of only up to 22 mol%. We also observed a tetragonal distortion at about 26 mol%. This
difference in solubility limit of YF3 in SrF2 could be due to the difference in heat treatment:
Sobolev et al. [12] quenched the samples from high temperature, whereas our samples were
slow-cooled. Hence, more YF3 can be retained in the SrF2 lattice at higher temperature.
After the composition YSr2F7 was obtained, further addition of YF3 was attempted to
give compositions such as Y1.1Sr1.85F7, Y1.3Sr1.55F7, Y1.4Sr1.40F7, Y1.5Sr1.25F7,
Y1.8Sr0.80F7, and Y2Sr0.5F7. The XRD patterns of these products are shown in Figure
1(F)–(L). The XRD patterns in (F) and (G) show that the doublets of the stoichiometric
phase (YSr2F7) start to merge, but an additional weak peak at about 2␪ Ϸ 42.5° appears.
This feature was persistent in Y1.1Sr1.85F7 and Y1.2Sr1.70F7. We indexed these patterns on
a cubic unit cell with a ϭ 11.411 and 11.435 Å, respectively. It is important to note that
Sobolev et al. [12] had reported the Y1.1Sr1.85F7 as a rhombohedral cell. Our XRD data
for these compositions could not be fitted using their lattice parameter, but the fit was
better on a cubic cell. The lattice parameter in our case was one-half of that mentioned
by them for the cubic supercell. Thus, we assume that the supercell has existence in a
very small range of composition (Table 1).
Further addition of YF3 to Y1.2Sr1.70F7 leads to its decomposition into two phases. This
indicates that the supercell has a saturation limit. These two phases could be observed at and
after the stoichiometry Y1.3Sr1.55F7. It was found that one set of the new lines that appeared
at and beyond Y1.3Sr1.55F7 match very well with the compound Y2Sr0.5F7, which was
synthesized independently. Y2Sr0.5F7 could be indexed on a hexagonal LaF3-type cell with
cell parameters a ϭ 6.882 Ϯ 0.002 and c ϭ 7.032 Ϯ 0.003 Å. The additional phase could
be indexed on a rhombohedral cell with lattice parameters a ϭ 8.99 Å and ␣ ϭ 107.3°, which
is similar to a rhombohedral supercell observed for Ca7ϩ␦Y6Ϫ␦F32Ϫ␦ (␦ ϭ 0.475) [15]. Thus,
we inferred that a similar kind of supercell exists in this system also. Surprisingly, though the
two phases coexist up to Y1.8Sr0.80F7, they do not show significant variation in lattice
parameters on going from Y1.3Sr1.55F7 to Y1.8Sr0.80F7. But the rhombohedral phase to
hexagonal phase ratio decreases as we proceed from Y1.3Sr1.55F7 to Y1.8Sr0.80F7. The
summary of the phase equilibria is given in Table 1, and the typical fits of some of the XRD
patterns are given in Tables 2–6.

766
S.J. PATWE et al.
Vol. 34, No. 5
TABLE 3
Typical Fit for the Powder XRD Data of
YSr2F7
dobs (Å)
dcal (Å)
I/I0 ϫ 100
h k l
3.321
3.303
2.874
2.858
2.484
2.380
2.297
2.172
2.033
2.024
2.019
1.733
1.723
1.651
1.649
1.436
1.428
3.323
3.302
2.872
2.858
2.494
2.383
2.300
2.175
2.032
2.024
2.018
1.732
1.722
1.651
1.649
1.436
1.427
58
100
17
3
0 0 4
3 0 2
2 0 4
3 2 2
3 3 2
4 2 2
3 3 3
1 0 6
2 1 6
4 2 4
4 4 0
4 3 5
5 4 2
6 0 4
6 3 2
4 0 8
8 0 0
01
02
01
02
20
27
27
11
22
02
03
02
02
Tetragonal, a ϭ 11.416 Ϯ 0.004 Å and c ϭ
13.291 Ϯ 0.007 Å.
DISCUSSION
The solid solubility studies on this system show that towards the SrF2 side, it adopts a
SrF2 (fluorite)-type structure, but there is a decrease in the lattice parameter (Table 1).
This range exists up to the Y0.7Sr2.45F7. We found the solubility limit to be 22 mol% of
YF3. The composition YSr2F7 could be unequivocally indexed on a tetragonal lattice
with a larger unit cell. This larger unit cell could be due to the ordered state of the double
TABLE 4
Typical Fit for the Powder XRD Data of
Y1.2Sr1.70F7
dobs (Å)
dcal (Å)
I/I0 ϫ 100
h k l
3.305
2.863
2.128
2.021
1.723
1.650
1.429
3.301
2.859
2.123
2.021
1.724
1.650
1.429
100
26
03
45
27
04
03
2 2 2
4 0 0
5 2 0
4 4 0
6 2 2
4 4 4
8 0 0
Cubic, a ϭ 11.435 Ϯ 0.005 Å.

Vol. 34, No. 5
YTTRIUM STRONTIUM FLUORIDES
767
TABLE 5
Typical Fits for the Powder XRD Data of
Y1.3Sr1.55F7
dobs (Å)
dcal (Å)
I/I0 ϫ 100
h k l
4.604
3.301
3.283
2.847
2.432
2.306
2.197
2.138
2.106
2.054
2.016
2.009
1.905
1.879
1.829
1.721
1.716
1.649
1.642
1.426
4.604
3.300
3.284
2.847
2.433
2.306
2.198
2.138
2.107
2.051
2.017
2.010
1.904
1.879
1.827
1.721
1.716
1.650
1.642
1.424
02
51
100
41
02
03
03
02
02
01
40
32
01
01
01
12
24
02
05
05
1
1
1
1
0
1
1
0
2 Ϫ2
3 Ϫ1
3 Ϫ2 Ϫ2
3 Ϫ3
4 Ϫ2 Ϫ1
4 Ϫ2
2 Ϫ2
3 Ϫ3
2 Ϫ1
4 Ϫ3 Ϫ1
4 Ϫ3
4 Ϫ3 Ϫ2
1
0
3
2
3
1
3
4
2
1
0
0
2
2
2
0
4 Ϫ3
2
4 Ϫ4
6 Ϫ2
2
Rhombohedral, a ϭ 8.994 Ϯ 0.002 Å and ␣ ϭ
107.34°.
fluoride, i.e., a fluorite-type superlattice, which has been observed [16–18] in the
compounds Ca2HoF7, Ca2TmF7, and Ca2YbF7, which have a tetragonal cell with Z ϭ 10.
The lattice parameter calculations and density measurement in our case suggest the unit
cell to have Z ϭ 10. Although Sobolev et al. [12] found a tetragonal cell, they did not
consider the weak reflections at about 2␪ Ϸ 40°, 41°, etc. But we could index these
reflections with different lattice parameters. On further moving towards the YF3-rich
side, we ended again in a cubic phase, namely, Y1.1Sr1.85F7 and Y1.2Sr1.70F7. Sobolev et
al. [12] found a rhombohedral phase at about 36 mol% of YF3 corresponding to
Y1.1Sr1.85F7. But in our case, we found this to be a cubic phase. The higher-YF3-
containing compositions could not be retained as single phase but consisted of a
rhombohedral supercell phase and a hexagonal phase of the LaF3 (tysonite)-type struc-
ture. A possible explanation for the LaF3-type structure is that the average ionic size at
the Y3ϩ site in YF3 increases by Sr2ϩ substitution. The rhombohedral phase and
hexagonal phase did not show any appreciable lattice parameter variation, but their ratio
decreased and ultimately resulted in a single phase product with hexagonal lattice. This
hexagonal phase is similar to Ca0.5Y2F7 [14]. The structural elucidation of these two
phases is in progress and will be reported subsequently. Measurement of ionic conduc-
tivity in this system is also in progress.

768
S.J. PATWE et al.
Vol. 34, No. 5
TABLE 6
Typical Fit for the Powder XRD Data of
Y2Sr0.5F7
dobs (Å)
dcal (Å)
I/I0 ϫ 100
h k l
3.517
3.448
3.095
2.463
1.987
1.937
1.759
1.729
1.671
1.565
1.546
1.386
3.516
3.441
3.091
2.459
1.987
1.937
1.758
1.730
1.671
1.566
1.543
1.387
60
33
100
04
27
40
04
15
02
01
01
02
0 0 2
1 1 0
1 1 1
1 1 2
0 3 0
1 1 3
0 0 4
0 3 2
2 2 1
1 1 4
2 2 2
2 2 3
Hexagonal, a ϭ 8.882 Ϯ 0.002 Å and c ϭ
7.032 Ϯ 0.003 Å.
CONCLUSIONS
From this study, we conclude that AB2F7-type phases exist in the SrF2–YF3 system as in
other rare-earth analogs. The stability of YSr2F7 is in a narrow range of composition. The
difference between the heat treatment used by us and that by Sobolev et al. [12] is responsible
for a few disagreements between the results of these two studies: our study was on
slow-cooled samples, while theirs was on high-temperature quenched samples. The Sr2ϩ-rich
double fluorides adopt a fluorite lattice, whereas Y3ϩ-rich double fluoride composition shows
a separation of one phase into two phases with rhombohedral and hexagonal lattices.
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