BaSr(NH4)Mg5F15, A tetragonal tungsten bronze structure with ammonium barium disorder and its solid solutions Ba xSr2-x(NH4)Mg5F15 (x = 1.8-0.6)

Article abstract of DOI:10.1002/zaac.200600380

The new fluoride BaSr(NH₄)Mg₅F₁₅ precipitates as a nanocrystalline powder from aqueous solutions of the alkaline earth ions and ammonium fluoride. The compound crystallizes in the tetragonal tungsten bronze structure type with lattice parameters of a,b = 12.4492(14) A and c = 3.9421(4) A (space group P4/mbm [Nr. 127], structure refinement from powder data, R_Bragg = 4.0%). Corner-linked magnesium-fluoride octahedra form a channel structure which contains one empty triangular channel, one channel filled with Sr²⁺ [CN = 8+4] and one channel with disordered Ba²⁺ and NH⁴⁺ (ratio 1:1) with CN = 7+8. Solid solutions between the composition limits Ba(Ba_0.8Sr _0.2)(NH₄)Mg₅F₁₅ and (Ba _0.6Sr_0.4)Sr(NH₄)Mg₅F₁₅ were obtained. The crystallite size increases from 15 to 75 nm with increasing barium concentration. The thermal stability between 350 to 450°C depends on the amount of absorbed (NH₄)F. Heated samples show luminescence and phosphorescence.

Full text of DOI:10.1002/zaac.200600380

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DOI: 10.1002/zaac.200600380
BaSr(NH4)Mg5F15, a Tetragonal Tungsten Bronze Structure with Ammonium
Barium Disorder and its Solid Solutions BaxSr2؊x(NH4)Mg5F15 (x ؍ 1.8؊0.6)
F. Kubel* and M. Pantazi
Vienna / Austria, TU Vienna, Institute of Chemical Technologies and Analytics
Received December 23rd, 2006.
Professor Joachim Strähle zum 70. Geburtstag gewidmet
Abstract. The new fluoride BaSr(NH4)Mg5F15 precipitates as a na-
nocrystalline powder from aqueous solutions of the alkaline earth
ions and ammonium fluoride. The compound crystallizes in the
tetragonal tungsten bronze structure type with lattice parameters
between the composition limits Ba(Ba0.8Sr0.2)(NH4)Mg5F15 and
(Ba0.6Sr0.4)Sr(NH4)Mg5F15 were obtained. The crystallite size
increases from 15 to 75 nm with increasing barium concentration.
The thermal stability between 350 to 450 °C depends on the
amount of absorbed (NH4)F. Heated samples show luminescence
and phosphorescence.
˚
˚
of a,b ϭ 12.4492(14) A and c ϭ 3.9421(4) A (space group P4/mbm
[Nr. 127], structure refinement from powder data, RBragg ϭ 4.0 %).
Corner-linked magnesium-fluoride octahedra form a channel struc-
ture which contains one empty triangular channel, one channel
filled with Sr2ϩ [CN ϭ 8ϩ4] and one channel with disordered
Keywords: Nanocrystals; Solid solutions; Disorder; Barium; Stron-
tium; Ammonium; Fluorides
ϩ
Ba2ϩand NH4 (ratio 1:1) with CN ϭ 7ϩ8. Solid solutions
Introduction
were made and allowed to study the relationship between
crystallite size changes and optical properties.
Optical properties such as luminescence and persistent
phosphorescence can be influenced by varying the type of
luminescent centers (defects or doping with rare earth ele-
ments) along with the chemistry of the host matrix, local
disorder or crystal size. Fluorides have been observed to be
good host systems. Recently, a fully fluorinated pyrochlore
compound with composition NaSrMg2F7 [1] has been ob-
tained as a phase with sodium-strontium structural dis-
order. Disorder combined with fluorescence can be as-
sumed to be a reason for a broader colour spectrum in a
system, giving rise to new white phosphors as observed for
BaNa,Eu(II)7F12Cl2 [2 and references therein]. Recently, other
(nano)crystalline fluorides have been studied to understand
the influence of crystal size on optical properties. Crystals
of SrMgF4 with an average crystallite size of ϳ3 nm were
precipitated. Upon heating at ϳ300 °C crystals grow to
30 nm and show luminescence and phosphorescence with-
out doping [3]. The origin of this property depending on
crystallite size and defect concentration is not elucidated in
detail and should be studied on related systems. Preliminary
studies on (BaxSr1Ϫx)MgF4 powders obtained by precipi-
tation using different synthesis methods and concentrations
During precipitation of (BaxSr1Ϫx)MgF4 with an excess
of MgF2 Ϫ to obtain a core-shell system Ϫ with (NH4)F, a
powder pattern was obtained revealing the formation of a
new phase. Analysis of the crystal structure and the physical
properties of this phase are the subject of this study.
Experimental Details
Nanocrystalline BaSr(NH4)Mg5F15 was obtained by precipitation
of dissolved Ba(OH)2·8H2O, Sr(OH)2·8H2O, Mg(CH3COO)2 in di-
luted acetic acid (pH ϳ6) and (NH4)F (all reagent grade). In a
typical reaction, 2.9243 (NH4)F (12 % excess) in 15 ml H2O were
added to
a solution of 1.4824 g Ba(OH)2·8H2O, 1.2488 g
Sr(OH)2·8H2O and 5.0393 g Mg(CH3COO)2 dissolved in 35 ml
H2O. The colourless precipitate was treated by two different prep-
aration steps. Powder 1 was obtained by centrifuging and drying
over 3 d at 140 °C in a compartment drier. Powder 2 was obtained
after an Oswald ripening period of 5 hours in the reaction liquid,
followed by decanting the liquid phase from the solid. Both pow-
ders were then heated at 140 °C in a compartment drier. Under
these conditions the by-product, ammonium acetate, decomposes
and sublimates. (NH4)F should sublimate, too. Residuals of
(NH4)F are physically absorbed to both compounds, but in differ-
ent amounts. Sample 1 consists of 98 wt% BaSr(NH4)Mg5F15 and
2 wt% of physisorbed (NH4)F, sample 2 consists of 94 wt%
BaSr(NH4)Mg5F15 and 6 wt% (NH4)F. Both samples differ in ther-
mal stability.
* Prof. Dr. F. Kubel
Institute of Chemical Technologies und Analytics
TU Wien, Getreidemarkt 9/164-SC
A-1060 Vienna / Austria
Acetates and acetic acid can be replaced by diluted nitric acid
(adjusted to pH 6), the decomposition temperature of the sample
e-mail: frank.kubel@tuwien.ac.at
752
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Z. Anorg. Allg. Chem. 2007, 633, 752Ϫ756

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BaSr(NH4)Mg5F15
Table 1 Crystallographic data and details of data collection for
Table 2 Atomic positions, isotropic vibrational parameters and
BaSr(NH4)Mg5F15 and Ba1.8Sr0.2(NH4)Mg5F15
population parameters of BaSr(NH4)Mg5F15 and Ba1.8Sr0.2
(NH4)Mg5F15 with e.s.dπs in parentheses. Uiso is given in A ; pp ϭ
2
˚
Chemical formula
BaSr(NH4)Mg5F15
Ba1.8Sr0.2(NH4)Mg5F15
population parameter
Cell setting, space group
Temperature/ K
Mr
tetragonal, P4/mbm [127]
293
649.5
a, b ϭ 12.4492 (14)
c ϭ 3.9421 (4)
610.96 (15)
2
BaSr(NH4)Mg5F15
689.31
Atom
Wyckoff
x
y
z
Uiso
pp
˚
Lattice constants/A
12.5296 (5)
3.98012 (18)
624.84 (6)
2
Mg1
Mg2
F1
2d
8i
8i
0.0
0.5
0.0
0.0
0.0
0.0
0.0
0.5
0.5
0.5
0.5
0.5
0.62
0.38
0.0329(11)
0.0329(11)
0.0329(11)
0.0329(11)
0.0329(11)
0.0329(11)
0.0329(11)
0.0542(14)
0.0542(14)
0.0105(9)
0.02530
3
˚
Unit cell volume/A
0.0746(4)
0.3414(4)
0.1265(6)
0.2721(4)
0.0
0.2995(5)
0.16481(15)
0.16481(15)
0.0
0.2175(4)
0.0058(5)
0.0706(5)
0.7721(4)
0.5
0.4238(7)
0.66481(15)
0.66481(15)
0.0
Number of formula units Z
Calculated density/Mg mϪ3 3.531
3.664
F2
8i
F3
F4
F5
4g
2c
8j
Diffractometer
Radiation type Cu Kα1,2
Specimen form and size/mm flat, 12 ϫ 12 ϫ 4
PANanalytical XPert PRO, X’Celerator detector
˚
˚
1.5444 A
1.5406 A,
Ba2
N
Sr1
H1
H2
4h
4h
2b
16l
16l
0.50
0.50
Measuring time/min
Data collection:
Scan length/°
32
2θmin ϭ 5, 2θmax ϭ 100,
2.546
0.23
0.105
0.67
0.67
0.25
0.25
0.02530
Intensity data convertet into 0.02
step /°
Ba1.8Sr0.2(NH4)Mg5F15
Refinement package
Refinement on
Profile function
No. of parameters
R factors (%), Rp
Rwp
TOPAS P Bruker
intensity
fundamental parameter approach
Atom
Wyckoff
x
y
z
Uiso
pp
17
17
Mg1
Mg2
F1
2d
8i
8i
0.0
0.5
0.0
0.0
0.0
0.0
0.0
0.5
0.5
0.5
0.5
0.5
0.5
0.62
0.38
0.0074(8)
0.0074(8)
0.0074(8)
0.0074(8)
0.0074(8)
0.0074(8)
0.0074(8)
0.0287(9)
0.0287(9)
0.0183(7)
0.0183(7)
0.02530
2.4
3.1
1.9
4.0
3.7
4.8
2.5
6.9
2.0
0.0767(3)
0.3460(4)
0.1390(5)
0.2734(4)
0.0
0.2899(4)
0.16764(14)
0.16764(14)
0.0
0.2194(3)
0.0062(5)
0.0693(5)
0.7734(4)
0.5
0.4222(5)
0.66764(14)
0.66764(14)
0.0
Rexp
RBragg
F2
8i
F3
F4
F5
4g
2c
8j
Goodness-of-fit
(∆/σ)max
Refined average crystallite
size/nm
1.7
0.01
27.1(3)
83.2(1.1)
417420
Ba2
N
Ba1
Sr1
H1
H2
4h
4h
2b
2b
16l
16l
0.50
0.50
0.80
0.20
0.25
0.25
ICSD number
417419
0.0
0.23
0.105
0.0
0.67
0.67
0.02530
diminishes however below 200 °C due to decomposition caused by
absorbed NH4NO3.
It should be noted that small changes in the synthesis procedure,
as for example a stoichiometric precipitation without 12 wt% excess
of (NH4)F or a precipitation in stronger diluted systems have not
made it possible to obtain the title compound; instead,
Ba0.5Sr0.5MgF4 and MgF2 precipitate as nanocrystalline powders.
It is assumed that the excess of ammonium fluoride is necessary to
stabilize the crystallization germs of the tungsten-bronze phase.
Results and Discussion
The collected X-ray diffraction powder pattern was similar
to a pattern observed for an orthorhombic variant of a tet-
ragonal tungsten bronze crystal structure, K3Fe5F15 [4, 5],
so a structural relationship with the tetragonal tungsten
bronze type was assumed. The refinement confirmed this
assumption but indicated a higher symmetry. Starting par-
ameters for the Rietveld refinement were therefore taken
from the literature [6]. Refinement conditions and lattice
parameters are given in Table 1 for BaSr(NH4)Mg5F15 and
for one selected solid solution, Ba1.8Sr0.2(NH4)Mg5F15,
heated up to 300 °C to obtain a crystallite size more suitable
for the refinement procedure.
The structure refinement allowed to refine the positions
of the magnesium fluoride octahedra and the alkaline earth
atoms. Occupation parameters of the heavy alkaline earth
ions were refined and converged at ϳ1.0 for Sr2ϩ in Wyck-
off position 2b and ϳ0.5 for Ba2ϩ in position 4h. The re-
sults agree with the synthesis conditions, so final refine-
ments were made using fixed populations. The structure
Solid solutions of BaxSr2Ϫx(NH4)Mg5F15 were formed by the same
procedures as mentioned above. X-ray patterns showing only one
crystalline tungsten bronze phase can be obtained for samples
with composition between Ba(Ba0.8Sr0.2)(NH4)Mg5F15 and
BaSr(NH4)Mg5F15. For compositions (Ba0.8Sr0.2)Sr(NH4)Mg5F15
and (Ba0.6Sr0.4)Sr(NH4)Mg5F15 only 83 % and 60 %, respectively,
of the title compound were observed; none of this phase has been
observed for the synthesis with Ba:Sr ϭ 2:8. Precipitation with HF,
NaF or KF instead of (NH4)F has not been successful in obtaining
the tetragonal tungsten-bronze phase.
After drying, the samples were heated successively up to 300, 450,
500 and 750 °C, respectively and analyzed by X-ray diffraction.
For X-ray diffraction analysis, the PANanalytical X’Pert PRO pow-
der diffractometer (CuKα1,2) equipped with a X’Celerator detector
was used. Rietveld refinements were made using the software pack-
age TOPAS P (Bruker) and the fundamental parameter approach.
The background was calculated using a 20 terms Chebychev poly-
nom. TG and DSC measurements were made on a Mettler Toledo
unit. IR measurements were performed using the spectrometer Per-
kin Elmer Spectrum One FT-IR with a specac ATR diamond cell.
ϩ
was completed by inserting NH4 into the structure. Geo-
metrical reasons and size considerations as well as an im-
proved refinement suggest a solid solution with Ba2ϩ
.
Amorphous (NH4)F was excluded from the refinements. X-
ray patterns (measured, calculated and difference) for two
Z. Anorg. Allg. Chem. 2007, 752Ϫ756
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F. Kubel, M. Pantazi
˚
Table 3 Selected interatomic distances in A for BaSr(NH4)Mg5F15
and Ba1.8Sr0.2(NH4)Mg5F15 with e.s.dπs in parentheses.
BaSr(NH4)Mg5F15
Ba1.8Sr0.2(NH4)Mg5F15
Mg1i Ϫ F4ii
Mg1i Ϫ F4i
Mg1i Ϫ F1iii
Mg1i Ϫ F1iv
Mg1i Ϫ F1v
Mg1i Ϫ F1vi
1.9711(2)
1.9711(2)
1.976(5)
1.976(5)
1.976(5)
1.976(5)
1.99006(9)
1.99006(9)
1.931(5)
1.931(5)
1.931(5)
1.931(5)
Mg2i Ϫ F1v
Mg2i Ϫ F2i
Mg2i Ϫ F5vii
Mg2i Ϫ F5viii
Mg2i Ϫ F3ix
Mg2i Ϫ F2iv
F2iv Ϫ Sr/Ba1v
F5viii Ϫ Sr/Ba1iv
F2iv Ϫ Sr/Ba1iv
1.837(7)
1.940(8)
1.9826(9)
1.9826(9)
2.026(7)
2.134(8)
1.896(7)
2.036(7)
1.9935(4)
1.9935(4)
1.996(6)
2.088(7)
Fig. 1 Powder refinement (observed, calculated and difference
pattern) of BaSr(NH4)Mg5F15 and Ba1.8Sr0.2(NH4)Mg5F15. Refine-
ment package TOPAS P (Bruker), CuKα1,2 radiation.
2.672(5)
2.670(7)
2.672(5)
2.783(4)
2.807(5)
2.783(4)
F1i Ϫ Sr/Ba2ii
F3vi Ϫ Sr/Ba2vii
F1i Ϫ Sr/Ba2iii
F4iv Ϫ Sr/Ba2iii
F4iv Ϫ Sr/Ba2iv
F2iv Ϫ Sr/Ba2vi
F5viii Ϫ Sr/Ba2vii
F2iv Ϫ Sr/Ba2vii
F5viii Ϫ Sr/Ba2iii
F3vi Ϫ Sr/Ba2vi
2.795(5)
2.730(4)
2.795(5)
2.9016(19)
2.9016(19)
3.466(6)
3.255(9)
3.466(6)
3.437(8)
2.730(4)
2.843(5)
2.734(4)
2.843(5)
2.9705(18)
2.9705(18)
3.369(5)
3.234(6)
3.369(5)
3.436(6)
2.734(4)
Symmetry codes:
(i) xϩ1/2, Ϫyϩ1/2, Ϫzϩ1; (ii) x, y, zϩ1; (iii) yϩ1/2, xϪ1/2, Ϫzϩ1;
(iv) Ϫyϩ1/2, Ϫxϩ1/2, Ϫzϩ1; (v) Ϫxϩ1, Ϫy, zϩ1; (vi) xϩ1/2, Ϫyϩ1/2,
Ϫzϩ2; (vii) Ϫyϩ1, x, zϩ1; (viii) Ϫyϩ1, x, z; (ix) Ϫxϩ1, Ϫyϩ1, zϩ1;
(x) Ϫxϩ1/2, yϪ1/2, Ϫzϩ2; (xi) Ϫxϩ1/2, yϪ1/2, Ϫzϩ1; (xii) Ϫxϩ1, Ϫyϩ1, z.
Substitution expands the channel as expected and increases
the heavy alkaline earth to fluorine distances of about 4 ϫ
˚
˚
0.14 A and 8 ϫ 0.11 A. A complete substitution with bar-
ium has not been possible under the given synthesis con-
ditions. In contrast, the barium/ ammonium channel toler-
ates only small amounts of strontium. As the crystallite size
diminishes with increasing strontium concentration (see
Fig. 3) the system might be destabilized and other already
known phases form.
ϩ
Fig. 2 Drawing of the structure of BaSr(NH4)Mg5F15. NH4 is
represented as a sphere.
compositions with different crystallite sizes are given in
Figure 1.
Lattice parameters of the refined solid solutions follow
Vegard’s law [7] as can be seen from Figure 4 and confirm
the incorporation of the bigger barium ion into the struc-
ture. Refinements of all solid solutions were made with
fixed population parameters based on the synthesis compo-
sition. Further details of the refinements may be obtained
from the authors on request.
The thermal stability depends on the temperature and the
amount of absorbed (NH4)F. For small amounts attached
to the nanocrystals, the decomposition starts at ϳ250 °C
as can be seen from new diffraction intensities (patterns of
(Ba,Sr)MgF4 appear). Samples with ϳ6 % absorbed
(NH4)F start to decompose at ϳ440 °C. Thermogravimetric
measurements in Figure 5 show for sample 1 (2 % absorbed
(NH4)F) a decomposition temperature of ϳ300 °C (upper
dark curve) whereas sample 2 (6 % absorbed (NH4)F) re-
leases the absorbed ammonium fluoride first and starts to
decompose at elevated temperatures (ϳ440 °C, lower grey
Structure description
Figure 2 shows a drawing of the structure along the c-axis.
The strong distortion (∆max ϭ 0.3 A) of the Mg2F6 oc-
tahedra can be seen. The average Mg-F distance of 1.977 A
˚
˚
is similar to the distance observed for the pyrochlor com-
˚
pound (1.992 A).Three types of channels, a triangular
empty one, a quadratic and a distorted pentagonal channel
are formed. The smaller alkaline earth ion occupies the tet-
ragonal channel, whereas the bigger channel is occupied by
ϩ
both Ba2ϩ and NH4 ions. The position of the hydrogen
atoms of NH4ϩ are hypothetic and related to the symmetry
of the space group. They have to be confirmed by neutron
diffraction. Refined atomic positions and selected in-
teratomic distances are given in Tables 2 and 3.
The strontium ion occupies the intermediate channel; in
this position, a solid solution with barium can be formed.
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BaSr(NH4)Mg5F15
Fig.
5 Thermogravimetric measurements of two samples of
BaSr(NH4)Mg5F15 heated at 140 °C for 9 and 3 days. They show
the loss of (NH4)F from the sample surface and of (NH4)F released
during decomposition at ϳ300 °C and ϳ440 °C respectively.
Fig. 3 Crystallite size of BaxSr2Ϫx(NH4)Mg5F15 (xϭ1.8-0.6) Be-
low xϭ1 more than one phase has been observed, below xϭ0.6,
no tetragonal tungsten-bronze can be assigned. Standard deviations
correspond to the size of the dots given in the figure. TOPAS refine-
ments of the crystallite sizes are based on the use of the Lorentz
function in the Scherrer analysis [10]
Fig. 6 Infrared spectrum of BaSr(NH4)Mg5F15 heated at 140 °C
(sample 1), 450 °C and 500 °C showing NH signals from the sample
surface and from the structure. No signals belonging to acetate
groups are observed.
these phases continue to react to other Ba/Sr/Mg-fluoride
phases.
Infrared spectroscopy measurements (see Fig. 6) confirm
Fig. 4 Lattice parameters of BaxSr2Ϫx(NH4)Mg5F15 (x ϭ 1.8-0.6).
Below x ϭ 1 more than one phase was found, below x ϭ 0.6, no
tetragonal tungsten-bronze has been found. Standard deviations are
smaller than the dots given in the figure.
ϩ
the presence of NH4 (1460 cmϪ1 (δasym) and 3280 cmϪ1
(νasym)) [9] for samples heated at 140 °C (3 d), 450 °C (3h;
type 2) and at 500 °C (1.5h). Shoulders at 800 and
1700 cmϪ1 might indicate the IR response of physically
adsorbed NH4ϩ, whereas the sharp lines belong to both
vibrations, absorbed and incorporated NH4ϩ. For samples
ϩ
curve). These results are confirmed by the X-ray data and
DSC measurements. Interestingly and in contrast to
SrMgF4, only a small crystallite size increase from 23.7(1.1)
to 27.6(9) nm was observed on heating. The vertical arrows
(1) and (2) correspond to the theoretical amount of (NH4)F
liberated during decomposition. Thermal stability is very
important in the preparation of phosphors with rare earth
doping, as reduction of EuIII to EuII needs elevated tem-
perature, so systems have to be coated to stabilize the com-
pounds even at higher temperature. At 500 °C, samples de-
compose to Ba0.55Sr0.45MgF4 [8], SrF2 and MgF2 corre-
sponding to the theoretical weight percentages. At 700 °C,
heated at 450 °C, only signals from NH4 as part of the
tetragonal tungsten bronze can be detected.
Preliminary experiments show yellow-white luminescence
and phosphorescence (ϳ5 sec) for samples prepared from
acetic acid and heated at 300 °C, when activated by a com-
mercial UV mercury lamp. The origin of this properties
might be related to the origin of phosphorescence and
luminescence observed for heated SrMgF4 [3].
Acknowledgement. Dr Hans Hagemann, Institute of Physical Chem-
istry of Geneva, is gratefully acknowledged for measurements and
his help in the interpretation of the IR spectra.
Z. Anorg. Allg. Chem. 2007, 752Ϫ756
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