Crystal structure and thermal behaviour of K2[CrF5 · H2O]

Article abstract of DOI:10.1002/zaac.200500458

K₂[CrF₅ · H₂O] is monoclinic: a = 9.6835(3) A, b = 7.7359(2) A, c = 7.9564(3) A, β = 95.94(1)°, Z = 4, space group C2/c (n^o 15). Its crystal structure was solved from its X-ray powder pattern recorded on a powder diffractometer, using for the refinement the Rietveld method. It is built up from isolated octahedral [CrF₅ · OH₂]^2- anions separated by potassium cations. The dehydration of K₂[CrF₅ · H₂O] leads to anhydrous orthorhombic K₂ ^-CrF₅: a = 7.334(2) A, b = 12.804(4) A, c = 20.151(5) A, Z = 16, space group Pbcn (n^o 60), isostructural with K₂FeF₅.

Full text of DOI:10.1002/zaac.200500458

DOI: 10.1002/zaac.200500458
Crystal Structure and Thermal Behaviour of K2[CrF5 ·H2O]
Capucine Sassoye and Ariel de Kozak*
`
´
´
Paris/France, Chimie de la Matiere Condensee de Paris et Cristallochimie du Solide de l’Universite Pierre-et-Marie-Curie, UMR-CNRS
no 7574
Received October 27th, 2005.
˚
Abstract. K2[CrF5 ·H2O] is monoclinic: a ϭ 9.6835(3) A, b ϭ
dration of K2[CrF5 ·H2O] leads to anhydrous orthorhombic
˚
˚
˚
˚
˚
7.7359(2) A, c ϭ 7.9564(3) A, β ϭ 95.94(1)°, Z ϭ 4, space group
C2/c (no 15). Its crystal structure was solved from its X-ray powder
pattern recorded on a powder diffractometer, using for the refine-
ment the Rietveld method. It is built up from isolated octahedral
[CrF5 ·OH2]2Ϫ anions separated by potassium cations. The dehy-
K2CrF5: a ϭ 7.334(2) A, b ϭ 12.804(4) A, c ϭ 20.151(5) A, Z ϭ
16, space group Pbcn (no 60), isostructural with K2FeF5.
Keywords: Potassium; Chromium; Fluorides; Crystal structure
Introduction
a hydrofluoric acid solution placed in a Teflon beaker until
dissolution. Then solid potassium carbonate is slowly
added in the solution until neutralisation. The solution is
stirred 24 hours and during this period a weak quantity of
CrF3 ·3H2O precipitates. After filtration and elimination of
the precipitate, the remaining light green solution is concen-
trated by evaporation at 80 °C in 24 hours. A large quantity
of water is then added to the concentrated solution and
immediately an abundant amount of a light green solid:
K2[CrF5 ·H2O], precipitates. It is recovered, washed with
water and dried at room temperature.
In order to synthesize fluorinated perovskites at room tem-
perature by “soft chemistry“, such as KMgF3, KMnF3 or
KCrF3, new preparation methods were investigated using
hydrofluoric acid and water as reactive medium. During
this study several powders were obtained and among them
a green powder of a hitherto unknown compound that was
disclosed to be K2[CrF5 ·H2O]. The crystal structure of this
new fluoride was solved from its X-ray powder pattern,
using for the structure refinement the Rietveld method.
Synthesis
X-ray powder pattern and comparison with
K2[GaF5 ·H2O]
K2[CrF5·H2O] was synthesized running the following process:
First, in order to increase the reactivity of chromium(III), a
dark green hydrochloric solution of CrCl3 ·6H2O is reduced
within 15 minutes into a blue chromium(II) solution by a
zinc/mercury amalgam in a decantation bulb, which is
shaken and placed under inert nitrogen atmosphere. Then
a hot potassium fluoride solution is introduced in the bulb
and immediately a light green powder of K2[CrF5 ·H2O]
precipitates. The mixture is stirred 15 minutes before the zinc/
mercury amalgam is removed. The precipitate is then reco-
vered, washed with water and dried at room temperature.
Another preparation method was also carried out, using
only CrCl3 ·6H2O, potassium carbonate and hydrofluoric
acid, without the intermediate reduction step of CrIII by the
zinc/mercury amalgam. Solid CrCl3 ·6H2O is introduced in
The X-ray powder pattern of K2[CrF5 ·H2O] (Fig. 1, Table 1) was
collected on a Siemens D5000 diffractometer (copper anticathode
˚
Kα: λ ϭ 1.5418 A). The indexation of the diffractogram was per-
* Prof. Dr. A. de Kozak
`
´
Chimie de la Matiere Condensee de Paris et
Cristallochimie du Solide, UMR-CNRS no 7574
´
Universite Pierre-et-Marie-Curie
Case courrier 176, 4 place Jussieu
F-75252 Paris Cedex 05 / France
E-mail: adk@ccr.jussieu.fr
Fig. 1 Observed (circles) and calculated (full curve) X-ray powder
diffractograms and difference curve for K2[CrF5 ·H2O]. The short
vertical lines mark the positions of all possible Bragg reflections.
Tel-33(1)44275543 & Fax-33(1)44272548
Z. Anorg. Allg. Chem. 2006, 632, 445Ϫ448
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
445

C. Sassoye, A. de Kozak
Table 1 Refined X-ray powder diffractogram of K2[CrF5 ·H2O].
d
h k l
I/I0
d
h k l
I/I0
4.954
4.816
4.654
3.475
3.411
3.225
3.016
2.965
2.913
2.868
2.766
2.491
2.477
2.408
2.361
1 1 Ϫ1
2 0 0
1 1 1
100
5
71
6
2.327
2.271
2.159
1.978
1.934
1.879
1.862
1.841
1.765
1.760
1.724
1.628
1.607
1.570
1.543
2 2 2
3 1 2
4 0 Ϫ2
0 0 4
0 4 0
22
6
12
29
13
16
16
10
5
10
7
6
0 2 1
1 1 Ϫ2
2 0 Ϫ2
2 2 0
3 1 0
2 0 2
3 1 Ϫ1
0 2 2
1 3 0 / 3 1 Ϫ2
2 2 Ϫ2
4 0 0
18
19
38
24
39
15
32
51
20
18
20
0 4 1
5 1 Ϫ1
3 3 Ϫ2
2 4 Ϫ1
5 1 Ϫ2
3 1 Ϫ4
5 1 2
2 2 4
1 3 Ϫ4
5 3 0
13
8
11
1 1 3
Fig. 2 Thermogravimetric and differential thermal analysis curves
for K2[CrF5 ·H2O].
Table 2 X-ray powder diffractogram of K2[GaF5 ·H2O]. The dmeas
values are those of Chassaing [2].
dmeas
dcalc
h k l
I/I0
peak height
dmeas
dcalc
h k l
I/I0
peak height
EDS analysis. The following elements were found to be present in
the powder: K, Cr, F and O with respectively 32.3, 22.1, 38.3 and
7.3 mass %. This composition is in good agreement with the for-
mula K2[CrF5 ·H2O] assumed for the precipitate: K: 32.2, Cr: 21.4,
F: 39.1, O: 6.6 and H: 0.8 mass %.
4.924
4.782
4.628
3.466
3.396
3.206
2.999
2.945
2.900
2.849
2.756
2.482
2.469
2.391
2.353
2.319
4.935
4.784
4.638
3.467
3.400
3.209
3.004
2.947
2.901
2.851
2.759
2.485
2.468
2.392
2.354
2.319
1 1 Ϫ1
2 0 0
1 1 1
100
12
70
8
2.262
2.145
1.972
1.930
1.894
1.874
1.852
1.835
1.760
1.748
1.733
1.716
1.603
1.566
1.557
2.261
2.145
1.973
1.930
1.893
1.874
1.849
1.834
1.760
1.749
1.733
1.716
1.602
1.565
1.556
3 1 2
4 0 Ϫ2
0 0 4
0 4 0
2 0 Ϫ4
0 4 1
5 1 Ϫ1
3 3 Ϫ2
2 4 Ϫ1
5 1 Ϫ2
0 4 2
3 1 Ϫ4
2 2 4
1 3 Ϫ4
0 4 3
4
8
16
8
2
14
8
8
6
8
0 2 1
1 1 Ϫ2
2 0 Ϫ2
2 2 0
3 1 0
2 0 2
3 1 Ϫ1
0 2 2
1 3 0
2 2 Ϫ2
4 0 0
1 1 3
14
16
30
16
25
12
25
25
16
16
16
8
Chromium titration. Chromium was precisely titrated in the powder
using a classical method based on redox reactions. 21.2 mass % of
chromium was found, showing the good agreement with the for-
mula K2[CrF5 ·H2O] that theoretically contains chromium for 21.4
mass %.
4
4
8
6
Thermogravimetry/DTA. Figure 2 shows the thermogravimetric and
DTA curves for K2[CrF5 ·H2O] heated under dry argon atmosphere
up to 450 °C (heating rate: 5° minϪ1). A 7.7 mass loss %, corre-
sponding to one water molecule per formula unit (th.: 7.4 mass %)
is observed between 230° and 340 °C. This shows clearly that the
water molecule is strongly bonded to the chromium cation and be-
longs to its coordination sphere in the structure. The DTA curve
exhibits then three other peaks: the first one, diffuse, at around
490-500 °C is exothermic and corresponds to the recrystallization
of the dehydrated powder into K2CrF5, as the second one, sharp,
at 853 °C is characteristic of its peritectic decomposition into
K3CrF6 and a liquid, as indicated at 855 °C on the phase diagram
of the binary system CrF3/KF [3]. The last peak at about 1230 °C
corresponds to the melting point of the mixture.
4
2 2 2
formed using the DICVOL04 software [1]. The symmetry was
found to be monoclinic and the reflections conditions: h k l, h ϩ
k ϭ 2n and h 0 l, l ϭ 2n, lead to the space groups Cc or C2/c. The
refined cell parameters are: a ϭ 9.6835(3) A, b ϭ 7.7359(2) A, c ϭ
7.9564(3) A, β ϭ 95.94(1)°, Z ϭ 4.
˚
˚
˚
It was observed that the powder pattern of Table 1 is
very similar to the unindexed one given by Chassaing for
the gallium fluoride hydrate K2[GaF5 ·H2O] [2]: therefore
the chemical formula K2[CrF5 ·H2O] was presumed for the
green powder. Table 2 gives the indexed pattern of
K2[GaF5 ·H2O] using the d values of Chassaing [2]: a ϭ
Structure determination
˚
˚
˚
9.638(7) A, b ϭ 7.731(4) A, c ϭ 7.945(4) A, β ϭ 95.81(1)°,
Z ϭ 4.
The crystal structure of K2[CrF5 ·H2O] was solved in the space
group C2/c (no 15). The FullProf-Suite software that integrates
WinPLOTR and FullProf for the Rietveld method was used [4].
First a full pattern matching of the powder diffractogram was per-
formed, then direct methods were applied using the software
WingX [5]. Potassium, chromium and most of the fluorine atoms
were localized, whereas the remaining anions were located with dif-
ference Fourier maps using the GFourier software [6]. The exper-
imental conditions are listed in Table 3. After refinement of all
atomic parameters, including isotropic thermal motion for each
Formula determination
The chemical formula of the green precipitate was assumed to be
K2[CrF5 ·H2O] in reason of the strong similitude of its X-ray pow-
der pattern with that of K2[GaF5 ·H2O] [2]. In order to confirm
the formula, several analyses were carried out, such as a semi-quan-
titative chemical analysis of the powder using a scanning electron
microscope equipped with an EDS-microanalyzer, classical chro-
mium titration and water titration by thermogravimetry, coupled
with DTA.
atom, the final agreement factors converged to Rp ϭ 11.0, Rwp
ϭ
14.8, Rexp ϭ 13.2 and RBragg ϭ 5.60. Table 4 gathers the atomic
parameters and the Biso values for the structure.
446
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 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 445Ϫ448

Crystal Structure and Thermal Behaviour of K2[CrF5 ·H2O]
Table 3 Crystal data, data collection and refinement characteristics
for K2[CrF5 ·H2O].
Chemical formula
System, space group
Refined cell parameters
K2[CrF5 ·H2O]
monoclinic, C2/c (No 15)
˚
˚
a ϭ 9.6835(3) A, b ϭ 7.7359(2) A
˚
c ϭ 7.9564(3) A, β ϭ 95.94(1)°
3
˚
Volume
V ϭ 592.8 A
Molecular weight, Z
Density (calculated)
Wavelength
2θ data range and step
Counting time
M ϭ 243.20 g.molϪ1, Z ϭ 4
2.73 g.cmϪ3
˚
Cu-Kα1,α2, λ ϭ 1.54051/1.54433 A
6 to 108° by 0.02°
50 s.stepϪ1
Number of independent reflections 387
Indices limits
Number of refined parameters
Ϫ global parameters
Ϫ10 Յ h Յ 10, 0 Յ k Յ 8, 0 Յ l Յ 8
31
1
Ϫ profile parameters
12
18
Ϫ intensity-dependant parameters
Number of parameters / number of
intensity parameters
1.72
Gaussian profile parametersa
U - V - W
0.0642 - 0.0079 - 0.0016
Rietveld reliability factors (%)
Rp - Rwp - Rexp - RBragg
χ2
11.0 - 14.8 - 13.2 - 5.60
1.44
a The pseudo-Voigt function was used to refine the diffraction peak profiles.
The Gaussian component of the halfwidth H is given by: H2 ϭ U tan2θ ϩ
V tanθ ϩ W ϩ P/cos2θ. In this refinement P is nil.
Fig. 3 Perspective view of the crystal structure of K2[CrF5 ·H2O].
The octahedral [CrF5 ·OH2]2Ϫ anions are in grey. Each is surroun-
ded by 8 Kϩ cations, represented by large black circles. Small black
circles represent the fluorine anions and small white circles the
oxygen atoms from the water molecules.
Table 4 Atomic coordinates and isotropic thermal motion para-
meters Biso (with e.s.d. in parentheses) for K2[CrF5 ·H2O].
2
˚
Atom/site
x
y
z
Biso /A
K(1)/8f
Cr(1)/4e
F(1)/8f
F(2)/4e
F(3)/8f
O(w)/4e
0.2986(1)
0
0.0611(3)
0
0.8161(3)
0
0.9970(3)
0.2062(2)
0.2044(5)
0.4510(6)
0.2023(5)
0.9449(7)
0.4736(2)
1/4
0.4838(5)
1/4
0.2939(4)
1/4
2.21(5)
1.15(4)
2.17(11)
1.32(14)
2.86(11)
2.27(19)
Description of the structure
The crystal structure of K2[CrF5 ·H2O] can be simply de-
scribed. It is built up from isolated [CrF5 ·OH2]2Ϫ octa-
hedral anions separated by potassium cations. A perspective
view of the structure is given in Figure 3. It is clearly evi-
denced that the water molecule is the sixth ligand of the
chromium cation and belongs to its coordination sphere.
The interatomic distances and bond angles are given in
Table 5. The [CrF5 ·OH2]2Ϫ coordination octahedron exhib-
˚
its a mean bond length Cr-F(O) of 1.900(4) A, which is
Fig.
4
10-coordination around the potassium cation in
˚
˚
K2[CrF5 ·H2O]: 7 K-F(O) distances are within 2.68 Ϫ 2.82 A (full
Table 5 Interatomic distances /A and bond angles /° in the
[CrF5 ·OH2] octahedron.
˚
lines) and 3 are longer, within 3.25 Ϫ 3.45 A (dotted lines). The
Kϩ cation is surrounded by 4 [CrF5 ·OH2]2Ϫ anions arranged in a
roughly tetrahedral coordination.
Cr(1)
F(3)
F(3)
F(1)
F(1)
F(2)
O(w)
F(3)
F(3)
F(1)
F(1)
F(2)
O(w)
1.850(4)
178.1
91.3
88.6
90.9
3.700
1.850(4)
88.6
91.3
90.9
2.678
2.615
1.893(4)
179.2
90.4
2.615
2.678
3.786
1.893(4)
90.4
2.669
2.669
2.687
2.687
1.894(5)
180.0
2.718
2.718
2.759
2.759
3.915
2.021(4)
really close to the balanced sum of the ionic radii of Cr3ϩ
,
FϪ and O2Ϫ: 1.911 A [7].
˚
The potassium cation ensures the electroneutrality of the
structure, as generally observed for fluorides. It exhibits a
common 10-coordination for a K-F(O) interatomic distance
89.1
89.1
89.6
89.6
˚
Average < Cr - F(O) > ϭ 1.900(4) A
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C. Sassoye, A. de Kozak
˚
shorter than 3.50 A (Fig. 4, Table 6). It is noticeable that 7
K2CrF5 obtained from the dehydration of K2[CrF5 ·H2O]
is not very well crystallized, therefore the best way for its
synthesis remains a direct solid state reaction method.
Anhydrous K2CrF5 is isostructural with orthorhombic
˚
of these distances are shorter than 2.82 A and 3 longer than
3.25 A. Nevertheless the mean K-F(O) bond length:
˚
˚
2.943(3) A, is in agreement with the balanced sum of the
˚
˚ ˚
ϭ 7.4059(4) A, b ϭ 12.877(1) A, c ϭ
ionic radii: 2.882 A [7].
K2FeF5:
a
˚
˚
The crystal structure of K2[CrF5 ·H2O] is in agreement
with that of K2[FeF5 ·H2O] previously solved on a single
crystal with analogous cell parameters: a ϭ 9.71(1) A, b ϭ
7.79(1) A, c ϭ 7.95(1) A, β ϭ 96.5(2)°, Z ϭ 4, the same
space group C2/c and analogous atomic positions. More
details on the structure description are given by Edwards,
who discussed also the existence of hydrogen bonds [8].
20.428(1) A, Z ϭ 16 [10] and K2GaF5: a ϭ 7.344 A, b ϭ
˚
˚
12.832 A, c ϭ 20.29 A [11], also with one peculiar form of
K2AlF5 [12]. The previous indexation of the powder pattern
of K2CrF5 (PDF-027-1351) [9], based on the indexation of
the X-ray pattern of this unusual form of K2AlF5 [12], must
be reconsidered according to the latest crystal structure
determination of K2FeF5 [10]. Therefore a new powder
pattern of K2CrF5, synthezised by direct solid state reaction
as described above, was collected on a vertical PW10/40
Phillips diffractometer. It is indexed in an orthorhombic
˚
˚
˚
˚
Table 6 Interatomic distances /A in the 10-coordinated potas-
sium polyhedron.
˚
˚
˚
cell: a ϭ 7.334(2) A, b ϭ 12.804(4) A, c ϭ 20.151(5) A, Z ϭ
16, space group Pbcn (no 60). This new indexed pattern is
given in Table 7.
˚
˚
F,O atoms
d K-F(O) /A
F,O atoms
d K-F(O) /A
F(1)
F(3)
F(3)
F(3)
F(2)
2.684(4)
2.706(4)
2.731(4)
2.792(4)
2.795(1)
F(1)
F(2)
O(w)
F(1)
F(3)
2.812(3)
2.813(1)
3.259(1)
3.398(4)
3.442(4)
Conclusion
The crystal structure of K2[CrF5 ·H2O] is established from
its X-ray powder pattern. Its homologues K2[GaF5 ·H2O]
and K2[FeF5 ·H2O] are isostructural. It is noticeable that
these fluorides are not “hydrates“, as the water molecule is
entirely part of the octahedral coordination sphere of the
metallic cations. This is an exceptional feature for fluorides,
which are water sensitive compounds.
The dehydration of K2[CrF5 ·H2O] under inert argon at-
mosphere leads to anhydrous K2CrF5. Both crystal struc-
tures of K2[CrF5 ·H2O] and K2CrF5 are very different and
no evident link can be made between them. In K2CrF5,
isostructural with K2FeF5, the chromium cation lies in an
octahedral environment of fluorine anions with bridging
fluorine leading to infinite zigzag chains of [CrF6] oc-
tahedra, whereas in K2[CrF5 ·H2O] only independent
[CrF5 ·OH2]2Ϫ octahedra are evidenced.
˚
Average < K - F(O) > ϭ 2.943(3) A
Table 7 X-ray powder diffractogram of anhydrous K2CrF5.
dmeas
dcalc
h k l
I/I0
dmeas
dcalc
h k l
I/I0
peak height
peak height
6.34
6.36
1 1 0
1 1 1
0 2 2
0 0 4
1 2 2
1 0 4
1 2 3
2 1 1
0 2 5
2 1 2
0 4 0
0 4 1
2 2 1
2 1 3
2 2 2
20
26
26
4
42
34
7
15
16
12
36
35
19
35
48
2.289
2.270
2.228
2.178
2.144
2.113
2.076
2.069
2.049
2.023
2.001
1.965
1.960
1.938
1.889
1.845
1.794
1.737
1.724
1.680
1.670
1.663
1.633
1.615
1.591
1.582
1.547
2.289
2.270
2.230
2.180
2.142
2.112
2.076
2.069
2.049
2.023
2.000
1.965
1.959
1.938
1.890
1.844
1.793
1.736
1.723
1.679
1.668
1.662
1.632
1.616
1.590
1.581
1.548
2 3 5
2 4 3
2 1 7
1 5 4
2 3 6
1 1 9
2 0 8
2 4 5
2 1 8
3 3 3
2 3 7
0 6 4
2 4 6
2 5 4
2 1 9
2 6 0
3 3 6
4 2 2
4 0 4
0 0 12
3 5 4
4 3 2
2 3 10
2 7 2
0 4 11
0 8 2
4 4 3
10
26
8
48
4
6.063
5.383
5.024
4.339
4.142
3.912
3.468
3.405
3.324
3.196
3.156
3.140
3.118
3.030
2.970
2.886
2.679
2.654
2.623
2.568
2.535
2.400
2.382
2.372
2.346
2.309
6.068
5.403
5.038
4.350
4.152
3.917
3.472
3.411
3.327
3.201
3.161
3.143
3.121
3.034
2.974
2.888
2.681
2.653
2.625
2.570
2.535
2.401
2.382
2.372
2.345
2.310
5
39
17
21
4
2
6
12
45
11
6
4
14
6
9
5
9
14
11
8
8
4
0 2 6 100
2 1 4
2 3 2
2 1 5
0 2 7
2 3 3
1 4 4
3 1 0
1 0 8
1 4 5
2 4 2
2 2 6
42
51
10
14
5
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37, 724.
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0820.
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Dehydration of K2[CrF5 ·H2O] into K2CrF5
´
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K2[CrF5 ·H2O] was dehydrated under dry argon atmos-
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350 °C. The X-ray powder diffractogram obtained for anhy-
drous K2CrF5 is analogous to that reported in the literature
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state reaction between 2 KF and 1 CrF3 [9]. Nevertheless
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