KLiSiF6 and CsLiSiF6 – A Structure Investigation

Article abstract of DOI:10.1002/ejic.202000867

KLiSiF₆ and CsLiSiF₆ were synthesized via a high-pressure/high-temperature synthesis route. Even though, both substances crystallize in the orthorhombic crystal system with space group Pbcn (no. 60), they show different crystal structures. Main motifs of KLiSiF₆ are an [SiF₆]^2? and an [LiF₆]^5? unit. In contrast, the main motifs of CsLiSiF₆ are an [SiF₆]^2? and an [LiF₅]^4? entity. Within both substances, these units are interconnected and form 3-dimensional networks. The substances were characterized via single-crystal and powder X-ray diffraction, as well as infrared spectroscopy and EDX measurements. Additionally, Mn⁴⁺-doped KLiSiF₆ was analyzed by means of luminescence spectroscopy. It displays line emission in the red spectral region. The maximum emission wavelength is λ_max=631 nm.

Full text of DOI:10.1002/ejic.202000867

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KLiSiF6 and CsLiSiF6 – A Structure Investigation
Christiane Stoll,[a] Markus Seibald,[b] Dominik Baumann,[b] Jascha Bandemehr,[c]
Florian Kraus,[c] and Hubert Huppertz*[a]
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KLiSiF6 and CsLiSiF6 were synthesized via a high-pressure/high-
temperature synthesis route. Even though, both substances
crystallize in the orthorhombic crystal system with space group
Pbcn (no. 60), they show different crystal structures. Main motifs
of KLiSiF6 are an [SiF6]2À and an [LiF6]5À unit. In contrast, the
main motifs of CsLiSiF6 are an [SiF6]2À and an [LiF5]4À entity.
Within both substances, these units are interconnected and
form 3-dimensional networks. The substances were character-
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ized via single-crystal and powder X-ray diffraction, as well as
infrared spectroscopy and EDX measurements. Additionally,
Mn4+-doped KLiSiF6 was analyzed by means of luminescence
spectroscopy. It displays line emission in the red spectral region.
The maximum emission wavelength is λmax =631 nm.
1. Introduction
crystallizes in space group Pnma (no. 62).[14] For the substances
KLiSiF6, NaLiSiF6, RbLiSiF6, and CsLiSiF6, Skarulis and Seibert
predicted the crystal systems as well as the lattice parameters
in 1970.[15] For KLiSiF6, they determined an orthorhombic unit
cell with the lattice parameters a=982.3, b=580.5, and c=
756.0 pm. For CsLiSiF6, a hexagonal unit cell with the lattice
parameters a=1133.4 and c=927.1 pm was measured.[15]
As detailed crystal-structure analyses of both substances,
KLiSiF6 and CsLiSiF6, are still missing, we took the chance to
resolve these structures by means of single-crystal diffraction.
Salts of the hexafluoridosilicic acid are known since the
beginning of the nineteen hundreds, with K2SiF6 first mentioned
in 1882.[1] Recently, many of the ternary alkali metal hexafluor-
idosilicates A2SiF6 (A=Li, Na, K, Rb, Cs),[2–5] as well as the ternary
alkaline earth metal hexafluoridosilicates A’SiF6 (A’=Ca, Ba, Sr,
Eu)[6–8] were synthesized and are mentioned in the literature.
Additionally, magnesium and strontium hexafluoridosilicates
are known as hydrates (Mg(SiF6)(H2O)6 and Sr(SiF6)(H2O)2).[9–10]
Nevertheless, a detailed structure description is limited to alkali
metal hexafluoridosilicates and a few other hexafluoridosilicates
A’SiF6 (A’=Ca, Ba, Sr, Eu).[2–8] All of those described hexafluor- 2. Results and Discussion
idosilicates have a common building block, a [SiF6]2À unit. This
unit recently gained lots of attention due to the possibility to
partially substitute Si for Mn which results in a narrow line
emission in the red spectral region.[11–13] Anyway, this unit can
also be observed within the quaternary hexafluoridosilicate
KNaSiF6.[14] However, it is surprising that just a few quaternary
hexafluoridosilicates, namely ALiSiF6 (A=Na, K, Rb, Cs), KNaSiF6,
and Na3Li(SiF6)2 are mentioned in the literature.[14–15] Currently,
the only known crystal structure is the one of KNaSiF6, which
2.1. Crystal structure of KLiSiF6
KLiSiF6 crystallizes in the orthorhombic crystal system with
space group Pbcn (no. 60) and is isostructural to (NH4)MnFeF6
(Pearson code: oP72 (without H); Wyckoff sequence: c2d8).[16] The
lattice parameters of KLiSiF6 are 747.50(3), 1158.58(5), and
979.77(4) pm for a, b, and c, respectively, with a volume of V=
0.8485(1) nm3. The single crystal was measured at 203(2) K. It is
noticeable that the b parameter is twice as large than the one
predicted by Skarulis and Seibert (a=982.3, b=580.5, and c=
756.0 pm, V=0.431 nm3).[15] This is likely a result of an oversight
during the indexing of the powder data in 1970. Pictures of the
reflections in 0kl, 1kl, h0l, and hk0 are depicted in Figure S1. The
extinction conditions for the b, c, and n glide reflections can be
noticed in 0kl, h0l, and hk0, respectively. Upon examination of
1kl, it becomes evident that there are reflections marking the
larger b-axis, therefore the unit cell parameters of 747.50(3),
1158.58(5), and 979.77(4) pm for a, b, and c, respectively are
found to be correct. The volume of the unit cell amounts to
0.8485(1) nm3. The unit cell contains eight formula units and
thus 72 atoms. Within the unit cell, there are two potassium
positions, which are located at the special Wyckoff position 4c,
as well as one lithium, one silicon, and six fluorine positions,
which are located at the general Wyckoff position 8d. Further
information on the crystal structure refinement can be found in
Table 1. Selected distances and angles, as well as atom
[a] Dr. C. Stoll, Prof. Dr. H. Huppertz
Institut für Allgemeine, Anorganische und Theoretische Chemie
Universität Innsbruck
Innrain 80–82, 6020 Innsbruck, Austria
E-mail: hubert.huppertz@uibk.ac.at
https://www.uibk.ac.at/aatc/mitarbeiter/hub/
[b] Dr. M. Seibald, Dr. D. Baumann
OSRAM Opto Semiconductors GmbH
Mittelstetter Weg 2, 86830 Schwabmünchen, Germany
[c] J. Bandemehr, Prof. Dr. F. Kraus
Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Str. 4, 35032 Marburg, Germany
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202000867
© 2020 The Authors. European Journal of Inorganic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original work is
properly cited, the use is non-commercial and no modifications or adap-
tations are made.
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Table 1. Crystal data and structure refinement of KLiSiF6 and CsLiSiF6.
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Empirical formula
Molar mass, gmolÀ 1
Crystal system
KLiSiF6
188.13
orthorhombic
Pbcn (no. 60)
CsLiSiF6
281.94
orthorhombic
Pbcn (no. 60)
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Space group
5
6
Powder data
Powder diffractometer
Radiation
Temperature, K
a, pm
Stoe Stadi P
Mo-Kα1 (λ=70.93 pm)
293(2)
755.27(2)
1159.61(2)
Stoe Stadi P
Mo-Kα1 (λ=70.93 pm)
293(2)
926.92(3)
1146.07(4)
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b, pm
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c, pm
982.21(2)
0.8602(1)
978.33(3)
1.0393(1)
V, nm3
Single-crystal data
Single-crystal diffractometer
Radiation
a, pm
Bruker D8 Quest Photon 100
Mo-Kα (λ=71.07 pm)
747.50(3)
1158.58(5)
979.77(4)
0.8485(1)
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2.945
203(2)
1.576
720
Bruker D8 Quest Photon 100
Mo-Kα (λ=71.07 pm)
922.08(5)
1142.9(1)
974.01(5)
1.0265(1)
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3.649
173(2)
7.48
1008
b, pm
c, pm
V, nm3
Formula units per cell, Z
Calculated density, gcmÀ 3
Temperature, K
Absorption coefficient, mmÀ 1
F(000), e
2θ range, deg
Range in hkl
6.5-75.8
5.7-65.3
�12, �19, �16
�13, �17, �14
Total no. of reflections
Independent reflections/ref. parameters/Rint
Reflections with I>2 σ(I)
Goodness-of fit on Fi2
Absorption correction
Final R1/wR2 (I�2σ(I))
Final R1/wR2 (all data)
Largest diff. peak/hole, e ÅÀ 3
32109
43802
2289/84/0.035
1979
1.080
semi-empirical (from equivalents)
0.0188/0.0420
0.0255/0.0438
0.45/À 0.47
1877/83/0.043
1490
1.055
semi-empirical (from equivalents)
0.0184/0.0406
0.0297/0.0452
1.13/À 0.59
coordinates and displacement parameters are reported in the
Supporting Information (Tables S1–S4).
obvious (Figure 2, right). Every second layer along [100] is
shifted, so that [SiF6]2À and [LiF6]5À units alternate.
The main structural motifs of KLiSiF6 are two octahedral
building blocks represented by an [SiF6]2À and an [LiF6]5À unit
(Figure 1, top). The SiÀ F bond lengths within the [SiF6]2À entity
vary from 167.6(1) to 170.1(1) pm (Table S3). This is in
accordance with the SiÀ F bond lengths described for K2SiF6
(dSiÀ F =168.3(2) to 170.6(9) pm).[3] The FÀ SiÀ F angles confirm a
distorted octahedral coordination sphere (Table S4). Within the
second building unit [LiF6]5À , the LiÀ F bond lengths vary from
193.0(1) to 219.4(1) pm (Table S3). This corresponds with the
values given in the literature for Li2SiF6 (dLiÀ F =195.5(2) to
216.6(4) pm).[2] The bond angles are in accordance with a
distorted octahedral coordination sphere (Table S4).
These octahedra are connected via a common edge and
thus form the fundamental building block (Figure 1, bottom).
The fundamental building blocks are connected via a common
corner (F3) and arranged in zigzag lines along [001]. Along
[010], the chains are additionally interconnected via a common
corner (F5). This leads to a formation of six membered rings,
which are formed by the centers of six octahedra within the bc-
plane (Figure 2, right), creating channels along [100] for the
Charge distribution (Chardi)[17], as well as bond-length/
bond-strength calculations (BLBS)[18–19] confirmed the crystal
structure (Table S9). The cation composition of KLiSiF6 was
verified by EDX analysis to be 1:1.0(3) for K:Si. An SEM image is
depicted in (Figure S2, left).
2.2. Crystal structure of CsLiSiF6
CsLiSiF6 also crystallizes in the orthorhombic crystal system with
the space group Pbcn (no. 60). The cell parameters are
922.08(5), 1149.2(1), and 974.01(5) pm for a, b, and c, respec-
tively. The volume of the cell amounts to 1.0265(1) nm3. This
differs from the unit cell (hexagonal, a=1133.4 and c=
927.1 pm, V=1.031 nm3) determined by Skarulis and Seibert.
Transformed into an orthohexagonal projection, the cell
parameters amount to a=1133.4, b=1963.05, and c=
927.1 pm. Therefore, they can be set into relation with the
newly determined cell parameters: aold =bnew, bold =cnew ×2,
c
old =anew. As the powder data published by Skarulis and Seibert
(ICDD: 00-023-0895) matches the powder data recorded for the
sample prepared during this work (Figure S3), the different
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potassium cations. Viewed along [010], the interconnection via
common corners (F6 and F1) between these layers becomes
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Figure 1. Octahedral coordination sphere of silicon (top-left) and lithium (top-right) in KLiSiF6. The connection via a common edge is shown in the bottom of
the figure.
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Figure 2. 2×2 unit cells of KLiSiF6 viewed along [100] (left) and along [010] (right).
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assignment of the crystal system can likely be attributed to a
misinterpretation during indexing of the powder data.
Pearson code of oP72. Even though, the Wyckoff sequence as
well as the Pearson code are equal with the one of KLiSiF6,
CsLiSiF6 exhibits its own structure type. Information about the
crystal structure refinement can be found in Table 1. Further-
more, selected distances, as well as angles, the atom coordi-
nates, and displacement parameters are shown in Tables S5–S8.
Main structural motifs of CsLiSiF6 are a [SiF6]2À octahedron
(Figure 3, top left) and a [LiF5]4À distorted square pyramid
(Figure 3, top right). Within the [SiF6]2À octahedra, the SiÀ F
bond lengths range from 166.7(2) to 169.9(2) pm (Table S7),
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The unit cell of CsLiSiF6 consists of eight formula units
including 72 atoms. The asymmetric unit contains two cesium,
one lithium, one silicon, and seven fluorine positions. Of these,
only the two cesium positions are located at special Wyckoff
sites (4c). All other atoms are located at general Wyckoff
positions (8d). This leads to a Wyckoff sequence of c2d8 and a
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which is in accordance with the literature (for K2SiF6: dSiÀ F
=
168.3(2) to 170.6(9) pm).[3] The bond angles confirm an octahe-
dral coordination (Table S8). Within the [LiF5]4À unit, the LiÀ F
bond lengths range from 190.7(4) to 210.4(4) pm (Table S7).
This unit is also present in Li2Ta2O3F6 (dLiÀ F =187(1)–
224(2) pm).[20] The next closest fluorine atom, F1 (dLiÀ F
=
335.0 pm), is not part of the coordination sphere according to
Maple[21–23] calculations (Table S11). Additionally, the fluorine
atom (F6), which would be needed for an octahedral coordina-
tion around lithium, is located 401.7(4) pm away from lithium.
This is approx. two times the usual bond-length of a LiÀ F bond.
Therefore, a distorted square pyramidal coordination instead of
an octahedral one is present within CsLiSiF6.
These two units are connected via a common edge
(Figure 3, bottom) building up the fundamental building block
of this structure. These building blocks are connected in a
similar manner than in KLiSiF6 and thus build six membered
�
rings within the bc-plane (Figure 4, left). Viewed along [010] or
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[001], the main structural differences between CsLiSiF6 and
KLiSiF6 become evident. In CsLiSiF6, lithium is not coordinated
octahedrally like in KLiSiF6 and thus is missing one of the
connections to the [SiF6]2À units. Nevertheless, both units
[SiF6]2À and [LiF5]4À are interconnected in an alternating manner
Figure 3. Octahedral coordination sphere of silicon (top-left) and distorted
square pyramidal coordination of lithium (top-right) in CsLiSiF6. The
connection via a common edge is shown in the bottom of the figure.
2À
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forming a network, if viewed along [010]. Hereby, an [SiF6]
unit is connected via F1 to the tip of an [LiF5]4À entity. Two of
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Figure 4. 2×2 unit cells of CsLiSiF6 viewed along [100] (left) and along [010] (right).
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these connected units share common corners (F2) and thus
2.3. Powder data
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form four membered rings within the ac-plane. These four
membered rings are further interconnected via F3 to form a
Both substances can be synthesized with good yields. Powder
analysis via the Rietveld technique determined the powder
composition of the KLiSiF6 sample (Figure 6) to be 93.0(1) wt.%
KLiSiF6, with 2.3(1) wt.% Li2SiF6 and 4.7(1) wt.% K2SiF6 as
impurities. The powder sample of CsLiSiF6 (Figure 7) exhibits
CsLiSiF6 as the main phase with 95.8(2) wt.%, and Li2SiF6 and
Cs2SiF6 as side phases with 0.3(2) wt.% and 4.0(1) wt.%,
respectively.
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netlike arrangement. Viewed along [001] and in comparison to
KLiSiF6 (Figure 5), an opening of channels for the cesium cations
is noticeable. This can likely be attributed to the significantly
larger cation radius of Cs+ in contrast to the radius of the
potassium cation (rCs =202 pm, CN=12; rK =178 pm, CN=
12).[24] Therefore, to the best of our knowledge, CsLiSiF6 forms
its own structure type.
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Chardi[17] and BLBS[18–19] calculations confirmed the structural
model (Table S10). The cation composition was verified via EDX
measurements to be 1:1.2(4) for Cs:Si. An SEM image is
depicted in Figure S2, right.
�
Figure 5. 2×2 unit cells of CsLiSiF6 (left) and KLiSiF6 (right) viewed along [001].
Figure 6. Rietveld plot of the recorded powder pattern (black crosses) of KLiSiF6. The calculated curve is depicted in red and the difference curve in blue.
Bragg positions of KLiSiF6 are marked in green and the Bragg positions of the side phases Li2SiF6 and K2SiF6 are marked in orange and purple, respectively.
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Figure 7. Rietveld plot of the recorded powder pattern (black crosses) of CsLiSiF6. The calculated curve is depicted in red and the difference curve in blue.
Bragg positions of CsLiSiF6 are marked in green and the Bragg positions of the side phases Li2SiF6 and Cs2SiF6 are marked in orange and purple, respectively.
2.4. Vibrational spectroscopy
with the ones given for the [SiF6]2À anion in K2SiF6 (first
harmonics:
ν4 �484 cmÀ 1
(deformation),
ν3 �744 cmÀ 1
FT-IR spectra of both substances (Figure 8) were measured in
the range of 400 to 4000 cmÀ 1. Both show two absorption
bands at similar positions (KLiSiF6: 482 cmÀ 1, 692 cmÀ 1; CsLiSiF6:
471 cmÀ 1, 702 cmÀ 1). These absorption bands are in accordance
(valence)).[25–27]
Figure 8. Normalized FT-IR spectra of KLiSiF6 (black) and CsLiSiF6 (red) in the range of 400 to 4000 cmÀ 1
.
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2.5. Luminescence Spectroscopy
3. Conclusion
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Mn4+ doping of KLiSiF6 was achieved by a ball-milling experi-
ment. A comparison of the powder pattern before and after the
ball-milling step is depicted in Figure S5. There is no difference
in the powder pattern, which shows that the majority of crystals
did not undergo any structural change during this synthetic
step. The product of the ball-milling step, KLiSiF6 :Mn4+, exhibits
red luminescence upon excitation with blue light. The sub-
stitution during ball-milling is likely to take place at the surface
of the material. Within KLiSiF6 :Mn4+, Mn4+ could be substituted
onto either, the Si4+ or the Li+ site because of the octahedral
coordination. Although we think that the substitution is more
likely to take place on the Si4+ site, because there is no need for
charge equalization in the surrounding. An emission spectrum
of the polycrystalline sample is shown in Figure 9 (bottom). In
comparison to K2SiF6 :Mn4+, which does not exhibit a zero-
phonon-line, KLiSiF6 :Mn4+ exhibits a zero-phonon-line (ZPL)
and therefore seven emission lines at 598, 609, 613, 622, 631,
635, and 647 nm are noticeable. These belong to the spin and
parity forbidden transitions Eg!A2g and correspond to the
transitions of the vibronic modes ν3(t1u), ν4(t1u), ν6(t2u), ZPL,
ν6(t2u), ν4(t1u), and ν3(t1u), respectively. A ZPL can also be noticed
within Li2SiF6 :Mn4+, but in comparison the emission line with
maximum intensity in KLiSiF6 :Mn4+ (λmax =631 nm) is slightly
red-shifted to the one in Li2SiF6 :Mn4+ (λmax =630 nm).[12] Further
measurements of the substance KLiSiF6 :Mn4+ could not be
conducted due to the fact that the non-doped precursor
material KLiSiF6 was synthesized via a high-pressure/high-
temperature route, which limits the amount of sample available
for the doping step.
The substances KLiSiF6 and CsLiSiF6 were synthesized via a
high-pressure/high-temperature approach. Although both sub-
stances crystallize in the orthorhombic crystal system with
space group Pbcn (no. 60), they exhibit different crystal
structures. KLiSiF6 is isostructural to (NH4)MnFeF6,[16] whereas
CsLiSiF6 forms its own new structure type. Chardi and BLBS
calculations verify the structural model of both substances,
while FT-IR measurements confirm the existence of [SiF6]2À units
within both structures. At first glance, KLiSiF6 and CsLiSiF6 might
wrongly be described as a solid solution of Li2SiF6 and K2SiF6 or
Cs2SiF6 with cation disorder, respectively. Here we present the
crystal structure based on single-crystal diffraction data, show-
ing that KLiSiF6 exhibits a different structure than CsLiSiF6.
Surprisingly, both presented substances do not show the crystal
structure of the heavier boundary phase K2SiF6 or Cs2SiF6,
respectively. KLiSiF6 shows two types of building units, which
are connected by common vertices to create a 3D network.
CsLiSiF6 also shows a different structure, revealing a novel
structure type, which shows two different fundamental building
blocks that are connected to a 3D network. As shown by first
experiments, these units are interesting for partial substitution
with [MnF6]2À units, to yield a red luminescent material, as
already described for some hexafluoridosilicates.[11–12,28]
KLiSiF6 :Mn4+ exhibits a red luminescence with a maximum
emission wavelength of 631 nm. If the materials become easily
accessible, the luminescence behavior can be studied in more
detail.
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Figure 9. Emission spectrum of KLiSiF6 :Mn4+ in comparison with the emission spectra of K2SiF6 :Mn4+ (state of the art phosphor) and K2MnF6 (dopant).
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scope. For the single-crystal analysis, a Bruker D8 Quest diffractom-
Experimental Section
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eter (BRUKER, Billerica, USA) was used. An Incoatec microfocus X-
ray tube (Incoatec, Geesthacht, Germany) with Mo-Kα radiation (λ=
71.07 pm) and a Photon 100 detector enabled the detection of the
intensity data. The data were collected at 203(2) K and 173(2) K for
KLiSiF6 and CsLiSiF6, respectively. A multi-scan absorption correction
was applied to the intensity data, using Sadabs 2014/5.[32]
Synthesis
Single-crystals of KLiSiF6 and CsLiSiF6 were synthesized via a high-
pressure/high-temperature approach. Both syntheses were carried
out using a 1000 t multianvil press (Max Voggenreiter GmbH,
Mainleus, Germany) equipped with a Walker-type module (Max
Voggenreiter GmbH, Mainleus, Germany).
Based on the extinction conditions, the space group Pbcn (no. 60)
was considered for the structure solution (Shelxtl-xt-2014/4) and
found to be correct for the refinement of KLiSiF6. Parameter
refinement (full-matrix least-squares against F2) was carried out
with Shelxl-2013[33–34] (implemented in the WinGX-2013.3[35] suite).
The anisotropic refinement led to values of 0.0255 and 0.0438 for
R1 and wR2 (all data), respectively.
For the synthesis of KLiSiF6, the starting materials Li2SiF6 and K2SiF6
were weighed under argon atmosphere (glove-box, MBraun
Inertgas-System GmbH, Germany) with a ratio of 1:1. The starting
materials were firmly ground together in an agate mortar and
subsequently filled in a platinum capsule (99.95%, Ögussa, Vienna,
Austria). This step was followed by the insertion of the capsule into
a boron-nitride crucible (Henze Boron Nitride Products AG, Lauben,
Germany). The crucible, in turn, was placed into an 18/11 assembly
(further details on the assembly are described in literature[29–31]). The
compression of the assembly was achieved by eight tungsten
carbide cubes (Hawedia, Marklkofen, Germany), which were placed
into the Walker-type module and therefore in the heart of the
1000 t multi-anvil press. Compression of the sample to 5.5 GPa was
carried out within 145 min and followed by the heating program,
during which the sample was kept at 5.5 GPa. The sample was
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For CsLiSiF6, the space group Pbcn (no. 60) was considered for the
structure solution (Shelxtl-xt-2014/4) based on the extinction
conditions. Structure refinement in the same space group was also
found to be correct. Parameter refinement (full-matrix least-squares
against F2) was carried out with Shelxl-2013[33–34] (implemented in
the WinGX-2013.3[35] suite). The anisotropic refinement led to values
of 0.0297 and 0.0452 for R1 and wR2 (all data), respectively.
Deposition Numbers 2031521 (for KLiSiF6) and 2031520 (for
CsLiSiF6) contain the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinformationszen-
trum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/
structures.
°
heated to 750 C within 10 min and held at that temperature for
°
150 min, followed by cooling to 350 C within 180 min. Subse-
quently, the sample was quenched to RT and the decompression to
room pressure was carried out within 430 min. A colourless,
crystalline sample was recovered (Figure S4, left).
In a second step, KLiSiF6 was doped with Mn4+ by high-energy ball
milling with K2MnF6. For this purpose, 30 mg KLiSiF6 and 1.7 mg
K2MnF6 were weighed under argon atmosphere and subsequently
transferred into a zirconia vessel together with 200 mg of milling
balls (Ø=1 mm, ZrO2). The ball-milling process was carried out in a
planetary mill (Pulverisette 7, FRITSCH, Idar-Oberstein, Germany) at
300 rpm for 6×10 min with 15 min breaks in between. A small
amount of lightly yellow colored product was recovered.
EDX-Spectroscopy
Both substances were analysed by energy dispersive X-ray spectro-
scopy (EDX) using a SUPRATM35 scanning electron microscope (SEM,
CARL ZEISS, Oberkochen, Germany, field emission), which was
equipped with a Si/Li EDX detector (OXFORD INSTRUMENTS,
Abingdon, Great Britain, model 7426).
The synthesis of CsLiSiF6 was similar. For this synthesis, Li2SiF6 and
Cs2SiF6 were weighed in with
Vibrational spectroscopy
a ratio of 1:1 under argon
atmosphere. The sample was ground and placed into a platinum
capsule, which subsequently was inserted into a boron-nitride
crucible. This crucible in turn was placed into a 14/8 assembly
(further details on the assembly are described in[29–31]). Again, the
assembly was placed in the middle of eight tungsten carbide cubes,
which were inserted into the Walker-type module. The compression
of the sample to 5.5 GPa was carried out within 120 min. This step
FT-IR (Fourier Transformed InfraRed) spectra of KLiSiF6 and CsLiSiF6
were recorded by a Bruker Alpha-P spectrometer (BRUKER, Billerica,
USA). This spectrometer is equipped with a 2×2 mm diamond ATR-
crystal and a DTGS detector. The software OPUS 7.2 was used for
the handling of the data.
°
Luminescence Spectroscopy
was followed by heating of the sample to 950 C within 10 min. The
temperature was kept for 60 min and subsequently lowered to
The samples were excited by a blue laser diode with a wavelength
of λ=448 nm (THORLABS, Newton, USA). The emission spectra
were recorded by a CCD-Detector (AVA AvaSpec 2048, AVANTES,
Apeldoorn, Netherlands). This single-grain setup was calibrated for
spectral radiance prior to the experiments by a tungsten-halogen
calibration lamp. Handling of the data was carried out with the
software AVA AvaSoft full version 7.
°
400 C within 120 min. Afterwards the sample was quenched to
room temperature and the decompression to room pressure was
carried out within 330 min. A colourless, crystalline sample was
recovered (Figure S4, right).
X-ray analysis
For the analysis of the powder samples, a Stoe Stadi P powder
diffractometer was used. The diffractometer was equipped with a
Ge(111) primary beam monochromator, a Mythen 2 DCS4 detector,
and a molybdenum radiation source Mo-Kα1 (λ=70.93 pm). The
Acknowledgements
We want to express our gratitude to Assoc.-Prof. Dr. Gunter
Heymann for the collection of the single-crystal data and to both,
Assoc.-Prof. Dr. Gunter Heymann and Dr. Klaus Wurst for the help
with the crystal-structure refinement. We also want to thank
Christian Koch for the SEM-EDX measurements.
°
data were recorded within the 2θ range of 2.0–40.4 with a step
°
size of 0.015 .
The samples were covered in perfluoropolyalkylether and single-
crystals were mounted on glass fibers under a polarization micro-
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doi.org/10.1002/ejic.202000867
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Conflict of Interest
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There are no conflicts to declare.
Keywords: Hexafluoridosilicates · Luminescence · Solid-state
synthesis · Structure elucidation
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Manuscript received: September 13, 2020
Revised manuscript received: November 5, 2020
Accepted manuscript online: November 6, 2020
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© 2020 The Authors. European Journal of Inorganic Chemistry published
by Wiley-VCH GmbH