Single crystal growth and crystal structure of HgTeO2FOH-a layered fluorohydroxooxotellurate(IV) stabilized by weak F?OH interactions

Article abstract of DOI:10.1016/j.jallcom.2004.05.033

Light-yellow crystals of HgTeO₂FOH were obtained under hydrothermal conditions by reacting stoichiometric amounts of TeO₂ and HgO in diluted HF, or alternatively, by dissolution of TeO₂ and Hg(NO₃)₂·H₂O in 15 wt.% HF and subsequent slow evaporation of the solution. The crystal structure (space group Pca2₁, (#29), Z= 4, a = 7.8960(7), b = 6.7845(6), c = 6.8641(6) A?, 1044 structure factors, 56 parameters, R[F² >2σ(F²)] = 0.0186, Goof = 1.007) was determined from a single crystal diffractometer data set and is made up from distorted [Hg ^IIO₄F₂(OH)] polyhedra and (Te ^IVO₂F(OH))^2- anions. These main building units form a layered assembly parallel to (0 1 0). Upon heating, HgTeO₂FOH decomposes above 300°C in a two-step mechanism under formation of the mixed-valent α-Hg^II₂Te^IV/VI ₂O₇ and traces of Hg^II₂Te ^VIO₅ in the first step, and of formation of TeO ₂ in the second step.

Full text of DOI:10.1016/j.jallcom.2004.05.033

Journal of Alloys and Compounds 386 (2005) 119–124
Single crystal growth and crystal structure of HgTeO FOH—a layered
2
fluorohydroxooxotellurate(IV) stabilized by weak F · · · OH interactions
∗
Matthias Weil
Institute for Chemical Technologies and Analytics, Division of Structural Chemistry, Vienna University of Technology,
Getreidemarkt 9/164-SC, A-1060 Vienna, Austria
Received 24 March 2004; received in revised form 19 May 2004; accepted 24 May 2004
Abstract
Light-yellow crystals of HgTeO
2
FOH were obtained under hydrothermal conditions by reacting stoichiometric amounts of TeO
2
and HgO
in diluted HF, or alternatively, by dissolution of TeO
2
and Hg(NO O in 15 wt.% HF and subsequent slow evaporation of the solution. The
3
)
2
·H
2
2
crystal structure (space group Pca2
1
(#29), Z = 4, a = 7.8960(7), b = 6.7845(6), c = 6.8641(6) Å, 1044 structure factors, 56 parameters, R[F
2
II
>
2σ(F )] = 0.0186, Goof = 1.007) was determined from a single crystal diffractometer data set and is made up from distorted [Hg O
4
F
2
(OH)]
FOH
in the
IV
2−
polyhedra and (Te O
2
F(OH)) anions. These main building units form a layered assembly parallel to (0 1 0). Upon heating, HgTeO
2
◦
II
IV/VI
II
VI
decomposes above 300 C in a two-step mechanism under formation of the mixed-valent α-Hg
first step, and of formation of TeO in the second step.
2004 Elsevier B.V. All rights reserved.
2
Te
2 7 2 5
O and traces of Hg Te O
2
©
Keywords: Chemical synthesis; Crystal growth; Crystal structure and symmetry; X-ray diffraction
1
. Introduction
On this account it seemed appropriate to prepare new
non-centrosymmetric candidates in the system Hg–Te –O
IV
Inorganic materials with non-linear optical properties
and to determine their crystal structures. In a recent project,
IV/VI
have a growing impact in science and technology. Ap-
plication of these materials for frequency conversion, as
electro-optical and photonic devices, optical switches etc.
make the specific syntheses and crystal growth of new com-
pounds challenging, because they have to meet very special
criteria (thermal stability, transparency in the spectral range,
excellent optical quality etc.) and, most importantly, their
crystal structures have to be non-centrosymmetric. Unfortu-
nately, only about 15% of the known inorganic compounds
crystallize in non-centrosymmetric structure types, but in
certain structural families the absence of a centre of sym-
metry is more frequently observed. These families include
two polymorphs of the mixed-valent Te
compound
Hg2Te2O7 have been crystallographically full characterized
[1], and one of the polymorphs (the β-modification) is in-
deed non-centrosymmetric. Another approach for the prepa-
ration of related phases is to (partly) replace oxygen with
IV
fluorine and to investigate the system Hg–Te –O–F–(H)
for which no representatives are known up to now. During
these examinations the title compound HgTeO2FOH was
obtained, whose preparation, crystal structure and thermal
behaviour is reported in this article.
−
compounds with small asymmetrical ligands like SCN and
2. Experimental
−
OCN , or with cations having lone-pair electrons such as
Tl , Sn , Pb , Sb and Bi , or oxoanions like Se O3
I
II
II
III
III
IV
2−
,
2.1. Synthesis
IV
2−
V
−
Te O3
and I O3 , respectively, which can cause the
asymmetry in the structure.
Single crystals of HgTeO2FOH used for structure analy-
sis were obtained under hydrothermal conditions. For that
purpose, 0.12 g (0.75 mmol) TeO2 (Merck, 99.999%) and
0.163 g (0.75 mmol) HgO (Merck, p.A.) were charged in a
Teflon inlay with 10 ml capacity which was two-thirds filled
∗
Tel.: +43-1-58801-17122; fax: +43-1-58801-17199.
E-mail address: mweil@mail.zserv.tuwien.ac.at (M. Weil).
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.05.033

1
20
M. Weil / Journal of Alloys and Compounds 386 (2005) 119–124
with a 5 wt.% HF solution and then sealed in a steel auto-
clave. This device was heated up to 220 C within 12 h, left
2.2. Data collection and refinement
◦
at that temperature for five days and cooled down to room
temperature in the course of 48 h. Besides canary-yellow
Single crystal diffraction intensities from a hydrother-
mally grown crystal were collected using the ω-scan tech-
◦
Hg3TeO crystals [2], colourless to light yellow plates of
nique with 0.3 rotation width and 30 s exposure time per
6
the title compound had formed. Single phase HgTeO2FOH
was prepared by dissolution of 0.4 g (2.5 mmol) TeO2 and
frame on a SMART three-circle diffractometer (Siemens)
equipped with a CCD camera. Three independent sets of 606
frames were measured thus scanning the whole reciprocal
sphere. The measured intensities were corrected for Lorentz
and polarization effects and an absorption correction was
applied using the program HABITUS [3] by minimizing the
0
.86 g (2.5 mmol) Hg(NO3)2·H2O (Fluka, p.A.) in 50 ml of
a 15 wt.% HF solution (Merck, p.A.). This solution was
then allowed to evaporate slowly at room temperature. Af-
ter five days crystal growth started, and after the period of
two more weeks yellow crystals with a plate-like habit and
an edge-length of up to 0.3 mm had formed in the clear so-
lution. The crystals were filtered off, washed with ethanol
and acetone and dried in a desiccator over CaCl2 for two
days. The obtained crystals are stable under normal labo-
ratory conditions, but darken to a yellow-brownish colour
in daylight, which had no influence on the diffraction in-
tensities. The presence of fluorine and OH was determined
qualitatively by wet chemical methods and IR spectroscopic
investigations, respectively.
internal R value and optimizing the crystal shape. The so
i
derived habit was the basis of the numerical absorption cor-
rection. The crystal structure was solved by direct methods
and refined with the SHELX97 program package [4]. In the
final least-squares cycles all atoms were refined anisotropi-
cally. The H atom was not included in the refinement because
no reliable atom position could be found in the difference
map. No higher symmetry was suggested by the PLATON
program [5], and the Flack parameter [6] clearly indicates
the absence of any centre of symmetry.
Thermoanalytical analyses were performed in an open
system under a flowing N2 atmosphere on a Mettler-Toledo
Further details of data collection and structure refine-
ment are summarized in Table 1. Final atomic coordinates,
equivalent isotropic displacement parameters, anisotropic
displacement parameters and selected interatomic dis-
tances and angles are listed in Tables 2–4. The calculated
◦
◦
−1
TG50 (35-700 C, heating rate 5 C min , corundum cru-
◦
cibles) and a DSC 25 system (35–550 C, heating rate
5
◦
−1
C min , aluminium capsules).
Table 1
Crystallographic data and details of data collection, structure solution and refinement
Diffractometer
SMART (Siemens)
Radiation; wavelength λ (Å)
Mo K ␣¯ ; 0.71073
22(2)
◦
Temperature ( C)
Crystal dimensions (mm )
3
0.11 × 0.07 × 0.05
Crystal description
Space group
Light-yellow plate
Pca21 (no. 29)
Formula units
Lattice parameters (Å)
Z = 4
a = 7.8960(7)
b = 6.7845(6)
c = 6.8641(6)
Volume (Å3)
V = 367.71(6)
Formula weight (g mol 1)
−
396.20
−
1
µ (mm
)
49.506
7.157
X-ray density (g cm 3)
−
Range θmin – θmax
Range h; k; l
Structure solution and refinement
Measured reflections
3.96 → 30.47
−11 → 11; -9 → 9; −9 → 9
SHELX97 [4]
4132
Independent reflections
1044
Observed reflections (I >2σ(I))
Ri
978
0.0313
Absorption correction
HABITUS [3]
Coefficients of Transmission Tmin; Tmax
Flack parameter
0.0470; 0.2007
0.049(9)
Number of parameters
56
Extinction coefficient (SHELXL97)
Difference electron density (e Å 3) with distance (Å) to atom
R(F >2σ(F ); wR(F all)
Goof
0.0036(3)
−
ꢀρmax = 0.97 (0.45; OH); ꢀρmin = −1.04 (1.33; F)
2
2
2
0.0186; 0.0416
1.007
CSD-number
413078

M. Weil / Journal of Alloys and Compounds 386 (2005) 119–124
121
Table 2
2
Atomic coordinates and equivalent isotropic displacement parameters (Å )
Atom
Wyckoff position
x
y
z
Ueqa
Hg
Te
O1
O2
F
4 a
4 a
4 a
4 a
4 a
4 a
0.16948(3)
0.07746(5)
0.1594(7)
0.6480(6)
0.0143(7)
0.3818(6)
0.59541(4)
0.15284(6)
0.4111(7)
0.2102(9)
0.2313(8)
0.1497(6)
0.23399(8)
0.02773(7)
−0.0003(10)
0.4633(8)
0.2969(6)
0.2327(9)
0.01632(9)
0.01030(10)
0.0194(12)
0.0163(11)
0.0253(11)
0.0119(9)
OH
a
∗ ∗
i j
Ueq = (1/3)ꢁi ꢁj Uija a ai·aj.
Table 3
Anisotropic displacement parameters (Å )
2
Atom
U11
U22
U33
U23
U13
U12
Hg
Te
O1
O2
F
0.01919(14)
0.01061(18)
0.027(3)
0.008(2)
0.032(3)
0.01453(14)
0.00825(19)
0.010(3)
0.018(3)
0.025(3)
0.01525(13)
0.01206(18)
0.021(3)
0.022(3)
0.019(2)
−0.00400(13)
0.00118(14)
0.00067(9)
0.00076(15)
−0.0088(18)
0.001(2)
0.00005(19)
−0.001(2)
0.012(2)
−0.00006(17)
0.000(2)
−0.001(2)
0.009(2)
−0.002(2)
−0.0072(19)
−0.012(2)
OH
0.011(2)
0.016(2)
0.0093(19)
−0.001(3)
0.0040(16)
Table 4
Table 5
Bond valence analysis (v.u.)a
◦
Selected distances (Å) and angles ( )
Hg–O1
2.039(6)
2.061(5)
2.592(6)
2.696(6)
2.791(5)
2.855(5)
2.997(6)
3.174(6)
3.2840(6)
Te–O2#6
Te–O1
Te–F
Te–OH#6
Te–OH
Te–OH#7
1.875(5)
1.878(5)
1.987(4)
2.051(6)
2.785(5)
2.929(5)
Hg
Te
ꢁ
Expected
2
Hg–O2#1
Hg–O1#2
Hg–O2#3
Hg–F
Hg–OH#1
Hg–F#4
Hg–O1#5
Hg–Te#5
O1–Hg–O2#1
O1
O2
OH
0.834
1.307
2.328
0.187
0.786
0.141
1.317
2.244
2
0.092
0.819
0.911
(1.099)
2 (including the H atom)
F–OH#7
2.823(6)
(
(
0.112)
0.076)
172.7(2)
O2#6–Te–O1
O2#6–Te–F
O1–Te–F
O2#6–Te–OH#6
O1–Te–OH#6
F–Te–OH#6
96.3(2)
85.7(2)
86.1(2)
85.3(2)
81.6(2)
163.9(2)
F
0.066
0.038
0.940
1.044
1
ꢁ
2.144
2
4.383
(4.571)
Expected
4
Symmetry transformations used to generate equivalent atoms – #1: x −
a
Additional contributions resulting from weak bonding interactions
are given in brackets.
1
#
1
/2, −y + 1, z; #2: −x + 1/2, y, z + 1/2; #3: −x + 1, −y + 1, z − 1/2;
4: x + 1/2, −y + 1, z; #5: −x, −y + 1, z + 1/2; #6: −x + 1/2, y, z −
/2; #7: x − 1/2, −y, z.
of this formula type as well as the first member of this
structural family with mercury as a cation reported so far.
Other crystallographically well-characterized representa-
tives include H2Te2O3F4 [10,11], (NH4)TeO(OH)F2 [11],
MTe(OH)F4 (M = K, Rb) [12], as well as the double-salts
Na2Te(OH)F3(SO4) and (NH4)2Te(OH)F3(SO4) [13]. Fur-
ther known structures are the related fluorooxotellurate(IV)
KTeOF3 [11], and the tellurium(IV) oxidefluorides Te2O3F2
bond-valence sums [7], using the parameters provided by
Brese and O’Keeffe [8], are given in Table 5. Additional
crystallographic information on HgTeO2FOH is available
from the Fachinformationszentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen, Germany, email: crysdata@fiz-
karlsruhe.de, by quoting the literature citation, the name of
the author and the depository number listed at the end of
Table 1. Drawing of structural details were produced using
the program ATOMS [9].
[14] and TeOF2 [15].
The crystal structure of the title compound consists of
II
IV
2−
distorted [Hg O4F2OH] polyhedra and (Te O2FOH)
anions as the main building units. Like in many other
mercury(II) compounds, the Hg atom shows an explicit
3
. Results and discussion
two-coordination and is surrounded by two tightly bonded
¯
HgTeO2FOH crystallizes in a non-centrosymmetric
structure and is the first fluorohydroxooxotellurate(IV)
oxygen atoms with almost collinear bonds (d(Hg–O)
= 2.05 Å, ∠(O–Hg–O) = 172.7(2) ). Two remote oxygen
short
◦

1
22
M. Weil / Journal of Alloys and Compounds 386 (2005) 119–124
with the lone-pair E located at one equatorial corner of the
polyhedron. This stereochemical behaviour is quite charac-
IV
teristic for the Te atom and has been described in detail
for various tellurates(IV) and related compounds [18,19].
Both oxidic atoms O1 and O2 show the shortest Te-O dis-
tances of ca. 1.88 Å (and also the shortest Hg–O distances).
The fluorine atom and the hydroxy group are located at
considerably longer distances of 1.987(4) and 2.051(6) Å,
respectively. In all fluorinated oxo- or hydroxotellurates(IV)
mentioned above, there is no general tendency with respect
to the atom type X (O, OH, F) and the Te–X bond length,
since in some of these compounds the shortest bond is a
Te–O bond, a Te–F bond or even a Te–OH bond. In the
corresponding structures this scatter is caused by the dif-
ferent bonding interactions of these atoms to the additional
coordination partners (covalent bond, intra- or intermolec-
ular hydrogen bond etc.). The bond-valence analysis of
HgTeO2FOH (Table 5) clearly demonstrates the presence of
F and OH in the structure. Although a reliable assignment
of these atoms is hampered by the very similar scattering
factors of the F and O atoms, and to compound matters
that on the basis of the present X-ray data no location of
the proton was possible, refinements of the occupancies
support the present tentative model. The reliability factors
were somewhat higher when F and O were interchanged
(R = 0.0196, wR = 0.0440) or refined using a model of
statistical disorder (R = 0.0214, wR = 0.0459).
II
Each [Hg O4F2OH] polyhedron shares an edge (O1–O2)
and (O2’-O1’) with a long and a short Hg–O distance to con-
stitute infinite zig–zag chains extending parallel to the [0 0 1]
direction. Adjacent chains are connected by interstitial Te
atoms parallel to [1 0 0] (Fig. 2) which results in a layered as-
Fig. 1. Plot of the coordination around Hg (a) and Te (b), with displacement
ellipsoids drawn at the 90% probability level. Short Hg–O bonds are
given as solid lines, longer Hg–O bonds as well as Hg-F bonds and weak
Te–OH bonds as open lines; the symmetry codes refer to Table 4.
IV
sembly parallel to (0 1 0). The lone-pairs E of the Te atoms
are stereochemically active and are arranged oppositely to
each other. Like in many other tellurites(IV) [20] or Te(IV)
fluorinated compounds [21], the lone-pairs point towards
atoms are located at considerably longer distances of about
2
.65 Å. If an empirically derived bonding interaction be-
tween mercury and the corresponding ligands is consid-
ered for distances <3 Å [16], the coordination figure is
augmented by a hydroxy group and two fluorine atoms
at distances >2.8 Å (Fig. 1a). The observed bond length
distribution is in agreement with other fluorinated oxocom-
pounds of mercury where the Hg-O bonds are considerably
shorter than the Hg–F bonds [17]. The polyhedron around
mercury as a whole is extremely distorted and difficult to
derive in terms of a simple geometric description like for
other (undistorted) hepta-coordinate atoms (e.g. a pentago-
nal bipyramid or a monocapped prism). The tellurium atom
is surrounded by six coordination partners (two oxygen
atoms, one fluorine atom and three hydroxy groups) with
bond lengths ranging from 1.88 to 3.0 Å in a highly dis-
torted octahedron (Fig. 1b). As a matter of fact, only four
of these atoms with distances < 2.10 Å contribute signifi-
cantly to the electrostatic valence around the tellurium atom
Fig. 2. Projection of the crystal structure along [0 1 0]; the unit cell is
outlined. The TeO2FOH anion is plotted in the polyhedral representation;
interpolyhedral F · · · OH interactions are given as dotted lines.
(Table 5). Then the corresponding coordination figure can
be described as a truncated trigonal bipyramid TeO2F(OH)E

M. Weil / Journal of Alloys and Compounds 386 (2005) 119–124
123
Although held together only by weak F · · · OH interac-
◦
tions, the crystals are comparatively stable up to ca. 300 C.
The thermal decomposition proceeds in a two-step mech-
anism (Fig. 4) and might be formulated by the idealized
reaction Eqs. (1) and (2), showing a good agreement be-
tween observed and theoretical mass losses. The first step
involves a redox-reaction under formation of α-Hg2Te2O7
[
1] which is accompanied by an endothermal DSC ef-
◦
fect at ca. 455 C. Subsequent XRPD analysis of material
heated up to 480 C revealed the mixed-valent Te
compound as the main phase besides traces of Hg 2Te O
◦
IV/VI
II
VI
5
[2]. TeO2 was not detected by this method, indicating
that this phase is X-ray amorphous at that temperature. In
the second step the remaining mercury phases decompose
◦
completely above 620 C under formation of paratellurite,
Fig. 3. Projection of the crystal structure along [1 0 0]; the unit cell is
outlined. The TeO2FOH anion is plotted in the polyhedral representation;
interpolyhedral F · · · OH interactions are given as dotted lines.
α-TeO2, which was determined by XRPD as the only phase
present.
First step:
3
HgTeO FOHs → ␣-Hg Te2O7 + TeO2s + Hg ↑
the cavities of the structure, in this case to the inter-sheet
spaces. Two neighbouring layers are connected perpendic-
ular to each other only by weak F · · · OH interactions at a
distance of 2.82 Å (Fig. 3), which explains consistently the
plate-like habit and the easy cleavage of the crystals.
2
2
s
g
+
3HFg ↑
(1)
Mass loss 22.5%, theory 21.9%
Second step:
An empirically derived correlation between the OH
stretching frequencies and O · · · O distances has been re-
ported for various OH bearing minerals and synthetic
compounds [22]. On the simplified assumption that this
correlation also holds for OH stretching frequencies and
the corresponding F · · · O interactions, the distance d(F–O)
␣
-Hg Te2O7s + TeO2s → 3TeO2s + 2Hg ↑ +2Hg ↑
2
g
g
1
+
1 O2g ↑
(2)
2
Mass loss referring to the second step: 49.1%, theory
51.6%
Overall reaction:
=
2.823(6) Å correlates with a calculated stretching fre-
−
1
quency of ν(OH)calc = 3430 cm which is in very good
1
2
HgTeO FOHs → TeO2s + Hg ↑ +HFg ↑ + O2g ↑
(3)
−
1
2
g
agreement with the observed value of ν(OH)obs = 3440 cm
powder specimen in a KBr matrix).
(
Mass loss 62.7%, theory 59.7%
Fig. 4. TG and DSC curves of the thermal decomposition of HgTeO2FOH.

1
24
M. Weil / Journal of Alloys and Compounds 386 (2005) 119–124
[3] W. Herrendorf, HABITUS, Program for Optimization of the Crys-
4
. Conclusion
tal Shape for the Numerical Absorption Correction, Universities of
Karlsruhe and Gießen, Germany, 1993–1997.
HgTeO2FOH has been synthesized and its structure de-
[4] G.M. Sheldrick, SHELX97, Programs for Crystal Structure Solution
termined by means of single crystal X-ray diffraction data.
and Refinement, University of Göttingen, Germany, 1997.
[5] A.K Spek, PLATON, a multipurpose crystallographic tool, Utrecht
University, Utrecht, The Netherlands, 2001.
◦
Upon heating above 300 C, the compound decomposes
in a two-step mechanism to form paratellurite, TeO2, with
α-Hg2Te2O7 (main phase) and Hg2TeO (traces) as inter-
mediate products.
[
[
6] H.D. Flack, G. Bernardinelli, Acta Crystallogr. A55 (1999) 908–915.
7] I. D. Brown, The Chemical Bond in Inorganic Chemistry, Oxford
University Press, 2002.
5
II
The structure is made up from distorted [Hg O4F2OH]
[8] N.E. Brese, M. O’Keeffe, Acta Cryst. B47 (1991) 192–197.
IV
2−
polyhedra and (Te O2FOH) anions as the main building
units which form a layered assembly parallel to (0 1 0). Ad-
jacent layers are connected only via F · · · OH interactions
at a distance of 2.823(6) Å which is in agreement with IR
spectroscopic data.
[9] E. Dowty, ATOMS for Windows, Version 5.1, Shape Software, 521
Hidden Valley Road, Kingsport, TN 37663, USA, 2000.
[10] J.C. Jumas, M. Maurin, E. Philippot, J. Fluorine Chem. 8 (1976)
329–340.
[11] Yu.E. Gorbunova, S.A. Linde, V.I. Pakhomov, Yu.V. Kokunov, Yu.A.
Buslaev, Koord. Khim. 8 (1982) 389–395.
Neutron diffraction experiments planned for the future are
considered to yield a reliable location of the proton position
and to achieve a more detailed understanding of the inter-
and intrapolyhedral hydrogen bonding.
[12] Yu.E. Gorbunova, S.A. Linde, V.I. Pakhomov, V.A. Sarin, N.N. By-
danov, Yu.V. Kokunov, M.P. Gustyakova, Yu.A. Buslaev, Koord.
Khim. 11 (1985) 270–276.
[
[
[
13] Yu.E. Gorbunova, S.A. Linde, V.I. Pakhomov, Yu.V. Kokunov, M.P.
Gustyakova, Yu.A. Buslaev, Koord. Khim. 9 (1983) 524–535.
14] A. Ider, J.P. Laval, B. Frit, J. Carre, J.P. Bastide, J. Solid State Chem.
1
23 (1996) 68–72.
15] L. Guillet, A. Ider, J.P. Laval, B. Frit, J. Fluorine Chem. 93 (1999)
3–38.
[16] D. Grdeni
Acknowledgements
3
c´ , Quart. Rev. Chem. Soc. 19 (1965) 303–328.
17] M. Weil, Acta Cryst. C58 (2002) i37–i39.
I thank Dr. E. Halwax for valuable comments to improve
the manuscript.
[
[
[
18] J. Zemann, Monatsh. Chem. 102 (1971) 1209–1216.
19] J. Galy, G. Meunier, S. Andersson, A. Åström, J. Solid State Chem.
1
3 (1975) 142–159.
20] V.A. Dolgikh, Russ. J. Inorg. Chem. (Eng. Trans.) 36 (1991) 1117–
129.
References
[
1
[
1] M. Weil, Z. Kristallogr. 218 (2003) 691–698.
[21] L. Guillet, J.P. Laval, B. Frit, J. Fluorine Chem. 107 (2001) 223–228.
[22] E. Libowitzky, Monatsh. Chem. 130 (1999) 1047–1059.
[2] M. Weil, Z. Anorg. Allg. Chem. 629 (2003) 653–657.