Synthesis, crystal structure and properties of K2Ta 2S10: A novel ternary tantalum polysulfide with TaS 8 polyhedra forming infinite anionic chains

Article abstract of DOI:10.1016/j.jssc.2005.02.010

The new ternary alkali tantalum polysulfide K₂Ta ₂S₁₀ has been synthesized by reacting TaS₂ with an in situ formed melt of K₂S₃ and S at 773 K. The compound crystallizes with four formula units in the monoclinic space group P2₁/n (No. 14) with lattice parameters of a=14.9989(13)A,b=6. 4183(4)A,c=15.1365(13)A,β=117.629(9)°. The structure contains two different zigzag chain anions [TaS₅]^-, running parallel to the crystallographic b-axis separated by potassium cations. The two crystallographically independent tantalum atoms are in a distorted bi-capped trigonal prismatic environment of eight sulfur atoms which was never observed before. The TaS₈ polyhedra share three S atoms on each side to form the anionic chains. The compound was characterized with FIR and Raman spectroscopy.

Full text of DOI:10.1016/j.jssc.2005.02.010

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Journal of Solid State Chemistry 178 (2005) 1569–1574
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Synthesis, crystal structure and properties of K2Ta2S10: A novel
ternary tantalum polysulfide with TaS8 polyhedra forming
infinite anionicchains
Ã
Yuandong Wu, Christian Nather, Wolfgang Bensch
Institut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t Kiel, Arbeitskreis Bensch, Olshausenstr. 40 (Otto-Hahn-Platz 7),
D-24108 Kiel, Germany
Received 17 November 2004; received in revised form 17 November 2004; accepted 13 February 2005
Abstract
The new ternary alkali tantalum polysulfide K2Ta2S10 has been synthesized by reacting TaS2 with an in situ formed melt of K2S3
and S at 773 K. The compound crystallizes with four formula units in the monoclinic space group P21/n (No. 14) with lattice
(
(
(
parameters of a ¼ 14:9989ð13Þ A; b ¼ 6:4183ð4Þ A; c ¼ 15:1365ð13Þ A; b ¼ 117:629ð9Þ1: The structure contains two different zigzag
chain anions [TaS5]ꢀ, running parallel to the crystallographic b-axis separated by potassium cations. The two crystallographically
independent tantalum atoms are in a distorted bi-capped trigonal prismatic environment of eight sulfur atoms which was never
observed before. The TaS8 polyhedra share three S atoms on each side to form the anionic chains. The compound was characterized
with FIR and Raman spectroscopy.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Ternary tantalum sulfide; Crystal structure; IR; Raman spectrum
1. Introduction
examples include discrete tetrahedral TaQ4 units like in
A3TaQ4 (A ¼ K, Rb, Cs, Q ¼ S, Se) [6–8], Ta2S11
groups like in A4Ta2S11 (A ¼ K, Rb, Cs) [13–15],
complex Ta4S22 units like in A6Ta4S22 (A ¼ K, Rb,
Cs) [16,18,19], and Ta4S25 like in Rb6Ta4S25 units [20].
The Ta2S11 unit is composed of two face sharing TaS7
The so-called reactive molten flux method [1–3]
employing alkali metal polychalcogenides is a powerful
tool to synthesize novel compounds at temperatures
between 523 and 773 K. The products are often obtained
as well developed single crystals that enable unambig-
uous characterization by single crystal X-ray diffraction.
pentagonal bipyramids and is
a building block
which may be interconnected through S22ꢀ and S52ꢀ
polyanions yielding the Ta4S22 and Ta4S25 unit.
Due to the relatively low reaction temperature com-
2ꢀ
pounds containing large Qx
anions are accessible
Anionichcains are observed in ATaQ
Q ¼ S, Se, Te) [21,23] and in K12Ta6Se35 [23]. The
(A ¼ K, Cs,
3
which cannot be obtained by ‘‘classical’’ high-tempera-
ture syntheses. Recently, a large number of ternary
group 5 chalcogenide compounds with a variety of metal
cations including alkali metals has been reported.
Depending on the reaction conditions and size of the
cations, structures with discrete molecular anions [4–20]
or chains [21–24] are formed. In the case of tantalum,
ꢀ
anionic[TaQ
]
chains are composed of face-sharing
3
TaS6 octahedra in which the Ta5+ ions are displaced
from the centers of the octahedra along the direction of
the chains in a pair-wise fashion. In K12Ta6Se35 the
anionichcains are arranged in layers parallel to the
(010)-plane and successive planes are rotated by 331
against each other. The Ta2Se11 units are interconnected
by Se22ꢀ and Se32ꢀ polyanions forming infinite
Ã
Corresponding author. Fax: +49 4341 880 1520.
E-mail address: wbensch@ac.uni-kiel.de (W. Bensch).
N[Ta6Se35]12ꢀ chains.
1
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.02.010

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In this paper, we report on the synthesis and
well as some refinement results are summarized in
Table 1, atomicocordinates and equivalent isotropic
displacement parameters are given in Table 2. Selected
bond distances are listed in Table 3.
Further details of the crystal structure investigation
can be obtained from the Fachinformationszentrum
Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
(fax: (49) 7247 808 666; E-mail: crysdata@fiz-karlsruhe.de)
on quoting the depository number CSD-414535.
characterization of the new ternary tantalum sulfide
compound K2Ta2S10. To the best of our knowledge this
compound has not been reported until now. It is the first
1
ternary tantalum polysulfide exhibiting N[TaS5]ꢀ anio-
nicchains built up by [Ta(S 2)2(S)4] groups sharing faces
through S2 and S22 anions.
2. Experimental
2.3. Optical property measurements
2.1. Synthesis
A Far-IR spectrum was recorded between 80 and
550 cmꢀ1 (resolution ¼ 2 c mꢀ1) on an ISF-66 device
(Bruker) with K2Ta2S10 pressed in polyethylen pellets.
The FT-Raman spectrum was measured on an ISF-66
spectrometer (Bruker) with an additional FRA 106
Raman modul. A Nd/YAG laser was used as source for
excitation (l ¼ 1064 nm). K2Ta2S10 was ground and
The compound K2Ta2S10 was prepared by reacting a
mixture of K2S3 (0.0927 g, 0.53 mmol), TaS2 (0.1734 g,
0.75 mmol), and S powder (0.0344 g, 1.06 mmol) in a
3:4:6 molar ratio. K2S3 was synthesized by the reaction
of stoichiometric amounts of the elements (K, 499%,
Chempur; S, 99.99%, Heraeus) in liquid ammonia under
argon atmosphere. The starting material TaS2 was
prepared by heating stoichiometric amounts of the
elements (Ta, 99.97%, Fluka) at 1123 K in an evacuated
silica tube. The starting material was thoroughly mixed
in a N2-filled glove box and loaded into a Pyrex glass
ampoule. After evacuation to 10ꢀ3 mbar the ampoule
was flame-sealed and placed in a computer-controlled
furnace. The ampoule was heated to 773 K at a rate of
30 K hꢀ1, kept at this temperature for 6 days, followed
by a cooling to 373 K with 2 K hꢀ1 and a rapid cooling
to room temperature. The solid products were washed
with DMF and acetone to remove residual polysulfide
flux. The reaction product consists of a mixture of dark
red needles of K2Ta2S10 (yield: ꢁ60% based on Ta) and
orange-yellow polyhedra (yield: ꢁ40%), which were
identified as K6Ta4S22 by single crystal X-ray diffrac-
tion. The new polysulfide K2Ta2S10 is stable in dry air
for several weeks. The X-ray powder pattern of the dark
red needles could successfully be refined on the basis of
the lattice parameters of K2Ta2S10. An EDX analysis
indicated the presence of all three elements (K, Ta, S) in
an approximate atomicratio of 1:1:5.3 which is in good
agreement with the composition derived from the
crystallographic structure determination.
Table 1
Technical details of data acquisition and some refinement results for
K2Ta2S10
Empirical formula
Formula weight/g/mol
Temperature
Wavelength/A
Crystal system
Space group
a/A
K2Ta2S10
760.80
180 K
0.71073
Monoclinic
P21/n
14.9989(13)
6.4183(4)
b/A
c/A
15.1365(13)
117.629(9)
1290.99(18)
b/1
V/A3
Z
4
3.914
Calculated density/g cmꢀ3
Crystal color
m/mmꢀ1
Dark red
19.154
F(0 0 0)
Crystal size/mm3
1376
0.13 ꢂ 0.11 ꢂ 0.095
y range
Index range
2.61–27.931
ꢀ19php19
ꢀ7pkp8
ꢀ19plp19
Reflections collected
Independent reflections
Rint
12035
3035
0.0257
97.9%
2.2. Crystallography
Completeness to y ¼ 27.931
Refinement method
Min./max. transm.
Full-matrix least square on F2
0.095/0.162
Data collection was performed on a STOE Imaging
Plate Diffraction System (IPDS) (MoKa radiation;
l ¼ 0.71073 A) equipped with a low-temperature device
from Oxford Cryosystems at 180 K. The raw intensities
were treated in the usual way applying a Lorentz,
polarization and a numerical absorption correction.
Structure solution was performed with SHELXS-97 [25].
Refinement was done against F2 using SHELXL-97 [26].
All atoms were refined with anisotropicdisplaecment
parameters. Technical details of the data acquisition as
Refl. with Fo44s(Fo)
Number of parameters
Goodness-of-fit on F2
Final R indices (Fo44s(Fo))a,b
R indices (all data)a,b
Extinction coefficient
Largest diff. peak and hole/e Aꢀ3
2796
128
1.033
R1 ¼ 0.0220, wR2 ¼ 0.0550
R1 ¼ 0.0247, wR2 ¼ 0.0561
0.00123(9)
1.626/ꢀ2.426
aR1 ¼ SJFo|–|FcJ/S|Fo|.
bwR2 ¼ [Sw(Fo2–Fc2)2/Sw(Fo2)2]1/2, w ¼ 1/[s(F2o)+(aP)2+bP], where
P ¼ (max (F2o,0)+2 ꢃ F2c)/3.

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Table 2
Atomic coordinates and equivalent isotropic displacement parameters
Ueq (A2 ꢂ 103) for K2Ta2S10
3. Results and discussions
3.1. Crystal structure
x
y
z
Ueq
The new tantalum polysulfide K2Ta2S10 crystallizes in
the monoclinic space group P21/n (conventional setting:
P21/c) with two unique K atoms, two crystallographi-
cally independent Ta atoms and ten crystallographically
independent S atoms all of them located in general
positions. In the structure of K2Ta2S10 two different
Ta(1)
Ta(2)
K(1)
K(2)
S(1)
S(2)
S(3)
S(4)
S(5)
S(6)
S(7)
S(8)
S(9)
S(10)
0.2436(1)
0.2047(1)
0.4360(1)
0.0850(1)
0.2395(1)
0.1201(1)
0.3339(1)
0.3781(1)
0.1232(1)
0.2454(1)
0.3103(1)
0.0514(1)
0.1082(1)
0.0997(1)
0.5803(1)
0.4506(1)
0.3538(2)
0.2081(1)
ꢀ0.2563(1)
0.0857(1)
6(1)
6(1)
18(1)
19(1)
9(1)
ꢀ0.1408(2)
0.2427(2)
0.8648(2)
0.6673(2)
0.2883(2)
0.3534(2)
0.1780(2)
0.7405(2)
0.6098(2)
0.4146(2)
0.3012(2)
ꢀ0.0680(1)
0.1251(1)
0.1637(1)
9(1)
9(1)
ꢀ0.1206(1)
ꢀ0.2344(1)
0.2525(1)
infinite N[TaS5]ꢀ chains running parallel to [010] are
1
10(1)
9(1)
9(1)
separated by the K+ cations (Fig. 1). The shortest inter-
chain S–S distance of 4.08 A is much larger than the sum
of van der Waals radii of S (3.6 A). Each chain is
surrounded by six other chains in a pseudo-hexagonal
fashion. The two unique Ta atoms are in an eightfold
coordination of S atoms. The resulting polyhedron may
be described as a strong distorted bi-capped trigonal
prism. Two rectangular faces are capped by S atoms of
the S22ꢀ anions (Fig. 2). We note that such a coordina-
tion environment was never observed before for Ta
atoms. The Ta–S distances range from 2.463(1) to
2.639(1) A with an average /Ta–SS of 2.527(1) A
for Ta(1) and from 2.455(1) to 2.654(1) A (average
/Ta–SS: 2.526(1) A) for Ta(2) (Table 3). These
distances are in good agreement with the values reported
for other potassium tantalum sulfides [13,14,16] and the
sum of ionicradii of Ta and S as well [27].
ꢀ0.1249(1)
0.1020(1)
10(1)
11(1)
11(1)
12(1)
ꢀ0.3937(1)
0.0530(1)
ꢀ0.1903(1)
Estimated standards deviations are given in parentheses. The Ueq is
defined as one third of the trace of the orthogonalized Uij tensors.
Table 3
Selected bond distances (A) for K2Ta2S10
Ta(1)–S(1)
Ta(1)–S(2)
Ta(1)–S(5)
Ta(1)–S(7)
Ta(2)–S(3)
Ta(2)–S(4)
Ta(2)–S(6)
Ta(2)–S(8)
S(1)–S(9)
2.4906(11)
2.4628(11)
2.5130(11)
2.4753(11)
2.4680(11)
2.4680(11)
2.5089(11)
2.5165(11)
2.0717(15)
2.0818(15)
3.3443(15)
3.4219(16)
3.3400(15)
3.3830(15)
3.2851(16)
3.2506(15)
3.3153(15)
3.5545(15)
3.3202(16)
3.4504(16)
Ta(1)–S(1)
Ta(1)–S(2)
Ta(1)–S(5)
Ta(1)–S(9)
Ta(2)–S(3)
Ta(2)–S(4)
Ta(2)–S(6)
Ta(2)–S(10)
S(4)–S(8)
2.6327(11)
2.4853(11)
2.6385(11)
2.5191(11)
2.4779(11)
2.6389(11)
2.6541(11)
2.4545(11)
2.0707(15)
2.0911(15)
3.2425(15)
3.5069(16)
3.3828(16)
3.1947(15)
3.7001(16)
3.2959(15)
3.3646(15)
3.2495(15)
3.4857(16)
The TaS8 polyhedra share three S atoms on each side
to form the one-dimensional chains, i.e. two edges of
neighbored TaS8 prisms and two S atoms which belong
to one of the S22ꢀ anions. All S22ꢀ ions are bound to the
Ta atoms in a m2-Z1Z2 mode whereas the S atoms
connect the Ta atoms in a m2-bridging mode (Fig. 3).
The description of the coordination mode of the
compound is K2[Ta(1)(m2-Z1Z2-S2)2(m2–S) Ta(2)(m2-
Z1Z2-S2)2(m2–S)] and the assignment of the formal
S(5)–S(7)
S(6)–S(10)
K(1)–S(3)
K(1)–S(4)
K(1)–S(8)
K(1)–S(7)
K(2)–S(1)
K(2)–S(2)
K(2)–S(5)
K(2)–S(7)
K(2)–S(9)
K(1)–S(1)
K(1)–S(3)
K(1)–S(6)
K(1)–S(8)
K(1)–S(10)
K(2)–S(2)
K(2)–S(4)
K(2)–S(6)
K(2)–S(9)
K(2)–S(10)
valences is K+, Ta5+, S22ꢀ and S2ꢀ
.
The chains propagate in a zigzag fashion as is
evidenced by angle Ta–Ta–Ta of 139.31 for the chain
prepared on Al sample holders. The measuring range
was 100–3000 cmꢀ1 with resolution of 2 cmꢀ1
a
.
Resonance Raman spectra were measured on a multi-
channel Spectrometer XY (Dilor), excitation was
obtained through Ar+ and Kr+ lasers (o 150 mW) at
about 10 K.
UV/Vis/near-IR spectroscopic investigations were
conducted at room temperature using a UV–VIS–NIR
two-channel spectrometer Cary 5 from Varian Techtron
Pty., Darmstadt. The compound was ground with KBr
in a N2-filled glove box and pressed into a transparent
pellet. The UV/Vis/near-IR absorption spectrum was
corrected for the scattering background.
Fig. 1. Crystal structure of K2Ta2S10 with view along [010].

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1572
zigzag chains. The S22ꢀ anions are coordinated to two
M5+ centers in the same m2-Z1Z2 mode. The main
difference is the coordination number of the M5+ ions
in the two compounds. In NaNbS6 the Nb atom is
surrounded by nine S atoms in an unusual Nb(S2)3(S)3
coordination. The distorted tri-capped trigonal NbS9
prisms are linked through three S atoms. Each corner of
the trigonal prism is occupied by a S atom of a S22ꢀ
dumbbell. The environment of Nb is completed by three
S atoms of S22ꢀ anions which cap the rectangular faces of
the distorted trigonal prisms. In contrast, in K2Ta2S10
the Ta5+ ions are coordinated by eight S atoms forming
the unusual Ta(S2)2(S)4 polyhedron. In the title com-
pound only three corners of the distorted trigonal TaS8
prisms are occupied by S atoms of S2 dumbbells. One
S22ꢀ anion forms the edge of one triangle of the prism
and one S atom of the other S22ꢀ anion is a capping
atom.
Fig. 2. The interconnection of the trigonal prisms via common edges.
Note: the prisms are bi-capped with S1 and S9 being the capping
atoms; the environment and interconnection around Ta(2) is very
similar.
3.2. Spectroscopy
The far-IR spectrum of K2Ta2S10 displays absorp-
tions at about 350 (s), 303 (s), 280 (vs), 238 (s), 195 (s),
174 (vw), 139 (vw), 115 (w) cmꢀ1. It is in good agreement
with the Raman spectra, shown in Fig. 4 in the spectral
range of 100–550 cmꢀ1. Due to the resonance Raman
effect, the Raman spectra display significant dependen-
cies on the excitation frequency. With the Nd–YAG
laser excitation line (1064 nm), which is far away from
the first electronic absorption band at 739 nm (Fig. 5), a
Fig. 3. Connection of TaS5 polyhedra into chains for Ta1 (top) and
Ta2 (bottom) (displacement ellipsoids are drawn at 90% probability
level).
containing Ta(1) atoms and 140.01 for chains with the
Ta(2) atoms. The Ta(1)–Ta(1) and Ta(2)–Ta(2) dis-
tances of 3.432(1) and 3.426(1) A in the two distinct
chains are about 0.7 A longer than in metallicTa
(2.86 A), too long for Ta–Ta interactions. The S–S bond
lengths in the S22ꢀ dumbbells range from 2.071(2) to
2.091(2) A, and are in the usual range.
The two crystallographically distinct potassium
atoms are in an irregular environment of nine S atoms
with the K–S distances ranging from 3.195(2) to
3.507(2) A (mean value: 3.345(2) A) for K(1) and
3.250(2)–3.555(2) A (mean value: 3.365(2) A) for K(2),
if a cutoff of 3.6 A was chosen. For K(1) the S atoms are
from two [Ta(2)S5]ꢀ and one [Ta(1)S5]ꢀ chains, and for
K(2) from two [Ta(1)S5]ꢀ and one [Ta(2)S5]ꢀ chains.
The average distances agree well with the sum of the
+
ionicradii of K (1.33 A) and S2ꢀ (1.84 A) [27].
The structure of K2Ta2S10 exhibits several similarities
to that of NaNbS6 [24]. In both structures the MSx
polyhedra are joined to form one-dimensional anionic
Fig. 4. Raman spectra of K2Ta2S10, (a) FT-Raman, (b–f) resonance
Raman, excitation line are 1064 nm (FT), 647, 568, 531, 515, and
455 nm, respectively.

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1573
of n1 decreases, while the intensities of some lower
energy Ta–S vibrational modes, especially that at
320 cmꢀ1, increase significantly. This observation in-
dicates that this electronic absorption may be assigned
to a S-Ta charge transfer of the Ta–S–S–Ta bridges, in
contrast to the former electronic band at 569 nm
involving mainly a S-Ta charge transfer of the
Ta–S–Ta bridge. It is not surprising that because of
the interfering Raman effects, the Raman spectra are
incomplete, since only modes are enhanced which couple
to the electronic transition in resonance. Hence, more
complete excitation profiles including selected radiation
within the contour of the other electronic absorption
bands are necessary to better understand both the
electronic absorption spectrum, that displays up to six
distinct transitions at 739, 686, 569, 445, 366, and
284 nm, and the vibrational spectra of K2Ta2S10.
In conclusion, a new one-dimensional compound
K2Ta2S10 containing TaS8 polyhedra with disulfide
anions in A/group5/Q (A ¼ alkali metals; Q ¼
chalcogen) system has been synthesized and character-
ized. The presence of one-dimensional chain anions in
the compound obtained from a polychalcogenide flux
suggests possible new phases formed by combinations of
different building units of group 5 elements.
Fig. 5. UV/visible/near-IR spectrum of K2Ta2S10
.
‘‘normal’’ or at best a pre-resonance Raman spectrum is
observed (Fig. 4a). The S–S stretching vibration (n(S–S))
at 511/505 cmꢀ1 of the very polarisable disulfide ligands
is the most intense Raman band as compared with the
Ta–S stretching vibrations at 352, 320, 293, 281, 256,
243 cmꢀ1 and the S–Ta–S deformation modes at 202,
148 and 118 cmꢀ1 of the TaS8 polyhedra. The assign-
ment of n(S–S) is sustained by the complementary
infrared studies. According to the selection rules, no
distinct absorption is detected around 500 cmꢀ1. The
large hypsochromic shift of n(S–S) of the coordinated
disulfide ligands correlates well with the distinctly
shorter S–S distance as compared with that of the
disulfide ion, e.g. in K2S2 [30].
Applying excitations with higher energy Kr+ and
Ar+ laser lines at 647.1, 568.2, 530, 514.5, and 454.7 nm
(Fig. 4b–f), which are mainly only within the contour of
the 569 nm electronic absorption band of the complex
ion (Fig. 5), typical resonance Raman spectra are
obtained. In coincidence of the excitation frequency
with the maximum of this electronic absorption band,
the resonance Raman spectrum in Fig. 4 is characterized
by a significant enhancement of the intensity of the
symmetric(Ta–S–Ta) strethcing vibration ( n1) of the
(Ta–S–Ta) bridge at 352 cmꢀ1, and the appearance of an
overtone progression reaching up to 3n1. On the other
hand, all the bands of the other Ta–S vibrations and
especially that of n(S–S) of the disulfide ligand at 511/
505 cmꢀ1 show rather low intensities. In addition, all
these low intense Ta–S vibrational modes form combi-
nations with n1, but no subsidiary progressions with
other possible Raman active Ta–S modes is observed.
The bathochromic shift of the Ta–S vibrations corre-
lates well with the longer Ta–S distances (shortest:
2.455(1) A) compared to those (about 2.2 A) found in
the complex anions [Ta2S11]4ꢀ in Tl4[Ta2S11] [28] and
t-K4[Ta2S11] [14] as well as in TlTaS3 [29].
Attempts to prepare the analogous Rb and Cs
compounds yielded the polysulfides A6Nb4S22 (A ¼ Rb,
Cs) [18,19]. This observation suggests that the ionicradii
of Rb+ and Cs+ are too large to stabilize the [TaS5]ꢀ
anionicchains.
Acknowledgment
Financial support by the state of Schleswig-Holstein is
gratefully acknowledged.
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