Low dimensional materials: Syntheses, structures, and optical properties of Rb2CuTaS4, Rb2CuTaSe4, RbCu 2TaSe4, K3Ag3Ta2Se 8, and Rb3AgTa2Se12

Article abstract of DOI:10.1515/znb-2004-0910

The new compounds Rb₂CuTaS₄ (1), Rb ₂CuTaSe₄ (2), RbCu₂TaSe₄ (3), K ₃Ag₃Ta₂Se₈ (4), and Rb ₃AgTa₂Se₁₂ (5) have been synthesized by the reactive flux method at 773 or 873 K. Their crystal structures were determined by single crystal X-ray diffraction. Crystal data for 1: space group Fddd, a = 5.598(1), b = 13.512(4), c = 23.854(5) ?, Z = 8; Crystal data for 2: space group Fddd, a = 5.782(1), b = 13.924(3), c = 24.653(5) ?, Z = 8; Crystal data for 3: space group C2cm, a = 5.7218(3), b = 19.2463(13), c = 7.7456(5) ?, Z = 4; Crystal data for 4: space group C2/c, a = 25.1374(19), b = 6.1007(3), c = 14.4030(11) ?, β = 119.703(8), Z = 4; Crystal data for 5: space group P2₁/n, a = 9.8186(6), b = 13.7462(11), c = 15.7368(9) ?, β = 96.681(7)°, Z = 4. The compounds 1 and 2 are built up of _∞¹[CuTaQ₄]^2- anionic chains which are formed by edge-sharing CuQ₄ and TaQ₄ tetrahedra. The rubidium cations are located between the chains. Compound 3 consists of _∞²[Cu₂TaSe₄]^- anionic layers separated by rubidium cations. The anionic layers are formed by _∞¹[CuTaSe₄]^2- chains which are connected by CuSe₄ tetrahedra that share common edges with the TaSe₄ tetrahedra of neighboring chains. In compound 4 _∞¹[Ag₃Ta₂Se₈] ^3- anionic chains are found which are separated by potassium cations. These chains are formed by successive corner sharing of AgSe₄ tetrahedra and edge sharing between AgSe₄ and TaSe₄ tetrahedra. All three structures are closely related with the sulvanite (Cu ₃VS₄) structure type. Compound 5 contains a one dimensional _∞¹[AgTa₂Se₁₂] ^3- anionic chain formed by interconnection of AgSe₄ tetrahedra and [Ta₂Se₁₁] units. In the structure three monoselenide, three diselenide, and one triselenide anions are found. Raman and far-IR spectroscopic data of compounds 1 and 4 were collected and an interpretation is presented.

Full text of DOI:10.1515/znb-2004-0910

Low Dimensional Materials: Syntheses, Structures, and Optical
Properties of Rb2CuTaS4, Rb2CuTaSe4, RbCu2TaSe4, K3Ag3Ta2Se8,
and Rb3AgTa2Se12
Yuandong Wu, Christian Na¨ther, and Wolfgang Bensch
Institut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t Kiel, Olshausenstraße 40,
D-24098 Kiel, Germany
Reprint requests to Prof. Dr. W. Bensch. Fax: +49-(0)431-880-1520.
E-mail: wbensch@ac.uni-kiel.de
Z. Naturforsch. 59b, 1006 – 1014 (2004); received July 14, 2004
Dedicated to Professor Kurt O. Klepp on the occasion of his 60th birthday
The new compounds Rb2CuTaS4 (1), Rb2CuTaSe4 (2), RbCu2TaSe4 (3), K3Ag3Ta2Se8 (4), and
Rb3AgTa2Se12 (5) have been synthesized by the reactive flux method at 773 or 873 K. Their crys-
tal structures were determined by single crystal X-ray diffraction. Crystal data for 1: space group
˚
Fddd, a = 5.598(1), b = 13.512(4), c = 23.854(5) A, Z = 8; Crystal data for 2: space group Fddd,
˚
a = 5.782(1), b = 13.924(3), c = 24.653(5) A, Z = 8; Crystal data for 3: space group C2cm,
˚
a = 5.7218(3), b = 19.2463(13), c = 7.7456(5) A, Z = 4; Crystal data for 4: space group C2/c,
◦
˚
a = 25.1374(19), b = 6.1007(3), c = 14.4030(11) A, β = 119.703(8) , Z = 4; Crystal data for 5:
◦
˚
space group P21/n, a = 9.8186(6), b = 13.7462(11), c = 15.7368(9) A, β = 96.681(7) , Z = 4. The
compounds 1 and 2 are built up of 1∞[CuTaQ4]2− anionic chains which are formed by edge-sharing
CuQ4 and TaQ4 tetrahedra. The rubidium cations are located between the chains. Compound 3 con-
sists of 2∞[Cu2TaSe4]− anionic layers separated by rubidium cations. The anionic layers are formed
by 1∞[CuTaSe4]2− chains which are connected by CuSe4 tetrahedra that share common edges with the
TaSe4 tetrahedra of neighboring chains. In compound 4 1∞[Ag3Ta2Se8]3− anionic chains are found
which are separated by potassium cations. These chains are formed by successive corner sharing of
AgSe4 tetrahedra and edge sharing between AgSe4 and TaSe4 tetrahedra. All three structures are
closely related with the sulvanite (Cu3VS4) structure type. Compound 5 contains a one dimensional
3−
1∞[AgTa2Se12
]
anionic chain formed by interconnection of AgSe4 tetrahedra and [Ta2Se11] units.
In the structure three monoselenide, three diselenide, and one triselenide anions are found. Raman
and far-IR spectroscopic data of compounds 1 and 4 were collected and an interpretation is presented.
Key words: Tantalum Compound, Quaternary Chalcogenides, Crystal Structures, Flux Reaction,
Optical Properties
Introduction
genides with compositions A2MIMVQ4 (A = K, Rb,
Cs; MI = Cu, Ag; MV = V, Nb, Ta; Q = S, Se) [1 – 9],
ACu2MVQ4 (A = K, Rb, Cs; MV = V, Nb, Ta; Q = S,
Se, Te) [10– 18], K3Cu3MV2Q8 (MV = Nb, Ta; Q = S,
Se) [19], and K3CuNb2Se12 [20] crystallize with low-
dimensional anionic building units. The structures of
Low-dimensional compounds exhibit many interest-
ing physical properties such as high temperature super-
conductivity, the formation of charge density waves,
and low-dimensional magnetism. In these compounds
either two-dimensional slabs are stacked along a crys- these compounds except K3CuNb2Se12 can be derived
from the sulvanite (Cu3VS4) type. Sequential substitu-
tion of A = K, Rb, Cs for Cu ions results in a decrease
of the dimensionality of the structures, from the three-
dimensional covalent network in Cu3VS4 via two-
dimensional metal chalcogen layers in ACu2MVQ4,
to infinite one-dimensional metal-chalcogen chains in
tallographic direction or chains are in a juxtaposition.
Within the slabs or chains ionic, covalent, or metallic
interactions dominate. Between them very often only
weak interactions such as van der Waals forces occur or
they are separated by alkaline or alkaline earth cations.
The quaternary alkali metal group 5 metal chalco-
c
0932–0776 / 04 / 0900–1006 $ 06.00 ꢀ 2004 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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1007
Table 1. Technical details
of data acquisition and re-
finements.
Compound
Formula
1
2
3
4
5
Rb2CuTaS4
Rb2CuTaSe4
RbCu2TaSe4
K3Ag3Ta2Se8 Rb3AgTa2Se12
Crystal system
Space group
orthorhombic orthorhombic orthorhombic monoclinic
Fddd
monoclinic
P21/n
Fddd
C2cm
C2/c
˚
a [A]
5.598(1)
13.512(4)
23.854(5)
90
1804.2(7)
8
5.7820(5)
13.9240(12)
24.6526(29)
90
1984.7(7)
8
5.7218(3)
19.2463(13)
7.7456(5)
90
852.97(9)
4
25.1374(19)
6.1007(3)
14.4030(11)
119.703(8)
1918.6(2)
4
9.8186(6)
13.7462(11)
15.7368(9)
96.681(7)
2109.5(2)
4
=
1/[σ2(Fo2) +
∗
w
˚
b [A]
(x · P)2 + y · P]; P =
˚
c [A]
2
2
β [◦]
(Max(Fo )+2·Fc )/3.
3
˚
V [A ]
Z
T [K]
293
4.003
26.031
1920
293
4.895
37.494
2496
293
5.524
40.376
1216
293
4.966
30.171
2464
293
5.270
38.829
4852
calcd. den. (g.cm−3
)
µ [mm−1
F(000)
]
2θ Range [◦]
6◦ ≤ 2θ ≤ 60◦ 7◦ ≤ 2θ ≤ 56◦ 5◦ ≤ 2θ ≤ 56◦ 6◦ ≤ 2θ ≤ 56◦ 5◦ ≤ 2θ ≤ 56◦
Index range
−1 ≤ h ≤ 5
−1 ≤ k ≤ 18
−1 ≤ l ≤ 33
585
−7 ≤ h ≤ 7
−7 ≤ h ≤ 7
−32 ≤ h ≤ 32 −7 ≤ h ≤ 7
−18 ≤ k ≤ 17 −25 ≤ k ≤ 25 −7 ≤ k ≤ 7
−25 ≤ k ≤ 25
−33 ≤ l ≤ 32 −10 ≤ l ≤ 10 −18 ≤ l ≤ 19 −10 ≤ l ≤ 10
Indep. refl.
597
1113
2291
4852
refl. [Fo > 4σ(Fo)] 481
488
0.0614
1036
0.0544
2059
0.0407
4164
0.0548
Rint
0.0242
Min/max. transm.
No. parameters
w∗
R1 for Fo > 4σ(Fo) 0.0186
wR2 for all data
GooF
0.1727/0.3345 0.0549/0.1746 0.0115/0.0581 0.0438/0.2464 0.0097/0.0846
22
22
47
76
164
0.0177
0.0408
0.0259
0.0657
1.049
–
0.0455
0.0272
0.0687
1.051
0.259
0.0778
0.0381
0.1016
1.041
–
0.0370
0.0274
0.0658
1.025
–
0.0520
1.190
–
BASF
∆ρ[eA
−3
˚
]
0.998/−0.938 1.274/−2.361 1.285/−2.591 2.665/−3.067 1.361/−1.771
A2MIMVQ4 and K3Cu3MV2Q8, and finally to isolated sphere. The tantalum powder was activated with ultrasonic
radiation in dry acetone.
MQ4 ions in A3MQ4. This series of compounds ob-
viously shows the influence of the size and distinct
bonding nature of the cations onto the dimension-
ality of the structures. K3CuNb2Se12 is another ex-
ample containing a one-dimensional bimetallic chain
which comprises CuSe4 tetrahedra and [Nb2Se11]
units with each Nb atom in a pentagonal bipyra-
midal environment. Here we describe the syntheses
and structures of the five quaternary alkali metal-
group 5 chalcogenides, Rb2CuTaS4, Rb2CuTaSe4,
RbCu2TaSe4, K3Ag3Ta2Se8, and Rb3AgTa2Se12. In
addition, we draw some simple connections between
the different formulations of the quaternary alkali
metal group 5 chalcogenides.
Rb2CuTaS4 (1): The starting materials Rb2Te3, Cu, Ta,
and S in a 2:1:1:12 molar ratio were thoroughly mixed
and loaded into a Pyrex tube in a nitrogen glove box. The
tube was evacuated and flame sealed at the pressure of ∼
10−3 mbar. The mixture was heated at 773 K for 5 days, fol-
lowed by cooling to 373 K at a rate of 3 K/h and the furnace
was turned off. The product was washed with DMF to re-
move excess rubidium sulfides and dried with anhydrous ace-
tone. It contained transparent colorless needles (yield: about
90% based on Ta) and black needles of tellurium. The prod-
uct is stable under dry air for several months.
Rb2CuTaSe4 (2): The preparation and isolation of 2 were
similar to those of 1. However, the mixture of Rb2Se3, Cu,
Ta, Se in a molar ratio of 4:1:1:8 was heated at 873 K for
5 days in a fused silica tube. After cooling the mixture with
3 K/h, transparent amber-colored platelet crystals were ob-
tained in a yield of about 100%. The compound is stable in
dry air for several weeks. The homogeneity of the product
was confirmed by comparison of the experimental and calcu-
lated X-ray diffraction powder patterns.
Experimental Section
Syntheses
The following reagents were commercially available: K
(Strem, 99.95%), Rb (Strem, 99+%), Cu (alfa, 99.5%), Ag
(Degussa, 99.5%), Ta (Strem, 99.98%), S (Heraeus, 99.99%),
Se (Riedel-De Hae¨n, 99.5%), Te (Retorte, 99.999%). The
RbCu2TaSe4 (3): The procedure for the preparation of 3 is
chalcogenides A2Q3 (A = K, Rb; Q = Se, Te) were synthe- the same as that for 2 except that the molar ratio Rb2Se3, Cu,
sized from reactions of stoichiometric amounts of elemental Ta, Se was changed to 3:6:4:7. The product contained trans-
A and Q dissolved in liquid ammonia under an argon atmo- parent yellow platelet crystals in a 80% yield. The crystals
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Y. Wu et al. · Low Dimensional Materials
˚
Table 2. Atomic coordinates and equivalent isotropic dis- Table 3. Selected bond lengths (A) for Rb2CuTaS4 (1) and
2
3
˚
Rb2CuTaSe4 (2).
placement parameters Ueq (A ·10 ). Estimated standards de-
viations are given in parentheses. The Ueq is defined as one
Rb2CuTaS4
2.278(1)
2.360(1)
3.448(2)
3.490(2)
3.557(2)
3.656(2)
Rb2CuTaSe4
2.405(1)
2.465(1)
3.547(1)
3.611(1)
3.629(1)
3.770(1)
third of the trace of the orthogonalized U tensor.
ij
Ta - Q ×4
Cu - Q ×4
Rb - Q ×2
Rb - Q ×2
Rb - Q ×2
Rb - Q ×2
x
y
z
Ueq
Rb2CuTaS4:
Ta
1/8
1/8
0.3629(3)
1/8
1/8
1/8
0.7224(1)
1/8
5/8 13(1)
1/8 23(1)
Cu
Se
Rb
0.1794(1) 28(1)
0.4472(1) 28(1)
˚
Table 4. Selected bond lengths (A) for RbCu2TaSe4 (3).
Rb2CuTaSe4:
Ta(1)
Ta(1)
Cu(1)
Cu(2)
Rb(1)
Rb(1)
Rb(1)
-
-
-
-
-
-
-
Se(1)
Se(2)
Se(2)
2.395(2) Ta(1)
2.454(1) Cu(1)
2.453(3) Cu(1)
-
-
-
-
-
-
Se(3) ×2 2.420(1)
Se(1) 2.454(3)
Ta
1/8
1/8
0.3662(1)
1/8
1/8
1/8
0.7249(1)
1/8
5/8 11(1)
1/8 21(1)
0.1810(1) 20(1)
0.4462(1) 26(1)
Cu
Se
Rb
Se(3) ×2 2.448(2)
Se(2) ×2 2.462(2) Cu(2)
Se(3) ×2 2.443(2)
Se(1) 3.537(2) Rb(1)
Se(2)
3.596(2)
RbCu2TaSe4:
Se(3) ×2 3.693(2) Rb(1)
Se(1) ×2 3.905(1)
Se(3) ×2 3.760(2)
Ta(1)
Cu(1)
Cu(2)
Se(1)
Se(2)
Se(3)
Rb(1)
0.5444(1)
0.0436(6)
0.5649(4)
0.2972(2)
0.2995(2)
0.7898(2)
0.7975(2)
0.1086(1)
0.1062(1)
0
0.2090(1)
0.0039(1)
0.1079(1)
0.3171(1)
1/4 16(1)
1/4 25(1)
0
25(1)
ing material was heated to 773 K, held there for 24 h, then
ramped to 973 K, kept there for 4 days, and cooled down to
373 K at a rate of 3 K/h. After removing excess K2Sex flux,
crystals suitable for X-ray diffraction studies were picked
out manually from the mixture of transparent yellow needles
(∼ 70% yield based on Ta) and black needles, which have
not yet been identified.
Rb3AgTa2Se12 (5): Compound 5 can be obtained under
the same reaction conditions used for 2 but with more se-
lenium (molar ratio: Rb2Se3: Ag: Ta: Se = 3:2:4:15). The
product was isolated by removing the excess Rb2Sex flux
with DMF followed by washing with acetone. The product
consists of black chunk crystals (yield: 90% based on Ta)
which are insoluble in water and stable in air. The by-product
consisting of transparent amber-colored platelet crystals was
identified as RbAg2TaSe4 by comparison of the calculated
and experimental powder patterns.
1/4 32(1)
1/4 19(1)
0.9955(1) 24(1)
1/4 37(1)
K3Ag3Ta2Se8:
Ta(1)
Ag(1)
Ag(2)
Se(1)
Se(2)
Se(3)
K(1)
0.1292(1)
0
0.6945(1)
0.6909(1)
0.1946(1)
0.9255(1)
0.4699(1)
0.4809(1)
0.4266(1)
0
0.2823(1) 8(1)
1/4 21(1)
0.1290(1)
0.1598(1)
0.0464(1)
0.2171(1)
0.1944(1)
0
0.2927(1) 20(1)
0.1812(1) 16(1)
0.1489(1) 16(1)
0.3995(1) 15(1)
0.1074(1) 24(1)
K(2)
0
22(1)
Rb3AgTa2Se12
:
Ta(1)
Ta(2)
Ag(1)
Rb(1)
Rb(2)
Rb(3)
Se(1)
Se(2)
Se(3)
Se(4)
Se(5)
Se(6)
Se(7)
Se(8)
Se(9)
Se(10)
Se(11)
Se(12)
0.1575(1)
−0.0149(1)
−0.1989(1)
0.2784(1)
0.2215(1)
0.1723(1)
0.0802(1) 14(1)
0.2712(1) 14(1)
0.4187(1) 28(1)
0.1590(1) 34(1)
0.2222(1) 33(1)
0.4927(1) 33(1)
0.3669(1) 25(1)
0.2456(1) 21(1)
0.3254(1) 22(1)
0.0736(1) 22(1)
0.1729(1) 18(1)
0.3612(1) 23(1)
0.1958(1) 23(1)
0.1626(1) 22(1)
0.1041(1) 22(1)
−0.0067(1) −0.1059(1)
0.5694(1)
0.0843(1)
−0.2240(1) −0.0850(1)
Single crystal X-ray diffraction
0.1966(1)
0.2354(1)
−0.0706(1)
0.4251(1)
0.0240(1)
−0.1339(1)
−0.2401(1)
0.2585(1)
−0.0495(1)
0.0509(1)
0.1614(1)
0.4932(1)
0.2793(1)
0.1836(1)
0.0618(1)
0.2704(1)
0.3892(1)
0.3525(1)
0.2863(1)
0.4301(1)
0.1613(1)
Single crystal investigation of compound 1 was per-
formed using a Philips PW1100 4-circle diffractometer, and
of compounds 2 – 5 using an Imaging Plate Diffraction Sys-
tem (IPDS) with monochromated Mo-Kα -radiation (λ =
˚
0.71073 A). The intensities were treated in the normal way
applying Lorentz and polarization correction. For compound
1 an empirical absorption correction based on ψ-scan reflec-
tions and for compounds 2 – 5 face indexed absorption cor-
rections were applied.
All structures were solved with direct methods using
SHELXS-97 and refined against Fo2 using SHELXL-97 [21].
All atoms were refined with anisotropic displacement param-
eters. The crystal of compound 3 was racemically twinned
0.3421(1) −0.0549(1) 23(1)
0.0933(1)
0.1252(1)
0.0611(1) 26(1)
0.0052(1) 34(1)
are stable in dry air for several weeks. The homogeneity of
the product was confirmed through X-ray powder diffraction.
K3Ag3Ta2Se8 (4): Crystals were prepared reacting a mix- and therefore, a twin refinement using SHELXL-97 was
ture of K2Se3, Ag, Ta, and Se with a molar ratio of 3:6:4:7. performed (BASF parameter: 0.26(3)). In order to compare
After being sealed in a silica tube under vacuum, the start- the structure of 3 with that of KCu2MSe4 (M = Nb, Ta)
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˚
Table 5. Selected bond lengths (A) for K3Ag3Ta2Se8 (4).
Ta(1)
Ta(1)
Ag(1)
Ag(2)
Ag(2)
K(1)
K(1)
K(1)
K(2)
K(2)
-
-
-
-
-
-
-
-
-
-
Se(1)
Se(3)
2.410(1) Ta(1)
2.395(1) Ta(1)
-
-
-
-
-
-
-
-
-
Se(2)
Se(4)
2.437(1)
2.447(1)
Se(2) ×2 2.642(1) Ag(1)
Se(4) ×2 2.650(1)
Se(1)
Se(3)
Se(1)
Se(3)
Se(4)
Se(2)
2.667(1) Ag(2)
2.636(1) Ag(2)
3.414(2) K(1)
3.363(2) K(1)
3.466(2) K(2)
3.420(1) K(2)
Se(2)
Se(4)
Se(1)
Se(3)
2.676(1)
2.629(1)
3.485(2)
3.546(2)
Se(1) ×2 3.572(1)
Se(2) ×2 3.733(1)
Se(4) ×2 3.556(1)
Fig. 1. View of the Rb2CuTaQ4 structure (Q = S (1), Se (2))
approximately along the a axis.
˚
Table 6. Selected bond lengths (A) for Rb3AgTa2Se12 (5).
Ta(1)
Ta(1)
Ta(1)
Ta(1)
Ta(2)
Ta(2)
Ta(2)
Ag(1)
Ag(1)
Rb(1)
Rb(1)
Rb(1)
Rb(1)
Rb(1)
Rb(2)
Rb(2)
Rb(2)
Rb(2)
Rb(3)
Rb(3)
Rb(3)
Rb(3)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Se(2)
Se(5)
Se(9)
2.931(1) Ta(1)
2.572(1) Ta(1)
2.654(1) Ta(1)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Se(4)
Se(8)
Se(10) 2.421(1)
Se(1)
Se(3)
Se(6)
Se(9)
Se(4)
2.643(1)
2.591(1)
Se(11) 2.563(1) Ta(2)
2.547(1)
2.440(1)
2.646(1)
2.740(1)
2.712(1)
Se(2)
Se(5)
Se(7)
Se(3)
Se(6)
Se(1)
Se(6)
Se(8)
2.588(1) Ta(2)
2.827(1) Ta(2)
2.546(1) Ta(2)
2.546(1) Ag(1)
2.737(1) Ag(1)
3.495(1) Rb(1)
3.552(1) Rb(1)
3.532(1) Rb(1)
Se(10) 2.547(1)
Se(3)
Se(7)
Se(9)
3.598(1)
3.862(1)
3.785(1)
Fig. 2. Structure of the 1∞[CuTaQ4]2− anionic chain in com-
pounds 1 and 2. Displacement ellipsoids are drawn at the
50% probability level.
Se(10) 3.640(1) Rb(1)
Se(11) 3.631(1) Rb(2)
Se(3)
Se(5)
Se(7)
Se(11) 3.620(1)
Se(2)
Se(4)
Se(6)
3.611(1)
3.639(1)
3.532(1)
compounds 1 and 4 pressed in polyethylene pellets. The
FT-Raman spectrum was measured on an ISF-66 spectrom-
eter (Bruker) with an additional FRA 106 Raman modul.
A Nd/YAG laser was used as source for excitation (λ =
1064 nm). The samples were prepared on Al-sample holders.
The measuring range was 100 to 3500 cm−1 with a resolu-
3.727(1) Rb(2)
3.334(1) Rb(2)
3.400(1) Rb(2)
Se(10) 3.676(1)
Se(12) 3.456(1) Rb(3)
Se(1)
Se(3)
Se(5)
Se(8)
3.456(1)
3.839(1)
3.716(1)
3.438(1)
Se(3)
Se(4)
Se(7)
3.768(1) Rb(3)
3.791(1) Rb(3)
3.437(1) Rb(3)
tion of 2 cm−1
.
Se(10) 3.604(1)
[13, 15, 16] directly, 3 was refined in the non-conventional
setting (C2cm) of space group Ama2. With the matrix (0 0
−1 0 1 0 1 0 0) the conventional setting Ama2 is achieved.
Technical details of the data acquisition as well as some re-
finement results are summarized in Table 1. Atomic coordi-
nates as well as equivalent isotropic displacement parameters
are listed in Table 2. Selected bond lengths are given in Ta-
bles 3 – 6.
Further details of the crystal structure investiga-
tion can be ordered referring to the No CSD-414271
(Rb2CuTaS4), CSD-414272 (Rb2CuTaSe4), CSD-414274
(RbCu2TaSe4), CSD-414270 (K3Ag3Ta2Se8), and CSD-
414273 (Rb3AgTa2Se12), the authors and the citation
of this paper at the Fachinformationszentrum Karls-
ruhe, Gesellschaft fu¨r wissenschaftlich-technische Informa-
tion mbH. D-76344 Eggenstein-Leopoldshafen (Germany).
E-mail: crysdata@fiz-karlsruhe.de.
Results and Discussions
Structures
The structures of the isostructural compounds
Rb2CuTaS4 (1) and Rb2CuTaSe4 (2) are shown in
Fig. 1. They consist of infinite 1 [CuTaQ4]2− chains
∞
running parallel to [100] which are separated by Rb+
ions. These chains are illustrated in Fig. 2. Within
the chains the Cu and Ta atoms are in a tetrahe-
dral coordination of four Q atoms. These tetrahedra
share two opposite edges, and CuQ4 and TaQ4 tetra-
hedra alternate along the chain. The Cu and Ta atoms
have crystallographically imposed 222 site symme-
try. Consequently, each atom must have single M – Q
distances and each can have three independent Q –
M – Q angles. The Cu – Q distances and Ta – Q dis-
˚
˚
Infrared and Raman spectroscopy
tances are 2.360(1) A (Cu – S), 2.278(1) A (Ta – S) in
FIR spectra were recorded between 80 and 550 cm−1
˚
˚
1 and 2.465(1) A (Cu – Se), 2.405(1) A (Ta – Se) in
(resolution = 2 cm−1) on an ISF-66 device (Bruker) with 2. The Q – Ta – Q angles are 108.45(7)◦, 109.38(7)◦,
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Y. Wu et al. · Low Dimensional Materials
Fig. 4. Structure of the 2 [Cu2TaSe4]1− layer in compound
∞
3 with atomic labeling. Displacement ellipsoids are drawn at
the 50% probability level.
are tetrahedrally coordinated by Se atoms. The TaSe4
and Cu(1)Se4 tetrahedra are sharing common edges
to form [Cu(1)TaSe4] chains along [100], like in the
one-dimensional chains in Rb2CuTaSe4. The Cu(2)Se4
tetrahedra share common corners with Cu(1)Se4 tetra-
hedra along [100] and have a common edge with TaSe 4
tetrahedra along [001]. This connection mode yields
the [Cu2TaSe4]− layers. The Ta – Se bond lengths
Fig. 3. Crystal structure of RbCu2TaSe4 (3) with view ap-
proximately along [100].
and 110.60(7)◦ in 1 and 109.11(3)◦, 109.35(3)◦ and
109.95(3)◦ in 2. The S – Cu – S angles are 103.09(7)◦,
112.18(7)◦, and 113.33(7)◦ in 1 and the Se – Cu – Se
angles are 105.26(3)◦, 111.91(3)◦, and 113.33(3)◦ in 2,
indicating distortions of the MQ4 tetrahedra which are
more pronounced for M = Cu. The shortest interchain
˚
range from 2.395(2) to 2.454(1) A, matching well with
those in Rb2CuTaSe4 (2). The Se – Ta – Se angles vary
from 109.07(6)◦ to 110.31(3)◦, close to the normal
tetrahedral values. The Cu – Se bonds from 2.443(2) to
˚
Q – Q distances of 3.778(1) A (S – S) in compound 1
˚
˚
and 3.803(1) A (Se – Se) in compound 2 are slightly
2.462(2) A are slightly shorter than the Cu – Se dis-
˚
˚
longer the sum of the van der Waals radii (3.6 A for
tance (2.465(1) A) in Rb2CuTaSe4 (2). The Se – Cu –
˚
S; 3.8 A for Se). Therefore, there are no interactions
Se angles about the two crystallographically indepen-
dent Cu atoms are different. The Se – Cu(1) – Se angles
are close to the normal tetrahedral value (107.1(1)◦ –
111.39(5)◦), while the Se – Cu(2) – Se angles scatter
from 103.8(1)◦ to 116.4(1)◦, which clearly indicates
the stronger distortion of the Cu(2)Se4 tetrahedron.
The Rb+ ions are coordinated by nine Se atoms choos-
between the anionic chains in these compounds. The
Rb+ ions are coordinated by eight S (Se) atoms that
belong to three different anionic chains. The Rb – S
˚
distances range from 3.448(2) to 3.656(2) A (aver-
˚
age: 3.538(2) A) and the Rb – Se distances are between
˚
˚
3.547(1) and 3.770(1) A (average: 3.639(1) A). These
distances agree well with those found in the isostruc-
tural compounds A2MIMVQ4 (A = K, Rb, Cs; MI =
Cu, Ag; MV = V, Nb, Ta; Q = S, Se) [1 – 9]. Due to
the long Q – Q distances there are no significant inter-
actions. The formal oxidation states Rb(+1), Cu(+1),
Ta(+5), and S(-2)/Se(-2) can be assigned.
˚
ing a cutoff of 4.0 A for the Rb – Se distances. The
˚
Rb-Se bond lengths range from 3.537(2) to 3.905(3) A
+
˚
(average: 3.698(2) A). The geometry around Rb is
similar to that of K+ in KCu2MSe4 (M = Nb, Ta)
[13, 16, 17] which is a slightly distorted tricapped trig-
onal prism, unlike the highly distorted trigonal prism of
the Cs cations in CsCu2MTe4 (M = Nb, Ta). The for-
mal oxidation states of all atoms in this compound can
be assigned as: Rb(+1), Cu(+1), Ta(+5), and Se(-2).
The structure of RbCu2TaSe4 (3) is shown in Fig. 3.
It is composed of two-dimensional [Cu2TaSe4]− lay-
ers which are parallel to the (010) plane (Fig. 4) and
Rb+ ions located between the layers. The shortest in-
The compound K3Ag3Ta2Se8 (4) is, to the best of
˚
terlayer Se – Se distance of 4.006(1) A is longer than our knowledge, the first silver compound in the fam-
the sum of the van der Waals radii of Se. Obviously ily with general composition K3MI3MV2Q8 (MI = Cu,
there are no interactions between the adjacent lay- Ag; MV = Nb, Ta; Q = S, Se). The main structural
ers. Within the [Cu2TaSe4]− layers Ta and Cu atoms feature is a one-dimensional [Ag3Ta2Se8]3− anionic
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Y. Wu et al. · Low Dimensional Materials
1011
Fig. 5. Structure of 1 [Ag3Ta2Se8]3− anionic chain in com-
∞
pound 4 with labeling and displacement ellipsoids drawn at
the 50% probability level.
¯
Fig. 7. Crystal structure of Rb3AgTa2Se12 (5) along [101].
Fig. 6. Crystal structure of K3Ag3Ta2Se8 (4).
3−
anionic chain in
compound 5 with atomic labeling and displacement ellipsoid
drawn at the 50% probability level (the dashed lines represent
Fig. 8. Structure of the 1 [AgTa2Se12
]
∞
chain separated by K+ ions (Fig. 5 and 6). The chains
are directed along [010] with the shortest interchain
˚
the long Se – Se distance of 2.838 A).
˚
separation of 3.748(1) A between Se(2) atoms of ad-
than the analogous bonds in K3Cu3Nb2S8. The tetra-
hedra surrounding Ta, Ag(1), and Ag(2) are strongly
distorted with the angles ranging from 104.81(3)◦ to
112.75(3)◦ for TaSe4, 100.41(2)◦ to 121.30(5)◦ for
Ag(1)Se4, and 98.12(3)◦ to 121.64(3)◦ for Ag(2)Se4,
while in K3Cu3Nb2S8 the NbS4 tetrahedra are some-
what more regular (107.98(7)◦ – 112.27(7)◦), Cu(1)S4
(105.27(7)◦ – 114.54(13)◦), and Cu(2)S4 (104.30(9)◦ –
115.47(9)◦).
jacent chains. Within the chains the unique Ta atom
and the two independent Ag atoms are tetrahedrally
coordinated by Se atoms. The TaSe4 and Ag(2)Se4
tetrahedra share common edges along [100] to form
infinite one-dimensional [AgTaSe4] chains which are
topologically identical with those found in compounds
1 and 2. Two adjacent chains are interconnected by
Ag(1)Se4 tetrahedra in such way that the Ag(1)Se4
tetrahedra have common edges with TaSe4 tetrahedra
along [100] and common corners with Ag(2)Se4 tetra-
hedra. Compared to compound 3, the structure of 4
can be viewed as a derivative removing one half of
the Cu(1)Se4 tetrahedra from the two-dimensional lay-
ers in 3. More precisely, the structure of the anionic
chain is an intermediate between those in compounds
1 – 3. Since there are no Se – Se interactions in this
compound the formal oxidation states K(+1), Ag(+1),
Ta(+5), and Se(-2) can be assigned. Similar copper –
group 5 metal – chalcogen chains were found pre-
viously in K3Cu3Nb2S8 [19]. As expected, the Ta –
The structure of Rb3AgTa2Se12 (5) (Fig. 7 and 8)
contains [AgTa2Se12]3− chains running along the
+
¯
[101] direction with Rb ions isolating these chains.
In the structure, there are three independent Rb atoms.
The Rb(1) ion is coordinated by nine Se atoms form-
ing an irregular polyhedron whereas both Rb(2) and
Rb(3) ions are coordinated to eight Se atoms. The
˚
Rb – Se distances range from 3.334(1) to 3.862(1) A.
In K3CuNb2Se12 the coordination numbers of the K+
ions are 10, 9, and 8.
Of particular interest in the structure of
Se and Ag – Se distances in compound 4 are longer Rb3AgTa2Se12 (5) is the fact that the two inde-
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1012
Y. Wu et al. · Low Dimensional Materials
˚
pendent Ta atoms are coordinated by seven Se atoms 2.542(3), and 2.726(3) A. In the present compound
each forming a TaSe7 pentagonal bipyramid. Two the comparable distances in compound 5 (Se(4)-
TaSe7 polyhedra share a common face yielding a Se(12)-Se(6)-Se(7)) amount to 2.400(1), 2.475(1), and
˚
[Ta2Se11] unit. We note that the Ta atoms are not in 2.838(1) A, the latter is much longer than a Se – Se sin-
˚
a tetrahedral environment as in compounds 1 – 4 and gle bond of about 2.35 A. In addition, the Se(7) atom is
2−
˚
˚
that the Se atoms form Se3 species in 5. The Ta(1) only 3.028(1) A apart from Se(9) and 3.011(1) A from
atom is surrounded by two Se22− and two Se2− anions Se(5), and both separations are significantly shorter
and has a bond to a Se atom of the Se2 dianion than the sum of the van der Waals radii of Se2− an-
2−
which is bound in a η2-mode to Ta(2). In contrast, ions. We note that in K3CuNb2Se12 similar short con-
Ta(2) has bonds to one Se22− anion, to three Se2− ions tacts are observed (3.041 and 3.108 A). The average
˚
2−
and to one Se atom of the two Se2 ions bound to Nb-Se bond length in the Nb compound is nearly iden-
Ta(1) in the η2-mode. The coordination mode can be tical with that observed in our compound. Hence, the
described as [Ta2(Se)3(µ2-η2, η1-Se2)3(µ2-η1, η1- structural differences within the anionic chains of the
Se3)]4−. Interestingly, the Se(7) atom is terminal two compounds must be due to the differing bonding
and has no bonds to Ag+ or Se2−. We note that interactions and geometrical features in the CuSe4 and
the connection mode within the [Ta2Se11] unit in AgSe4 tetrahedra, respectively.
Tl4Ta2Se11 is quite different and may be described as
[Ta2(µ–Se)(µ-η2, η1-Se2)2(η2-Se2)2(S e)2]4− [26].
Vibrational spectroscopy
Within the [AgTa2Se12]3− chain the [Ta2Se11]
The FIR and Raman spectra of Rb2CuTaS4 (1) in the
range of 80 to 475 cm−1 are shown in Fig. 9. The FIR
spectrum displays absorptions at about 419, 407, 228,
208, 150, 140, 121, and 88 cm−1. The Raman spectrum
shows resonances at about 435, 422, 414, 251, and
170 cm−1. Compared with the data of Cu3TaS4 and
Tl3TaS4 [27], the Raman shifts at 435, 422, 414 cm−1
can be assigned to Ta – S (Cu – S) stretching vibrations.
The resonance at 170 cm−1 is assignable to the S –
Ta(Cu) – S deformation mode of the Ta(Cu)S4 tetrahe-
dra. Because all sulfur atoms are edge-shared by TaS4
and CuS4 tetrahedra, it is impossible to separate the
Raman resonances of TaS4 and CuS4 tetrahedra ex-
actly. Due to the somewhat shorter Ta – S and Cu – S
bond lengths in compound 1, these vibrations are lo-
cated at higher energies. The shift at 251 cm−1 was
attributed to the Cu – S valence vibration. The absorp-
tions in the far-IR spectrum at 419, 407, 228, 208 cm−1
were assigned to Ta – S (Cu – S) asymmetric stretching
vibrations. The assignments of the bands at a lower en-
ergy from 150 to 80 cm−1 are not clear.
units share edges with the AgSe4 tetrahedra (Se(3)-
Se(6) and Se(4)-Se(10)). The Se – Se distances in
2−
the Se3
anion (Se(4)-Se(6)-Se(12)) range from
˚
2.385(1) to 2.475(1) A, typical values for single bonds.
The Ta – Se bond lengths are between 2.421(1) and
˚
2.931(1) A, somewhat longer than the Nb – Se dis-
tances in K3CuNb2Se12 [20]. The average distance
˚
dTa−Se = 2.62 A is in good agreement with the sum of
the ionic radii (Ta5+ (0.64 A), Se (1.98 A)) [24]. It is
2−
˚
˚
also in agreement with the average distance (dTa−Se
≈
˚
2.61 A) reported for K12Ta6Se35 [25] and Tl4Ta2Se11
[26]. But comparing compound 5 with K12Ta6Se35
and Tl4Ta2Se11 carefully, we found that the terminal
˚
Ta(2)– Se(7) distance (2.546(1) A) is much longer than
˚
those in K12Ta6Se35 (Ta – Seter: 2.360(1)– 2.376(1) A)
˚
and Tl4Ta2Se11 (Ta – Seter: 2.410(2)– 2.433(2) A). Ob-
viously, the Ta(2) – Se(7) bond cannot be described as
a double bond. One reason for the elongation of the
terminal Ta-Se(7) bond may be a weak interaction be-
tween Se(7) and Se(6) (see below).
The Ag – Se distances vary from 2.546(1) to
˚
2.737(1) A, comparing well with those in RbAg5Se3
Fig. 10 shows the FIR and Raman spectra of com-
(Ag – Se: 2.642(2)– 2.745(2) A) [22], RbAg3Se2 (Ag – pound 4 in the range of 80 to 350 cm−1. The FIR spec-
˚
˚
Se: 2.589(2)– 2.753(2) A) [23] and those in compound trum displays absorptions at about 277, 265, 248, 164,
4 (2.629(1) and 2.676(1) A). The AgSe4 tetrahedron is 140, 107, and 90 cm−1, and the Raman spectrum shows
˚
strongly distorted with Se-Ag-Se angles ranging from resonances at about 287, 276, 268, 249, 156, 142, and
86.38(3)◦ to 126.81(3)◦.
120 cm−1. As a result of the lower site symmetry of
According to Lu and Ibers [20], the isostructural the Ta, Ag(1), and Ag(2) atoms, the absorptions in
2−
compound K3CuNb2Se12 contains a Se4
polyse- the FIR and Raman spectra of compound 4 are split-
lenide anion with Se-Se distances between 2.389(3), ted. On the basis of previous reports [27], the peaks
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Y. Wu et al. · Low Dimensional Materials
1013
Fig. 9. FIR and Raman spectra of Rb2CuTaS4 (1) in the range
Fig. 10. FIR and Raman spectra of K3Ag3Ta2Se8 (4) in the
of 80 to 500 cm−1
.
range of 80 to 350 cm−1
.
in the Raman spectrum at 287, 268, and 249 cm−1
can be assigned to Ta(Ag) – Se symmetric stretch-
ing modes, the band at 276 cm−1 to an asymmet-
ric vibration. The resonances at lower energies (200–
100 cm−1) may be assigned to the Ag – Se valence
vibrations (156(νs) and 142 cm−1 (νas)) and the Se –
Ta(Ag)– Se deformation mode, respectively. The FIR
spectrum is in good agreement with the Raman spec-
trum. Again the peaks at 277, 265, 248, 164, 140 cm −1
are due to Ta(Ag) – Se stretching modes, the absorp-
tions at 107 and 90 cm−1 are assignable to deformation
modes of Se – Ta(Ag)– Se and the unit cell vibration,
respectively.
In conclusion, the five low-dimensional com-
pounds Rb2CuTaS4, Rb2CuTaSe4, RbCu2TaSe4,
K3Ag3Ta2Se8, and Rb3AgTa2Se12 have been syn-
thesized and characterized. The presence of different
low-dimensional anions in these compounds obtained
from a polychalcogenide flux suggests possible new
phases formed by combinations of various poly-
chalcogenide anions and different building units of Ta
atoms.
Acknowledgements
Financial support by the State of Schleswig-Holstein and
the Deutsche Forschungsgemeinschaft (DFG) is gratefully
acknowledged.
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1014
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