KBiMS4 (M=Si, Ge): Synthesis, structure, and electronic structure

Article abstract of DOI:10.1016/j.jssc.2010.05.003

Two new bismuth sulfides KBiSiS₄ and KBiGeS₄ have been synthesized by means of the reactive flux method. They adopt the RbBiSiS₄ structure type and crystallize in space group P2₁/c of the monoclinic system. The structure consists of ₂^∞[Bi M S₄^-] (M=Si, Ge) layers separated by bicapped trigonal-prismatically coordinated K atoms. The M atom is tetrahedrally coordinated to four S atoms and the Bi atom is coordinated to a distorted monocapped trigonal prism of seven S atoms. The optical band gap of 2.25(2) eV for KBiSiS₄ was deduced from the diffuse reflectance spectrum. From a band structure calculation, the optical absorption for KBiSiS₄ originates from the ₂^∞[BiSiS₄^-] layer. The Si 3p orbitals, Bi 6p orbitals, and S 3p orbitals are highly hybridized near the Fermi level. The orbitals of K have no contributions on both the upper of valence band and the bottom of conduction band.

Full text of DOI:10.1016/j.jssc.2010.05.003

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Journal of Solid State Chemistry 183 (2010) 1640–1644
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
KBiMS4 (M¼Si, Ge): Synthesis, structure, and electronic structure
Dajiang Mei a,b, Zheshuai Lin a, Lei Bai a, Jiyong Yao a,n, Peizhen Fu a, Yicheng Wu a
a Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new bismuth sulfides KBiSiS4 and KBiGeS4 have been synthesized by means of the reactive flux
Received 27 February 2010
Received in revised form
26 April 2010
method. They adopt the RbBiSiS4 structure type and crystallize in space group P21/c of the monoclinic
1
system. The structure consists of ½BiMSꢀ4 ꢁ (M¼Si, Ge) layers separated by bicapped trigonal-
2
prismatically coordinated K atoms. The M atom is tetrahedrally coordinated to four S atoms and the Bi
Accepted 2 May 2010
Available online 6 May 2010
atom is coordinated to a distorted monocapped trigonal prism of seven S atoms. The optical band gap of
2.25(2) eV for KBiSiS4 was deduced from the diffuse reflectance spectrum. From a band structure
1
calculation, the optical absorption for KBiSiS4 originates from the ½BiSiSꢀ4 ꢁ layer. The Si 3p orbitals, Bi
Keywords:
Bismuth
2
6p orbitals, and S 3p orbitals are highly hybridized near the Fermi level. The orbitals of K have no
contributions on both the upper of valence band and the bottom of conduction band.
& 2010 Elsevier Inc. All rights reserved.
Sulfide
Synthesis
Crystal structure
Optical
Band structure
1. Introduction
adopt centrosymmetric structures although the Bi3+ cations
possess a structurally active lone pair of electrons and Si and Ge
adopt tetrahedral coordination in the structures. However several
reports have shown that the size of the alkali metal has great
influence on the crystal symmetry. For example, LiAsS2 is non-
centrosymmetric while NaAsS2 is centrosymmetric and the non-
centrosymmetric structure of Li1ꢀxNaxAsS2 holds up to 40%
sodium [16]. Another example is the A3Ta2AsS11 (A¼K, Rb, Cs)
series of compo1unds: the larger Cs+ favors the centrosymmetric
smaller Rb+ and K+ cations favor non-centrosymmetric packing
[17]. These results inspire us to reinvestigate the A/Bi/M/Q
(A¼alkali metal, M¼Si, Ge; Q¼chalcogen) with emphasis on
smaller alkali metals (i.e. K, Na, Li) in the hope of discovering new
NCS structure with interesting properties. Our exploration has led
to the discovery of two new K compounds, namely KBiSiS4 and
KBiGeS4. Here we report the synthesis, structure, optical property,
and band structure of these two compounds.
Bismuth chalcogenides have received considerable attention
due to their amazing structural and compositional complexity [1]
and their interesting properties [2–8]. The stereochemical activity
of the 6s2 lone pair of electrons influences the structure type, the
electronic structure, and thus the properties of Bi chalcogenides.
In the past 20 years, exploratory research has led to the discovery
of many new bismuth chalcogenides with intriguing properties
including thermoelectric and nonlinear optical properties [6–8].
Many recent reports on bismuth chalcogenides have focused on
generating non-centrosymmetric (NCS) structures since Bi3+ is
susceptible to the second-order Jahn-Teller effect owing to the 6s2
lone pair of electrons [8–13]. The combination of Bi3+ with
cations in tetrahedral coordination geometry often results in NCS
structures. Examples include the compounds Cs5BiP4Se12 [8]
ABi2CuS4 (A¼K, Cs) [9,10], KBiP2S6 [11], K9Bi(PS4)4 [12], and
Cu2Pb2Bi6S12 [13]. NCS compounds are of great interest because
they may possess technologically important properties, such as
the piezoelectric, ferroelectric, and nonlinear optical (NLO)
properties [14]. For example, the compound Cs5BiP4Se12 exhibits
very large second harmonic generation signals [8].
3ꢀ
packing of the ½Ta2AsS ꢁ polymeric anionic chains, while the
1
11
2. Experimental section
In the A/M/Bi/Q (A¼alkali metal; M¼Si, Ge; Q¼chalcogen)
system, four compounds, namely RbBiSiS4, RbBiGeS4, CsBiSiS4,
and CsBiGeS4 have been reported [15]. Unfortunately, they all
2.1. Synthesis
The following reagents were used: K (Sinopharm Chemical
Reagent Co., Ltd., 98+%), Si (Sinopharm Chemical Reagent Co., Ltd.,
99.5%), Ge (Sinopharm Chemical Reagent Co., Ltd., 99.999%),
n Corresponding author. Fax: +86 10 82543452.
Bi (Sinopharm Chemical Reagent Co., Ltd., 99.999%), and
S
E-mail address: jyao@mail.ipc.ac.cn (J. Yao).
(Sinopharm Chemical Reagent Co., Ltd., 99.5%). K2S3, the reactive
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.05.003

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D. Mei et al. / Journal of Solid State Chemistry 183 (2010) 1640–1644
1641
flux [18] employed in the synthesis, was prepared by stoichio-
Table 2
2
˚
Atomic coordinates and equivalent isotropic displacement parameters (A ) for
KBiSiS4 and KBiGeS4.
metric reaction of the elements in liquid NH3. Reaction mixtures
of 1 mmol of K2S3, 1.5 mmol of M (M¼Si, Ge), 0.5 mmol of Bi, and
4.8 mmol of S were loaded into fused-silica tubes under an Ar
atmosphere in a glovebox. These tubes were sealed under a
10ꢀ3 Pa atmosphere and then placed in a computer-controlled
furnace. The samples were heated to 473 K in 20 h, kept at 473 K
for 10 h, then heated to 873 K in 24 h, kept at 873 K for 72 h,
slowly cooled at 4 K/h to 373 K, and then cooled to room
temperature.
The reaction mixtures were washed free of flux with N,
N-dimethylformamide and then dried with acetone. The products
consist of red plates of KBiSiS4 and KBiGeS4 with about 40% yield
(based on Bi). Analyses of these compounds with an EDX-
equipped Hitachi S-3500 SEM showed the presence of K, M, Bi,
and S. The compounds are moderately stable in air. Analysis of the
byproducts indicated that they were mainly ternary A/Bi/S
compounds.
Atom
x
y
z
Ueq
K
0.21028(13)
0.73492(2)
0.29847(16)
0.49453(13)
0.08739(14)
0.51388(14)
0.10759(14)
0.74518(12)
0.18058(2)
0.26957(15)
0.01032(13)
0.24036(14)
0.51006(14)
0.31249(13)
0.96350(5)
0.01060(18)
0.00508(5)
0.0049(2)
Bi
0.781160(8)
0.84677(6)
0.86274(5)
0.91834(6)
0.87375(5)
0.72050(5)
Si
S1
S2
S3
S4
KBiGeS4
K
0.00631(18)
0.00726(19)
0.00720(19)
0.00605(18)
0.2875(3)
0.23494(5)
0.20436(13)
0.0147(3)
0.4190(3)
0.0019(3)
0.4024(3)
0.7414(3)
0.67750(4)
0.26785(12)
0.0167(3)
0.2317(3)
0.5047(3)
0.3219(3)
0.03605(11)
0.282715(18)
0.15286(5)
0.0180(4)
0.00816(11)
0.00669(19)
0.0116(4)
0.0116(5)
0.0100(4)
0.0096(4)
Bi
Ge
S1
S2
S3
S4
0.37413(12)
0.07928(12)
0.36212(12)
0.28254(12)
Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
2.2. Structure determination
Single-crystal X-ray diffraction data were collected with the
Table 3
use of graphite-monochromatized Mo K
a radiation (l¼0.71073
˚
Selected bond lengths (A) for KBiMS4 (M¼Si, Ge).
˚
A) at 93 K on a Rigaku AFC10 diffractometer equipped with a
Saturn CCD detector. Crystal decay was monitored by re-
collecting 50 initial frames at the end of data collection. The
collection of the intensity data was carried out with the program
Crystalclear [19]. Cell refinement and data reduction were carried
out with the use of the program Crystalclear [19], and face-
indexed absorption corrections were performed numerically with
the use of the program XPREP [20].
KBiSiS4
KBiGeS4
KBiSiS4
KBiGeS4
M-S1
M-S2
M-S3
M-S4
Bi-S1
Bi-S1
2.1264(13)
2.1127(14)
2.0937(14)
2.1546(15)
2.6570(10)
3.311(2)
2.180(2)
2.196(2)
2.216(2)
2.239(2)
2.904(2)
3.377(2)
Bi-S2
Bi-S3
Bi-S3
Bi-S4
Bi-S4
2.7518(14)
2.8846(13)
3.305(2)
2.755(2)
2.640(2)
3.347(2)
2.664(2)
3.110(2)
2.6850(10)
3.0439(11)
The structures were solved with the direct methods program
SHELXS and refined with the least-squares program SHELXL of the
SHELXTL.PC suite of programs [20]. Each final refinement
included anisotropic displacement parameters and a secondary
extinction correction. The program STRUCTURE TIDY [21] was
then employed to standardize the atomic coordinates. Additional
experimental details are given in Table 1 and selected metrical
data are given in Tables 2 and 3. Further information may be
found in Supplementary material.
2.4. Theoretical calculation
The electronic property calculations were performed using the
first principles plane-wave pseudopotential method [22] imple-
mented in the CASTEP package [23]. Normal-conserving pseudo-
potentials [24,25] are adopted with the 1s, 2s, and 2p electrons for
potassium, silicon and oxygen treated as core electrons. For
bismuth, 5d, 6s, and 6p electrons are chosen as the valence
electrons. A kinetic energy cutoff of 700 eV is employed, and the
local-density approximation is chosen. Monkhorst–Pack [26] k
point meshes with a density of (4 ꢂ 4 ꢂ 2) points in the Brillouin
zone of the unit cell are used.
2.3. Diffuse reflectance spectroscopy
A
Cary 1E UV–visible spectrophotometer with a diffuse
reflectance accessory was used to measure the spectrum of
KBiSiS4 over the range 350 nm (3.54 eV)–850 nm (1.46 eV).
Table 1
a
Crystal data and structure refinements for KBiSiS4 and KBiGeS4
.
KBiSiS4
KBiGeS4
3. Results and discussion
fw
404.41
448.91
3.1. Synthesis
6.4769(13)
6.5981(13)
˚
a (A)
6.7371(13)
17.168(4)
6.8149(14)
17.284(4)
˚
b (A)
The synthetic conditions of the title compounds differed from those
used in the syntheses of the previously reported RbBiSiS4, RbBiGeS4,
CsBiSiS4, and CsBiGeS4 compounds [15]. The S/A2S3 (A¼K, Rb, Cs) ratio
was larger; this less basic reaction condition is important for the
crystallization of the two K compounds. The yield was about 40% in the
synthesis, which was not improved much after several different trials.
The other 60% were black noncrystalline melt consisting of ternary
A/Bi/S compounds, based on EDX. No powder diffraction data were
measured on the black melt since they would not provide much
information. Efforts to synthesize the Na and Li analogues were
unsuccessful, presumably because the Na+ and Li+ cations are too
small to sustain the anionic framework in the structure.
˚
c (A)
(deg)
108.14(3)
711.9(3)
108.46(3)
737.2(3)
b
3
˚
V (A )
rc (g/cm3)
3.773
4.045
m
(cmꢀ1
)
265.70
0.0147
0.0281
294.95
0.0296
0.0571
R(F)b
Rw(F2o)c
a
For both structures Z¼4, space group¼P21/c, T¼93(2) K, and
l
¼0.71073.
P
P
b
c
R(F)¼ jjFoj-jFcjj/ jFoj for F2o 42
s
(F2o).
ꢀ
ꢁ
ꢀ
ꢁ
P
P
1=2
2
2
2
2
4
2
2
Rw Fo ¼ f ½wðFo ꢀFc Þ ꢁ= wFo
g
for all data. wꢀ1
¼
s2 Fo þðzPÞ ,
where P¼(Max(F2o, 0)+2Fc2)/3; z¼0.02 for KBiSiS4 and 0.03 for KBiGeS4.

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˚
where the Bi–S bond lengths range from 2.598(3) to 3.178(4) A for
3.2. Structure
3+
˚
Bi1 and from 2.640(3) to 3.211(4) A for Bi2. Although the Bi
cation contains a stereoactive lone pair of electrons, the position
of this lone pair is not apparent, either from the distortion of the
Bi coordination sphere or1from the X-ray diffraction data.
The structure of the ½BiSiSꢀꢁ layer in KBiSiS4 is shown in
KBiMS4 (M¼Si, Ge) crystallizes in RbBiSiS4 structure type [15].
There are no S–S bonds or any detectable disorder among M, Bi, or
K atoms in the structure; thus, the oxidation states of 1+, 4+, 3+,
and 2ꢀ can be assigned to K, M, Bi, and S, respectively.
2
4
Fig. 3. The neighboring BiS7 polyhedra share opposite edges of the
rectangular planes of the prisms to form zigzag chains along
the b direction (purple lines). Two parallel polyhedral chains are
connected by the sharing of opposite edges of the SiS4 tetrahedra.
Each SiS4 tetrahedron is arranged in such a way that one of the S
atoms becomes the cap of an adjacent BiS7 polyhedron (blue
lines). Although the caps in a single layer point in the same
The structure of KBiMS4 (M¼Si, Ge) is illustrated in Fig. 1. It
1
consists of ½BiMSꢀ4 ꢁ (M¼Si, Ge) layers separated by K atoms.
2
Each K atom is coordinated to a bicapped trigonal prism of eight
S
atoms. The K–S bond lengths range from 3.2058(14) to
˚
˚
3.5239(14) A in KBiSiS4 and from 3.194(3) to 3.601(3) A in
KBiGeS4. Each M atom is coordinated to a slightly distorted
tetrahedron of four S atoms. The M–S bond lengths range from
˚
direction along the b axis, the caps of neighboring layers point in
2.0937(14) to 2.1546(15) A in KBiSiS4 and from 2.180(2) to
1
alternating directions. The st1ructure of the ½BiSiSꢀ4 ꢁ layer in the
˚
2.239(2) A in KBiGeS4 (Table 3). Each Bi atom is coordinated by
2
KBiSiS4 is similar to that of ½BiSiSꢀꢁ layer in CsBiSiS4 shown in
seven S atoms. The coordination geometry may be described as a
distorted monocapped trigonal prism comprising five close S
atoms at the corners of a square pyramid with Bi near the center
of the basal plane and the sixth and seventh S atoms further away
(Fig. 2). The S atom at the apex of the square pyramid is the cap of
the prism. The largest differences in Bi–S bond lengths within
these BiS7 polyhedra are 0.654(1) A in KBiSiS4 and 0.737(2) A in
KBiGeS4. Earlier calculation has shown that the two longest Bi–S
bonds are mainly ionic in character [15]. This kind of coordination
geometry is common for Bi atom, for example in CsBi3S5 [27],
2
4
1
Fig. 4 and the structure of the ½LnMSꢀ4 ꢁ layer in the ALnMS4
2
(A¼alkali metal, Ln¼rare earth; M¼Si, Ge) series of compounds
[28–39]. The differences between the structures of KBiSiS4 and
CsBiSiS4 (Fig. 3 vs. Fig. 4) include the disposition of the capping S
atoms (blue lines) and the connectivity between BiS7 polyhedra.
In the CsBiSiS4 structure the caps on two sides of a chain point in
the same direction along b axis but the caps on neighboring chains
in the same layer point in alternating directions and each BiS7
polyhedron in one chain is connected to another BiS7 polyhedron
˚
˚
c
b
b
a
c
a
Si
S
Si
S
K
Bi
Bi
Fig. 3. Structure of the 12 ½BiSiS4ꢀꢁ layer in KBiSiS4. Here and in Fig. 4 the blue lines
show the capping S atoms and the purple lines display the connectivity between
BiS7 polyhedra within a chain. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
Fig. 1. Unit cell of the KBiSiS4 structure type viewed along [010].
c
b
a
Si
S
Bi
Fig. 4. Structure of the 12 ½BiSiS4ꢀ
ꢁ
ADVANCE \l 4 layer in CsBiSiS4. (For interpretation of
Fig. 2. Coordination geometry of Bi in KBiSiS4 (the dashed hollow lines represent
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
the two longest Bi–S bonds, which are more ionic in character).

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1643
in a neighboring chain by edging sharing, while in the KBiSiS4
3.4. Band structure calculation
structure all the caps within the layer point in the same direc-
tion and the connection between BiS7 polyhedra is by corner
sharing.
In the ALnMS4 (A¼alkali metal, Ln¼rare earth; M¼Si, Ge)
series of compounds, the two common structure types are
represented by KLaGeS4 (space group P21) [33] and CsSmGeS4
The calculated band structures of the KBiSiS4 crystal in the unit
cell are plotted along the symmetry lines in Fig. 6. Obviously, the
energy band can be divided into three regions; the lower region is
located below ꢀ5 eV, the middle region is the valence band (VB) from
about ꢀ5 to 0 eV, and the upper one is the conduction band (CB) in
which a band of a dispersion spanning about 0.6 eV appears at the
bottom of its conduction bands on the B point. The calculated indirect
band gap is 2.022 eV. This value is a bit smaller than the experimental
optical band gap, which is usual for indirect semiconductors. The
further calculations with other kinds of pseudopotentials show that
the change of the results is not apparent.
Fig. 7 gives the partial DOS (PDOS) projected on the consti-
tutional atoms of the KBiSiS4 crystal, in which several electronic
characteristics can be seen: (i) The K 3s orbitals and the Bi 5d
orbitals are localized at ꢀ28 and ꢀ22 eV (not shown), respec-
tively, in the very deep region of the VB; they are not included in
the atomic bonds. (ii) The VB from ꢀ15 to ꢀ5 eV mainly consist
of the K 4p, Bi 6s, Si 3s and S 3s orbitals. At the very top of the VB
(from 0 to ꢀ5 eV), there is quite strong hybridization among the
p orbitals of Bi, Si, and S atoms, but the S 3p orbitals occupy the
VB maximum. The orbitals of potassium have no contributions on
(space group P212121),[34] while b-KCeSiS4 (space group P21/m)
[36] and CsCeSiS4 (space group Pnma) [39] adopt the other two
less common structure types. In contrast to KBiSiS4, the ALnMS4
compound exhibit much smaller differences in the Ln–S bond
lengths within the LnSn (n¼7, 8) polyhedra. For example, the
˚
˚
largest difference is 0.225(1) A in KLaGeS4 and 0.185(1) A in
CsSmGeS4. Furthermore, in KLaGeS4 the caps of neighboring
layers point in the same direction along the polar b axis, whereas
in CsSmGeS4 the two [SmGeS4ꢀ] slabs in the unit cell are related
by a 21 screw axis along c rather than a center of inversion. For the
two centrosymmetric
b-KCeSiS4 and CsCeSiS4 structures, all
cations are in positions of m symmetry and the rare-earth cations
are in an environment of bicapped trigonal prism. The obvious
difference between these two structures and the KBiSiS4 structure
lies in the coordination of the capping S atom. In both b-KCeSiS4
and CsCeSiS4, the capping S atoms are in an environment of m
symmetry and shared by two Ln atoms, while in KBiSiS4, the
capping S is in a general position and is only connected to one Bi
atom.
5
0
So far, six members of the ABiMS4 family (A¼alkali metal,
M¼Si, Ge) have been found. For the five isostructural compounds,
namely KBiSiS4, RbBiSiS4, KBiGeS4, RbBiGeS4, and CsBiGeS4, the
1
½BiMSꢀ4 ꢁ layers lie within the ab plane and are separated by
2
the A cations along c direction; thus, the a and b dimensions of the
unit cell are mainly determined by the M, Bi and S atom while
the alkali metal A mainly influence the cell dimension along c
direction. In these five members, the a and b axes of compounds
containing the same M are quite similar, so do the c axes of
compounds containing the same A. On the other hand, the
combination of the largest A(Cs) with the smallest M(Si) leads to
the formation of a different structure: CsBiSiS4. Clearly, the
relative sizes of cations have significant influence on the packing
of atoms.
-5
-10
-15
Z G
Y
A
B
D
E
C
3.3. Experimental band gap
Fig. 6. Band structure of KBiSiS4.
The diffuse reflectance spectrum of KBiSiS4 is shown in Fig. 5.
A band gaps of 2.25(2) eV was deduced by the straightforward
extrapolation method [40].
s
p
d
40
20
K
0
8
100
Bi
4
0
80
60
40
20
Si
S
4
0
20
0
350
450
550
650
750
850
-15
-10
-5
0
5
10
Wavelength (nm)
Energy (eV)
Fig. 5. Diffuse reflectance spectrum of KBiSiS4.
Fig. 7. Partial density of states of KBiSiS4.

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both the upper of VB and the bottom of CB. (iii) The CB are mainly
composed of the orbitals of Bi, Si and S atoms, but the bottom of
CB is determined by the Bi 6p orbitals. Moreover, it is found that
some Si 3s orbitals are unoccupied and located at about 4.3 eV due
to the sp3 hybridization in the (SiS4)4ꢀ group.
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Acknowledgment
This research was supported by the National Basic Research
Project of China (No. 2010CB630701). ZSL acknowledges the
Special Foundation of President of Chinese Academy of Sciences.
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Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2010.05.003.
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