Ba2AgInS4 and Ba4MGa5Se 12 (M = Ag, Li): Syntheses, structures, and optical properties

Article abstract of DOI:10.1039/c2dt11895e

The first two members in alkaline-earth/group XI/group XIII/chalcogen system, namely Ba₂AgInS₄ and Ba₄AgGa ₅Se₁₂, were synthesized along with a Li analogue Ba ₄LiGa₅Se₁₂. Ba₂AgInS₄ crystallizes in space group P2₁/c. It contains 2∞[AgInS ₄]^4- layers built from AgS₃ triangles and InS₄ tetrahedra with Ba²⁺ cations inserted between the layers. Ba₄AgGa₅Se₁₂ and Ba ₄LiGa₅Se₁₂ adopt two closely-related structure types in space group P42₁c with structural difference originating from the different positions of Ag and Li in them. The three-dimensional framework in Ba₄AgGa₅Se₁₂ is composed of GaSe₄ tetrahedra with the Ba and Ag atoms occupying the large and small channels respectively, whereas that in Ba₄LiGa ₅Se₁₂ is built from LiSe₄ and GaSe₄ tetrahedra with channels to accommodate the Ba atoms. As deduced from the diffuse reflectance spectra measurement, the optical band gaps were 2.32 (2) eV, 2.52 (2) eV, and 2.65 (2) eV for Ba₂AgInS₄, Ba ₄AgGa₅Se₁₂, and Ba₄LiGa ₅Se₁₂, respectively. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt11895e

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PAPER
Ba2AgInS4 and Ba4MGa5Se12 (M = Ag, Li): syntheses, structures, and
optical properties†
Wenlong Yin,a,b,c Kai Feng,a,b,c Dajiang Mei,a,b,c Jiyong Yao,*a,b Peizhen Fua,b and Yicheng Wua,b
Received 8th October 2011, Accepted 22nd November 2011
DOI: 10.1039/c2dt11895e
The first two members in alkaline-earth/group XI/group XIII/chalcogen system, namely Ba2AgInS4
and Ba4AgGa5Se12, were synthesized along with a Li analogue Ba4LiGa5Se12. Ba2AgInS4 crystallizes in
space group P21/c. It contains 2•[AgInS4]4- layers built from AgS3 triangles and InS4 tetrahedra with
Ba2+ cations inserted between the layers. Ba4AgGa5Se12 and Ba4LiGa5Se12 adopt two closely-related
¯
structure types in space group P421c with structural difference originating from the different positions
of Ag and Li in them. The three-dimensional framework in Ba4AgGa5Se12 is composed of GaSe4
tetrahedra with the Ba and Ag atoms occupying the large and small channels respectively, whereas that
in Ba4LiGa5Se12 is built from LiSe4 and GaSe4 tetrahedra with channels to accommodate the Ba atoms.
As deduced from the diffuse reflectance spectra measurement, the optical band gaps were 2.32 (2) eV,
2.52 (2) eV, and 2.65 (2) eV for Ba2AgInS4, Ba4AgGa5Se12, and Ba4LiGa5Se12, respectively.
ionic [Ga4Se10]10- cluster composed of two Ga(Se)4 tetrahedra and
Introduction
two Ga(Ga)(Se)3 tetrahedra with Ga in the 2+/3+ valence states.
In this paper, we extend our exploration to the quaternary
A/M/M¢/Q (A = alkaline-earth metal; M = Cu, Ag, Au; M¢ =
Al, Ga, In; Q = S, Se, Te) system. Surprisingly, no compounds
have been reported in this system so far. The combination of
metal cations with different bonding nature, formal charge, and
size in the quaternary A/M/M¢/Q system would greatly increase
the possibility of forming new compounds with interesting sto-
ichiometries, structures, and physical properties. Our synthetic
efforts have led to the discovery of the first two members in
this family, namely Ba2AgInS4 and Ba4AgGa5Se12. We further
obtained a closely related compound Ba4LiGa5Se12 by replacing
Ag with Li in Ba4AgGa5Se12. They crystallize in three new types of
structure. In this paper, we report the syntheses, structural charac-
terizations, and optical properties of Ba2AgInS4, Ba4AgGa5Se12,
and Ba4LiGa5Se12.
Over the past decades, a large number of multinary chalcogenides
with two or more metal cations have been synthesized, which
exhibited interesting structures and thermoelectric, magnetic,
transport, and optical properties.1–16 Most of these compounds
contained at least one alkali metal because the low-melting binary
alkali metal polychalcogenides A2Qn (A = alkali metal; Q = S,
Se, Te; n ≥ 1) can be used as a “reactive flux” to provide alkali
metal and aid in crystal growth.17 In contrast, the number of
multinary chalcogenides containing an alkaline-earth metal is
much lower, probably because the high-melting binary alkaline-
earth metal chalcogenides are poor reactive fluxes. However, recent
research demonstrated that the traditional high-temperature solid
state reaction is still a valid method to isolate new multinary
chalcogenides containing alkaline-earth metals, as indicated by
the successful synthesis of BaM4Se7 (M = Al, Ga),18,19 Ba5M2Se8
(M = Al, Ga),20 Ba12In4S19,21 Ba4M2S8 (M = Ga, In),21 Ba5Ga4Se10,22
and Ba2BiMS5 (M = Ga, In),23 which exhibit interesting structures
and physical properties.18–23 For example, the BaM4Se7 (M = Al,
Ga) and Ba2BiInS5 compounds exhibit strong non-linear optical
(NLO) response; Ba12In4S19 and Ba4M2S8 (M = Ga, In) possess the
disulfide S22- anions; and Ba5Ga4Se10 contains the novel highly an-
Experimental section
Syntheses
The following reagents were used as obtained: Li (Sinopharm
Chemical Reagent Co., Ltd., 98%), Ag (Sinopharm Chemical
Reagent Co., Ltd., 99.99%), Ba (Sinopharm Chemical Reagent
Co., Ltd., 98%), Ga (Sinopharm Chemical Reagent Co., Ltd.,
99.99%), In (Sinopharm Chemical Reagent Co., Ltd., 99.99%), S
(Sinopharm Chemical Reagent Co., Ltd., 99.99%), Se (Sinopharm
Chemical Reagent Co., Ltd., 99.99%), and BaS (Sinopharm
Chemical Reagent Co., Ltd., 98%).
aCenter for Crystal Research and Development, Technical Institute of Physics
and Chemistry, Chinese Academy of Sciences, Beijing, 100190, P.R. China.
E-mail: jyao@mail.ipc.ac.cn
bKey Laboratory of Functional Crystals and Laser Technology, Chinese
Academy of Sciences, Beijing, 100190, P.R. China
cGraduate University of Chinese Academy of Sciences, Beijing, 100049, P.R.
China
The binary starting materials, Ag2S, Ag2Se, BaSe, In2S3, and
Ga2Se3, were synthesized by the stoichiometric reactions of
elements at high temperatures in sealed silica tubes evacuated to
† CCDC reference numbers 847971–847973. For crystallographic data in
CIF or other electronic format for Ba2AgInS4 and Ba4MGa5Se12 (M = Ag,
Li) see DOI: 10.1039/c2dt11895e
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10-3 Pa, while Li2Se was prepared by the stoichiometric reaction of
Li with Se in liquid NH3 at 194 K. The ternary AgInS2, AgGaSe2,
and LiGaSe2 were synthesized by the stoichiometric reaction of
the binary chalcogenides (Ag2S and In2S3 for AgInS2; Li2Se and
Ga2Se3 for LiGaSe2) or the constituent elements (Ag, Ga, and Se
for AgGaSe2) at high temperatures in sealed silica tube evacuated
to 10-3 Pa.
Table 1 Crystal data and structure refinements for Ba2AgInS4,
Ba4AgGa5Se12, and Ba4LiGa5Se12
Ba2AgInS4
Ba4AgGa5Se12
Ba4LiGa5Se12
Fw
625.61
1953.35
1852.42
˚
a/A
8.343(2)
7.258(2)
14.544(5)
106.80(3)
843.1(4)
Monoclinic
P21/c
13.654(2)
13.654(2)
6.522(1)
90.00
13.591(2)
13.591(2)
6.515(1)
90.00
˚
b/A
˚
c/A
b (◦)
Ba2AgInS4. The reaction mixture of 1 mmol BaS and 0.5 mmol
AgInS2 was ground and loaded into a fused-silica tube under
an Ar atmosphere in a glovebox. The tube was sealed under a
10-3 Pa atmosphere and then placed in a computer-controlled
furnace. The reaction mixture was heated to 1273 K in 15 h,
kept at 1273 K for 48 h, followed by slow cooling to 793 K at
a rate of 4 K h-1, and finally cooled to room temperature by
switching off the furnace. The product consisted of orange single
crystals of Ba2AgInS4, which were manually selected for structure
characterization. Analyses of the crystals with an EDX-equipped
Hitachi S-4800 SEM showed the presence of Ba, Ag, In, and S in
the approximate molar ratio of 2 : 1 : 1 : 4. The crystals are stable
in air.
3
˚
V/A
1215.9(3)
Tetragonal
1203.3(3)
Tetragonal
Crystal system
Space group
Z
¯
¯
P421c
P421c
4
2
2
Meas./Indep. reflns
Rint
7051/2192
0.042
4.929
15.095
0.0358
10 337/1629
0.054
5.335
30.542
0.0281
0.0522
10 608/1764
0.061
5.113
30.071
0.0332
0.0660
rc/g cm-3
m/cm-1
R(F)a
2
b
RW(Fo
)
0.0825
ꢀ
ꢀ
2
a R(F) =
||Fo| - |Fc||/ |Fo| for Fo > 2s(Fo2). b Rw(Fo2) =
ꢀ
ꢀ
1
2
2
4
2
{
[w(Fo - Fc2)2]/ wFo
}
for all data. w-1 = s (Fo2) + (zP)2, where
P = (max(Fo2, 0) + 2Fc2)/3.
and the other is coordinated to a tetrahedron of four S with
Ba4MGa5Se12 (M = Ag, Li). Reaction mixtures of 1 mmol BaSe
and 1 mmol MGaSe2 (M = Ag, Li) were ground and loaded into
fused-silica tubes under an Ar atmosphere in a glovebox. These
tubes were sealed under a 10-3 Pa atmosphere and then placed in a
computer-controlled furnace. The reaction mixtures were heated
to 1273 K in 15 h, kept at 1273 K for 48 h, followed by slow
cooling to 793 K at a rate of 4 K h-1, and finally cooled to room
temperature by switching off the furnace. The products consisted
of yellow single crystals of Ba4MGa5Se12 (M = Ag, Li), which
were manually selected for structure characterization. Analyses of
the crystals with an EDX-equipped Hitachi S-4800 SEM showed
the presence of Ba, Ag, Ga, and Se in the approximate molar
ratio of 4 : 1 : 5 : 12 in Ba4AgGa5Se12 and the presence of Ba, Ga,
and Se in the approximate ratio of 4 : 5 : 12 in Ba4LiGa5Se12 (Li is
undetectable in EDX). The crystals are stable in air.
˚
the shortest M–S distance being about 2.42 A. Although it is
difficult for X-ray to distinguish between Ag and In, the differences
in the coordination preference and normal M–S bond distances
between Ag and In atoms give some clue to the assignment of atom
types: firstly, the triangle coordination environment is common
for the Ag atom,26–29 but rare for the In atom in chalcogenides;
secondly, the In–S is usually shorter than the Ag–S, as shown in
AgInS2.30 Thus we were able to assign the three-coordinated metal
position to Ag and the four-coordinated metal position to In. The
calculated bond valence sums31,32 were 0.98 and 3.12 for Ag and
In atoms respectively, which proved our assignment of the atom
types. The final refinement included anisotropic displacement
parameters and a secondary extinction correction. The program
STRUCTURE TIDY33 was then employed to standardize the
atomic coordinates. Additional details and structural data are
given in Tables 1–3 and further information may be found in
Supporting Information.†
Structure determination
Single-crystal X-ray diffraction data were collected with the use
Diffuse reflectance spectroscopy
˚
of graphite-monochromatized Mo-Ka (l = 0.71073 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.24 Cell
refinement and data reduction were carried out with the use of the
program Crystalclear,24 and face-indexed absorption corrections
were performed numerically with the use of the program XPREP.25
The structures were solved with Direct Methods implemented
in the program SHELXS and refined with the least-squares
program SHELXL of the SHELXTL.PC suite of programs.25
For the refinement of Ba4AgGa5Se12, the site occupancy of the
Ag position was allowed to refine in the beginning because
the isotropic displacement parameters were unusually large. The
resultant occupancy was close to 0.5 and it was then fixed to
be 0.5 to achieve charge balance in the subsequent refinements.
For Ba2AgInS4, there are two crystallographically unique metal
positions for Ag and In atoms. One is coordinated to a triangle
A Cary 1E UV-visible spectrophotometer with a diffuse re-
flectance accessory was used to measure the spectra of Ba2AgInS4,
Ba4AgGa5Se12 and Ba4LiGa5Se12 over the range 250 nm (4.96 eV)
to 2500 nm (0.50 eV).
˚
Table 2 Selected interatomic distances (A) in Ba2AgInS4
Ba2AgInS4
Distance
Ba2AgInS4
Distance
Ba1–S3
Ba1–S2
Ba1–S4
Ba1–S2
Ba1–S1
Ba1–S3
Ba1–S1
Ba2–S3
Ba2–S2
Ba2–S3
Ba2–S4
3.128(2)
3.143(2)
3.185(2)
3.203 (2)
3.210(2)
3.216(2)
3.295(2)
3.138(2)
3.146 (2)
3.195(2)
3.205(2)
Ba2–S2
Ba2–S1
Ba2–S4
In–S2
In–S1
In–S4
3.219(2)
3.231(2)
3.314(2)
2.419(2)
2.451(2)
2.458(2)
2.527(2)
2.518(2)
2.538(2)
2.544(2)
In–S3
Ag–S1
Ag–S4
Ag–S3
˚
of three S with the shortest M–S distance being about 2.52 A
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˚
Table 3 Selected interatomic distances (A) for Ba4LiGa5Se12 and
Ba4AgGa5Se12
Ba4LiGa5Se12
Ba4AgGa5Se12
Ba–Se1
Ba–Se1
Ba–Se1
Ba–Se1
Ba–Se2
Ba–Se2
Ba–Se3
Ba–Se3
Ag–Se1 ¥ 2
Ag–Se1 ¥ 2
Ga1–Se1
Ga1–Se2
Ga1–Se2
Ga1–Se3
Ga2–Se3 ¥ 4
3.3679(9)
3.388(1)
3.420(1)
3.693(1)
3.3026(9)
3.3408(9)
3.3135(9)
3.5216(9)
2.572(2)
2.590(2)
2.380(1)
2.403(1)
2.426(1)
2.389(1)
2.4003(7)
Ba–Se1
Ba–Se1
Ba–Se1
Ba–Se1
Ba–Se2
Ba–Se2
Ba–Se3
Ba–Se3
Li–Se3 ¥ 4
Ga1–Se1
Ga1–Se2
Ga1–Se2
Ga1–Se3
Ga2–Se3 ¥ 4
3.305(1)
3.376(1)
3.381(1)
3.405(1)
3.343(1)
3.387(1)
3.345(1)
3.705(1)
2.4938(8)
2.362(1)
2.404(1)
2.427(1)
2.402(1)
2.3997(8)
Fig. 2 Structure of 2•[AgInS4]4- layer in Ba2AgInS4.
34
˚
of 2.459(2) A in AgInS2. Ba1 and Ba2 are both coordinated to
a monocapped trigonal prism of seven S atoms with the Ba–S
Results and discussion
˚
distances ranging from 3.128(2) to 3.314(2) A, which resemble
Structure
35
˚
those in Ba4Cr2US9 (3.120(2)–3.510(2) A). As there is no S–S
bonding in the structure, the oxidation states of 2+, 1+, 3+, and
2- can be assigned to Ba, Ag, In, and S atoms, respectively.
Ba2AgInS4. Ba2AgInS4 crystallizes in a new structure type in
the centrosymmetric space group P21/c. The asymmetric unit con-
tains two crystallographically unique Ba atom, one Ag atom, one
In atom, and four S atoms, all at general positions. As illustrated
in Fig. 1, the structure of Ba2AgInS4 contains two-dimensional
2•[AgInS4]4- layers separated by Ba atoms. The interesting structure
Ba4MGa5Se12 (M = Ag, Li). They crystallize in two closely-
related new structure types in the non-centrosymmetric space
¯
group P421c of the tetragonal system. For Ba4AgGa5Se12, there
is one crystallographically independent Ba atom, one Ag atom,
two Ga atoms, and three Se atoms in the asymmetric unit.
The Ag atom lies in the Wyckoff position 4d with 2-fold axis
symmetry and 50% occupancy, while the Ga2 atoms lies at Wyckoff
of the 2 [AgInS4]4- layer is shown in Fig. 2. The AgS3 triangles
•
and InS4 tetrahedra are connected via corner-sharing to form
a
1•[AgInS5]6- chain along the a direction. Adjacent 1 [AgInS5]6-
•
chains are shifted by an AgS3 triangle (or an InS4 tetrahedron)
¯
position 2a with 4 axis symmetry. All other atoms are in general
along the a axis and further linked to each other by common
positions. The structural difference between Ba4LiGa5Se12 and
Ba4AgGa5Se12 originates from the position of Li in Ba4LiGa5Se12,
2
S atoms to produce the two-dimensional [AgInS4]4- layer. Such
•
connectivity also generates the 12-membered [Ag3Ga3S6] rings in
the layer. S1, S2, and S4 atoms are connected to both Ag and In
atoms, while S2 lying inside the [Ag3Ga3S6] ring is connected to
the In atom only. Such a structure is observed for the first time in
compounds containing M/M¢/Q (M = Cu, Ag, Au; M¢ = Al, Ga,
In; Q = S, Se, Te).
¯
which is found in the Wyckoff position 2a with 4 axis symmetry
and 100% occupancy, rather than the 4d position of Ag in
Ba4AgGa5Se12. Other atoms in Ba4LiGa5Se12 are located in similar
positions as those in Ba4AgGa5Se12. Considering the obvious
˚
larger Ag–Se distances (2.572(2)–2.590(2) A) than the Li–Se
˚
distances (2.4938(8) A), the position occupied by the Ag atom
in Ba4AgGa5Se12 may be too large for Li in Ba4LiGa5Se12 and
vice versa. Because there are no Se–Se bonds in either structure,
the oxidation states of 2+, 1+, 3+, and 2- can be assigned to Ba,
Ag(Li), Ga, and Se, respectively.
The structures of Ba4MGa5Se12 (M = Ag, Li) are shown in
Fig. 3 and Fig. 4. Despite belonging to the same space group,
their structures have some differences as a result of the different
positions of Li and Ag as discussed above. In the structure of
Ba4AgGa5Se12, The Ga1Se4 tetrahedra are connected by corner-
sharing to form zigzag 1 [GaSe3]3- chains along the c direction
•
and these chains are further connected by the Ga2Se4 tetrahedra
2
along the a direction to form a [Ga3Se10]11- layer parallel to the
•
(110) plane (Fig. 5), which are then joined together via Ga1Se4
tetrahedra along the [110] direction forming a three-dimensional
framework with large and small channels along the c direction
occupied by the Ba and Ag atoms, respectively (Fig. 4).
Fig. 1 Structure of Ba2AgInS4 viewed along [100].
Selected bond distances are listed in Table 2. The Ag–S distances
As for the structure of Ba4LiGa5Se12, the Ga2Se4 tetrahedra and
the LiSe4 tetrahedra are connected alternately to each other by
˚
of 2.518(2)–2.544(2) A are consistent with those of 2.506(2)–
29
1
2.650(2) A for the three-coordinated Ag in CsAg3S2, whereas
edge-sharing to form one-dimensional [LiGaSe4]4- chains along
˚
•
˚
the In–S distances of 2.419(2)–2.527(2) A are comparable to those
the c direction (Fig. 6). The Ga1Se4 tetrahedra also form zigzag
2274 | Dalton Trans., 2012, 41, 2272–2276
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Fig. 6 The 2•[Li2GaSe6]7- layer in Ba4LiGa5Se12.
Fig. 3 Structure of Ba4AgGa5Se12 viewed along [001].
those in BaGa4Se7 (3.429(2)–3.861(2) A).19 The Li–Se distances of
˚
˚
2.4938(8) A in Ba4LiGa5Se12 and the Ag–Se distances of 2.572(2)–
˚
2.590(2) A in Ba4AgGa5Se12 are comparable to those of 2.493(1)–
2.565(1) A in LiGaSe2 and 2.585(1) A in AgGaSe2,37 respectively.
36
˚
˚
˚
The Ga–Se distances range from 2.362(1) to 2.427(1) A in
˚
Ba4LiGa5Se12 and from 2.380(1) to 2.426(1) A in Ba4AgGa5Se12,
which are in agreement with those in BaGa4Se7 (2.361(2)–
19
˚
2.488(2) A).
Experimental band gaps
The diffuse reflectance spectra of the three compounds are shown
in Fig. 7. The optical band gaps were 2.32(2) eV, 2.52(2) eV,
and 2.65(2) eV for Ba2AgInS4, Ba4AgGa5Se12, and Ba4LiGa5Se12,
respectively, as deduced by the straightforward extrapolation
method.38 The band gap of the Ba2AgInS4 is about 0.4 eV larger
39
that of AgInS2 (1.9 eV)
and that of Ba4AgGa5Se12 is about
0.7 eV larger than that of AgGaSe2 (1.8 eV).40 In Ba4AgGa5Se12,
the three-dimensional framework is mainly composed of the GaSe4
tetrahedra, so Ga 4s and 4p orbitals will contribute more to the
bottom of the conduction band in Ba4AgGa5Se12, while the bottom
of the conduction band is mainly determined by the Ag 4d orbitals
in AgGaSe2.41 Considering the higher energy levels of Ga 4s and 4p
orbitals, it is understandable the Ba4AgGa5Se12 possesses a larger
Fig. 4 Structure of Ba4LiGa5Se12 viewed along [001].
Fig. 5 The 2•[Ga3Se10]11- layer in Ba4AgGa5Se12.
1•[GaSe3]3- chains along the c direction (Fig. 6). These two kinds
2
of chains are connected alternately to form a [Li2GaSe6]7- layer
•
parallel to the (110) plane (Fig. 6), which are then connected by
the Ga1Se4 along the [110] direction to form a three-dimensional
framework with channels along the c direction, which are occupied
by the Ba atoms (Fig. 4).
In both structures, the Ba atom is coordinated to a bicapped
trigonal prism of eight Se atoms with the Ba–Se distances ranging
˚
from 3.305(1) to 3.705(1) A in Ba4LiGa5Se12 and from 3.3026(9)
Fig. 7 Diffuse reflectance spectra of Ba2AgInS4 and Ba4MGa5Se12 (M =
Ag, Li).
˚
to 3.693(1) A in Ba4AgGa5Se12 (Table 3), which are close to
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band gap than AgGaSe2. As for Ba2AgInS4, orbitals of Ba atoms
should make contributions to the bottom of the conduction band
as differences in the energy level among the orbitals of the Ba, Ag,
and In atoms become small. Because orbitals of Ba still possess
higher energy levels than those of Ag and In atoms around the
Fermi level, such hybridization leads to the larger band gap in
Ba2AgInS4 than AgInS2.
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Exploration has led to the discovery of the first two members in
the unexplored quaternary A/M/M¢/Q (A = alkaline-earth metal;
M = Cu, Ag, Au; M¢ = Al, Ga, In; Q = S, Se, Te) system, namely
Ba2AgInS4 and Ba4AgGa5Se12, as well as a lithium analogue
Ba4LiGa5Se12. Ba2AgInS4 crystallizes in a new structure type in
space group P21/c. In the structure, the AgS3 triangles and InS4
tetrahedra are connected via corner-sharing to form 1 [AgInS5]6-
chains. Adjacent chains are shifted by an AgS3 triangle along the
•
a axis and are connected via the common S atoms to form the
two dimensional 2 [AgInS4]4- layers separated by Ba atoms. The
•
interesting connectivity within the 2 [AgInS4]4- layer is observed
•
for the first time in compounds containing M/M¢/Q (M = Cu,
Ag, Au; M¢ = Al, Ga, In; Q = S, Se, Te).
Ba4MGa5Se12 (M = Ag, Li) crystallizes in two closely-related
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¯
new structure types in non-centrosymmetric space group P421c
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of the tetragonal system. Their structural difference lies in the
positions of Ag and Li. The Ag atom lies in the Wyckoff position 4d
with 2-fold axis symmetry and 50% occupancy in Ba4AgGa5Se12,
¯
while the Li atom occupies the Wyckoff position 2a with 4 axis
symmetry and 100% occupancy in Ba4LiGa5Se12. As a result,
Ba4AgGa5Se12 possesses a three-dimensional framework built
from GaSe4 with the Ba and Ag atoms occupying the large and
small channels respectively, while the three-dimensional frame-
work in Ba4LiGa5Se12 is built from LiSe4 and GaSe4 tetrahedra
with channels along the c direction to accommodate the Ba atoms.
The band gaps of Ba2AgInS4 and Ba4AgGa5Se12 are more than
0.4 eV larger than those of AgInS2 and AgGaSe2. Clearly, the
introduction of strongly ionic Ba atom could not only break the
dense covalent-bonded Ag–M–Q framework (M = Ga, In; Q =
S, Se) in the Ag/M/Q compounds, generating channels or layers,
but also increase the optical band gaps.
Acknowledgements
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This research was supported by the National Basic Research
Project of China (No. 2010CB630701) and National Natural
Science Foundation of China (No. 51072203).
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2276 | Dalton Trans., 2012, 41, 2272–2276
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