Synthesis and characterization of quaternary chalcogenides InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9

Article abstract of DOI:10.1016/j.jssc.2009.03.013

Quaternary chalcogenides InSn₂Bi₃Se₈ and In_0.2Sn₆Bi_1.8Se₉ were synthesized on direct combination of their elements in stoichiometric ratios at T>800 °C under vacuum. Their struct

Full text of DOI:10.1016/j.jssc.2009.03.013

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Journal of Solid State Chemistry 182 (2009) 1450–1456
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Journal of Solid State Chemistry
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Synthesis and characterization of quaternary chalcogenides InSn2Bi3Se8 and
In0.2Sn6Bi1.8Se9
Ã
Ming-Fang Wang a, Shyue-Ming Jang b, Jih-Chen Huang b, Chi-Shen Lee a,
a Department of Applied Chemistry, National Chiao Tung University, 1001 University Rd., Hsinchu 30010, Taiwan.
b Industrial Technology Research Institute, N300, UCL/ITRI Bldg. 22, 321 Kuang Fu Rd Sec. 2, Hsinchu 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Quaternary chalcogenides InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 were synthesized on direct combination of
Received 27 June 2008
Received in revised form
13 March 2009
their elements in stoichiometric ratios at T4800 1C under vacuum. Their structures were determined
with X-ray diffraction of single crystals. InSn2Bi3Se8 crystallizes in monoclinic space group C2/m (No. 12)
3
˚
˚
˚
˚
with a ¼ 13.557(3) A, b ¼ 4.1299(8) A, c ¼ 15.252(3) A,
b
¼ 115.73(3)1, V ¼ 769.3(3) A , Z ¼ 2, and
Accepted 21 March 2009
Available online 29 March 2009
R1/wR2/GOF ¼ 0.0206/0.0497/1.092; In0.2Sn6Bi1.8Se9 crystallizes in orthorhombic space group Cmc21
3
˚
˚
˚
˚
(No. 36) with a ¼ 4.1810(8) A, b ¼ 13.799(3) A, c ¼ 31.953(6) A, V ¼ 1843.4(6) A , Z ¼ 4, and R1/wR2/
GOF ¼ 0.0966/0.2327/1.12. InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 are isostructural with CuBi5S8 and Bi2Pb6S9
phases, respectively. The structures of InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 feature a three-dimensional
framework containing slabs of NaCl-(311) type with varied thicknesses. Calculations of the electronic
structure and measurements of electrical conductivity indicate that these materials are semiconductors
with narrow band gaps. Both compounds show n-type semiconducting properties with Seebeck
Keywords:
Chalcogenide
Quaternary
Indium
Tin
Bismuth
coefficients ꢀ270 and ꢀ230
m
V/K at 300 K for InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9, respectively.
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
ZT ¼ 0.82 and 2.2, respectively. New techniques involving the
fabrication of nanometer-sized thermoelectric materials on a thin
film have been developed; the optimized ZT value is near 2.4
about 23 1C [1]. Some rational approaches of prospective solid-
state systems for materials with large ZT have been proposed
including clathrate [11–13] and skutterudite phases [14,15].
In a synthesis of new materials with useful thermoelectric
properties, heavy main-group elements, such as tin, lead,
antimony and bismuth, might enhance the electrical conductivity
and diminish the thermal conductivity [16]. The research of
new chalcogenides focused primarily on the (A/Ln)–T–Pn–Q,
(A/Ln)–T–Q and A–Tt–Pn–Q systems (A ¼ alkali, alkaline-earth
elements, Ln ¼ rare-earth elements; T ¼ transition metal; Tt ¼ Sn,
Pb; Pn ¼ Sb, Bi; Q ¼ Se, Te) [17,18]. For chalcogenide systems
Tr–Tt–Pn–Q (Tr ¼ Ga, In, Tl; Tt ¼ Si, Ga, Sn, Pb; Pn ¼ As, Sb, Bi;
Q ¼ S, Se), many sulfide compounds are known, and a few
quaternary selenide phases. Many naturally occurring minerals
and several synthetic quaternary chalcogenides are known,
such as Pb1.6In8Bi4S19 [19], Pb4In2Bi4S13 [20], Pb4In3Bi7S18 [21],
TlPbSbS3 [22], Tl2SnAs2S6 [23], TlPbAs3S6 [24], TlPbAs5S9 [25],
Tl2Pb(Sb/As)10S17 [26] and In0.5Sb0.4524Se0.0238Sn0.0238 [27]. Most
of these belong to sulfide compounds; only one solid-solution
phase In0.5Sb0.4524Se0.0238Sn0.0238 of ZnS structural type was
synthesized. No quaternary phase has been found in the In-Sn-
Bi-Se system.
Multinary metal chalcogenides that possess varied structural
types that differ markedly from oxides have been proposed for
prospective applications as thermoelectrics [1,2], non-linear
optical materials [3], photoelectrics [4], phosphors [5,6], and
solid-state electrolytes for lithium secondary batteries [7]. The
search for multinary chalcogenides containing heavy main-group
elements is attractive because of their abundant structural
features and distinctive physical properties that are applicable
in thermoelectric devices. The success of a thermoelectric device
depends on the figure of merit, ZT ¼
T ¼ temperature; S ¼ Seebeck coefficient;
tivity;
s
S2T/
k
s
(Z ¼ figure of merit;
¼ electrical conduc-
k
¼ thermal conductivity). The combinations of these
properties indicate that a semiconducting material is the best
candidate to attain a maximum value of ZT. Currently available
materials suffer a small efficiency or rate of energy transfer that
prohibits applications as thermoelectric devices. For instance,
Bi2ꢀxSbxTe3ꢀySey [8] is the best known thermoelectric material
that has long served for cooling applications from ambient
temperature (ZTꢁ1). Two new materials, CsBi4Te6 [9] and
AgPbmSbTe2+m [10], show effective thermoelectric properties
at low and high temperatures with optimized figures of merit
Ã
In this work, we investigated the quaternary chalcogeni-
des that contain groups 3–5 heavy main-group element. On
Corresponding author. Fax: +886 3 572 3764.
E-mail address: chishen@mail.nctu.edu.tw (C.-S. Lee).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.03.013

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1451
introducing heavy main-group elements such as Bi, an intriguing
feature is the stereochemical localization of their ns2 electrons
that might influence the structural type and electronic structure,
and consequently the electronic properties of the resulting
compound [28]. Here we report the synthesis, crystal structure,
physical properties and electronic structure of two new quatern-
ary selenides—InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9.
impurity from any chemical reactant was detected. This reaction
product is stable in air under ambient conditions.
The experimental X-ray powder-diffraction pattern (Figure S1)
of InSn2Bi3Se8 agreed satisfactorily with patterns simulated based
on single-crystal data. The compound In0.2Sn6Bi1.8Se9 contains
broad diffraction line shapes in powder X-ray diffraction,
indicative of poor crystallinity. All diffraction peaks were indexed
˚
˚
to an orthorhombic lattice with a ¼ 4.190(4) A, b ¼ 13.80(1) A and
˚
c ¼ 31.90(2) A [29]. A strong signal at 2
y
ꢁ31.11 corresponds to a
preferred orientation (400) from SnSe [30]. The thermogravimetry
(TG)/differential scanning calorimetry (DSC) measurements show
little contribution of SnSe (mp ¼ 880 1C), indicative of low
contribution in the as-synthesized product (o5%).
2. Experiments
2.1. Synthesis
All operations were performed in a glove box with a dry argon
atmosphere. Chemicals were used as obtained (from Alfa
Aesar)—Bi, 99.5%, powder; In, 99.99%, powder; Sn, 99.9%, powder;
Se, 99.95%, powder. The total masses of samples (all elements
combined) were about 0.5 g. All reactants in evacuated fused-
silica tubes were placed in resistance furnaces with a controlled
temperature.
2.2. Single-crystal X-ray diffraction
Single crystals of compounds InSn2Bi3Se8 (0.05 ꢂ 0.05 ꢂ 0.2
mm3) and In0.2Sn6Bi1.8Se9 (0.04 ꢂ 0.04 ꢂ 0.15 mm3) were mounted
on glass fibers with epoxy glue; intensity data were collected on a
diffractometer (Bruker APEX CCD) with graphite-monochromated
˚
MoK
a radiation (l ¼ 0.71073 A) at 298(2) K. The distances from
crystal to detector were 5.95 cm for InSn2Bi3Se8 and 5.00 cm for
2.1.1. InSn2Bi3Se8
In0.2Sn6Bi1.8Se9. Data were collected with a scan 0.31 in groups
InSn2Bi3Se8 was initially observed as a product from a reaction
intended to synthesize ‘In2Sn4Bi4Se13’. The reaction mixture was
heated from 23 to 800 1C over 8 h; the latter temperature was
maintained for 24 h followed by cooling to about 23 1C on simply
terminating the power. The product contained a molten part and
small particles with a metallic luster. Based on measurements
of powder X-ray diffraction (XRD), the product ‘In2Sn4Bi4Se13’ of
the reaction is a mixture of In2Se3, SnSe and an unknown phase.
The product was cracked and a small sample (ꢁ0.05 ꢂ 0.05 ꢂ 0.2
mm3) was chosen for measurements on a single crystal. After the
structure and composition were confirmed as InSn2Bi3Se8, a pure
phase was synthesized on direct combination of In:Sn:Bi:Se ¼
1:2:3:8 with the same heating conditions as specified above.
Analyses of powder X-ray diffraction patterns and energy-
dispersive spectra (EDX) show no detectable impurity. InSn2Bi3Se8
is stable in air under ambient conditions. Attempts to synthesize
analogues of ‘InSn2Bi3S8’ and ‘InSn2Bi3Te8’ failed, but yielded
instead mixtures of In2X3, SnX and Bi2X3 (X ¼ S, Te). To investigate
the possible phase width in this system, we performed reactions
on varying the ratios In/Sn and Sn/Bi, but the products from all
reactions contained mixtures of binary chalcogenides In2Se3 or
SnSe or Bi2Se3.
of 600 frames each at
duration of exposure was 60 s and 20 s/frame for InSn2Bi3Se8
and In0.2Sn6Bi1.8Se9, respectively. The 2 values varied between
f settings 01, 901, 1801 and 2701. The
y
2.251 and 28.351. Diffraction signals obtained from all frames of
reciprocal space images were used to determine the unit-cell
parameters. The data were integrated using the Siemens SAINT
program and were corrected for Lorentz and polarization effects
[31]. Absorption corrections were based on a function fitted to the
empirical transmission surface as sampled by multiple equivalent
measurements of numerous reflections. The structural model was
obtained with direct methods and subjected to full-matrix least-
square refinement based on F2 using the SHELXTL package [32].
2.2.1. InSn2Bi3Se8
The rod-shaped crystal of InSn2Bi3Se8 revealed a monoclinic
˚
˚
˚
unit cell (a ¼ 13.557(3) A, b ¼ 4.1299(8) A, c ¼ 15.252(3) A,
3
˚
b
¼ 115.73(3)1, V ¼ 769.3(3) A ) with a C-centered lattice. The
unit cells were refined in the Laue group 2/m during an integration
based on all reflections. Systematic absences indicated C2, C2/m,
and Cm as possible space groups. The centrosymmetric space
group C2/m was chosen for the smallest values of the reliability
factors. Eight crystallographic sites (M1–3, In4, and Se5–8) were
located. The structural refinement displayed exceptional thermal
displacement parameters for sites M1–3, indicative of positions
with mixed occupancy of Bi/Sn or Bi/In. The In4 site with ꢁ50 eꢀ/
site might be assigned to either Sn or In atom. Refinements
were performed on varying the distributions of In, Sn and Bi in
M1–M3 and In4 sites. A charge-balanced model was eventually
constructed in which M1–M3 sites were mixed Bi/Sn sites and an
In4 site was occupied 100% by In. These site distributions reveal a
charge-balanced formula (In3+)(Sn2+)2(Bi3+)3(Se2ꢀ)8. Final struc-
tural refinements produced R1/wR2/GOF ¼ 0.0206/0.0497/1.092.
2.1.2. In0.2Sn6Bi1.8Se9
In0.2Sn6Bi1.8Se9 was first observed on annealing a pressed
pellet sample of InSn2Bi3Se8 at 800 1C. Needle-shaped crystals
were observed on the cool side of the silica ampoule. After the
structure and composition were confirmed as In0.2Sn6Bi1.8Se9, we
attempted to synthesize
a pure phase using elements In:
Sn:Bi:Se ¼ 0.63:5.57:1.8:9 in stoichiometric proportions, which
were heated to 650 1C over 12 h and held there for 24 h, followed
by cooling at –10 1C/h to 550 1C, but the product contained
an impurity phase SnSe in small proportions based on powder
X-ray diffraction measurements. We subsequently obtained In0.2
Sn6Bi1.8Se9 on heating the InSn2Bi3Se8 powder in a pressed-pellet
form under vacuum in a tube furnace. The sample was heated to
650 1C over 12 h and held at 650 1C for 24 h, followed by cooling
to 23 1C naturally in a gradient furnace. Needle-shaped crystals
of In0.2Sn6Bi1.8Se9 were observed in the cool part of the silica
ampoule, with SnSe, and Se attached on the tube wall. The needle
crystals were separated manually. Energy-dispersive spectra were
recorded on the In0.2Sn6Bi1.8Se9 needle crystals as synthesized; no
2.2.2. In0.2Sn6Bi1.8Se9
Several crystals from the crushed product were used for single-
crystal X-ray diffraction, but the initial diffraction data to index
these crystals indicated that many of them were twinned crystals
with large standard deviations of the lattice parameters. The best
crystal data for the structure determination were selected from a
black pillar crystal of dimensions 0.04 ꢂ 0.04 ꢂ 0.15 mm3. Analysis
of the diffraction data showed that several reflections in the
small-angle region exhibited split line shapes and fitted the cell

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M.-F. Wang et al. / Journal of Solid State Chemistry 182 (2009) 1450–1456
poorly, which indicated the presence of closely aligned crystals.
Attempts to separate those split signals for structural refinement
were unsuccessful. X-ray measurements revealed an orthorhom-
Table 2
Atomic coordinates, site occupancies and isotropic displacement parameters (Ueq
,
2
10ꢀ3 A ) for InSn2Bi3Se8.
˚
˚
˚
˚
bic unit cell (a ¼ 4.1810(8) A, b ¼ 13.799(3) A, c ¼ 31.953(6) A,
a
Atom
M1
Site
4i
x
y
0
0
0
z
Ueq
Site occ.
3
˚
V ¼ 1843.4(6) A ) with a primitive lattice from indexing of all
0.3520(1)
0
0.1338(1)
0
22(1)
22(1)
36(1)
Bi
Bi 0.732(6)
Sn 0.268
reflections. The unit cells were refined in Laue group mmm during
integration based on all reflections. Analysis of the systematic
absence of reflections indicated six possible space groups: C222
(No. 21), Cmm2 (No.35), Cmc21 (No. 36), Cmc2 (No. 40), Cmcm
(No. 63), and Cmmm (No. 65). As the intensity statistics were
consistent with the non-centrosymmetric groups, space group
Cmc21 (No. 36) was eventually chosen. Seventeen crystallographic
sites (M1–5, Sn6–8, and Se9–17) were found. The cationic sites
M1–M5 have mixed occupation by In, Sn and Bi atoms because of
their atypical parameters for thermal displacement when refined
with full occupancy by Sn, Bi or In. The refined electronic density
of Sn6–Sn8 sites were near 100% Sn within standard derivation,
and were so assigned. All Se atoms show unreasonable anisotropic
displacement parameters, so were refined isotropically. Three
large signals (ꢁ9 e/A3) appeared in the electron density map in
positions near M4, Sn7 and Sn8 atoms, which might be due to
a contribution of twin components. The poor single crystal data
make it difficult to refine site occupancy of M1–M5 sites. Attempts
to collect the data from the twin component and to refine in
the HKL5 format did not improve the refinement with reduced
residuals. Assuming In3+, Sn2+, Bi3+ and Se2ꢀ, the structural
refinements produced a charged-balanced formula In0.2Sn6Bi1.8Se9
with R1/wR2/GOF ¼ 0.0966/0.2327/1.12.
Sn
Bi
M2
2a
4i
Bi 0.586(7)
Sn 0.414
Sn
Bi
M3
0.2838(1)
0.6430(1)
Bi 0.464(6)
Sn 0.536
Sn
In4
Se5
Se6
Se7
Se8
2d
4i
4i
4i
4i
0.5
0
0
0
0
0
0.5
26(1)
20(1)
23(1)
22(1)
19(1)
0.3353(1)
0.3385(1)
0.0081(1)
0.1183(1)
0.9338(1)
0.3211(1)
0.1916(1)
0.4589(1)
a
Ueq is defined as one third the trace of the orthogonalized Uij tensor.
Table 3
Interatomic distances for InSn2Bi3Se8.
˚
˚
Contacts
Distance (A)
Contacts
Distance (A)
M1
M3
Se5
2.960(1)
3.0824(8)
2.939(1)
2.8110(7)
Se6*2
Se8*2
Se8
2.8472(7)
3.1980(8)
2.729(1)
Se5*2
Se6
Se7*2
M2
In4
Se5*4
Se7*2
2.8826(7)
2.876(1)
Se6*2
Se8*4
2.656(1)
Crystallographic data and selected bond distances for
InSn2Bi3Se8 are given in Tables 1–3. Further details of the crystal-
structure investigation are obtainable from Fachinformations-
zentrum Karlsruhe, Eggenstein-Leopoldshafen, Germany (Fax: +49
7247 808 666; E-mail: crysdata@fiz.karlsruhe.de) on quoting the
depository number CSD-420150.
2.8463(6)
and thermogravimetry were performed with a thermal analyzer
(NETZSCH STA 409PC). A powder sample (approximately 40 mg)
was placed in an alumina crucible; Al2O3 powder served as a
reference sample. The sample was heated to 1400 1C at 10 1C/min
under a constant flow of N2.
2.3. Characterization
Energy-dispersive spectra (SEM/EDX, Hitachi S-4700I high-
resolution scanning electron microscope) were recorded on the
needle-like crystalline samples. Elements (In/Sn), Bi, and Se were
present in several crystals. The In:Sn ratio was indeterminate
because of severe overlap of signals due to In and Sn by the SEM/
EDX system. Measurements of differential scanning calorimetry
2.4. Calculation of the electronic structure
Band calculations with tight-binding linear muffin-tin orbitals
(LMTO) were undertaken to understand the electronic structures.
[33–37] The space groups P2 for InSn2Bi3Se8 and Pmc21 for
In0.2Sn6Bi1.8Se9 of low symmetry were used to simulate the
observed crystal structures containing mixed-occupancy sites
(vide infra). Integration in k space was performed with an
improved tetrahedron method on grids 4 ꢂ12 ꢂ 4 and 12 ꢂ 4 ꢂ 4
of unique k points in the first Brillouin zone for InSn2Bi3Se8 and
In0.5Sn6Bi1.5Se9 models, respectively. We analyzed the electronic
structure on extracting information from the band structure,
densities of states (DOS), and crystal orbital-Hamiltonian popula-
tion (COHP) curves [38].
Table 1
Crystal data and conditions of data collection for InSn2Bi3Se8.
Refined composition
Formula mass (g molꢀ1
InSn2.02Bi2.98Se8
1609.074
)
Instrument, temperature (K)
Smart CCD; 298(2)
0.71073
˚
Wave length (A)
Crystal system
Monoclinic
C2/m (No. 12), 2
13.557(3)
Space group, Z
˚
a (A)
˚
b (A)
4.1299(8)
˚
c (A)
(deg)
15.252(3)
115.73(3)
2.5. Physical property measurements
b
3
˚
V (A )
ymin ymax (deg)
Independent (Rint), observed reflections
dcalcd. (g m-3
769.3(3)
2.97, 28.26
,
We measured Seebeck coefficients on a cold-pressed bar
1070 (0.0237), 4492
6.954
(1 ꢂ1 ꢂ5 mm3) with
a commercial thermopower measure-
)
Absorption coefficient (mmꢀ1
Reflections collected
Refinement method
)
57.752
ment apparatus (MMR Technologies) in the temperature range
300–600 K under a dynamic vacuum (ꢁ10ꢀ2 Torr). Constantan
served as an internal standard, and silver conductive paint was
used to create electrical contacts. Measurements of DC conduc-
tivity were performed with a standard four-probe method and a
homemade device under vacuum (ꢁ10ꢀ2 Torr) in the temperature
range 30–300 K.
4479
Full-matrix least-squares on F2
1.092
Goodness-of-fit on F2
R1, wR2 (all data)a
0.0220, 0.0505
0.0206, 0.0497
R1, wR2 (I42s(I))
P
P
a
2
2
2
1=2
R1 ¼ ||F0|ꢀ|Fc||/ |F0| wR2 ¼ f
S
ðwðF20 ꢀ F2c Þ Þ=
S
ðwðF0Þ Þg
.

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1453
3. Results and discussion
expanded parallel to the b-axis, interconnected by Se atoms
running along the c-axis. Each slab is composed of fused
rectangular [M12Se12] rod units that are expanded along direction
(100) and connected to each other with M-Se contacts in a
distorted octahedral environment.
3.1. Crystal structure
3.1.1. InSn2Bi3Se8
The structures of InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 are defined
with topochemical cell-twinning symbols L(3, 1) and L(7,7) [41],
respectively; L pertains to a member of the lillianite series and
the number corresponds to the number of octahedra in each of
the two slabs [41]. The cell-twinning symbol L(n, n0) reflects
the cation/anion ratio (M/X) [48]. In the same way that the
topochemical cell-twinning symbol L(3, 1) for InSn2Bi3Se8 reflects
the ratio M/X ¼ 0.750, the symbol L(7, 7) for In0.2Sn6Bi1.8Se9
reflects the ratio M/X ¼ 0.889. The polysynthetic twinning in
NaCl-(311) type exhibits a ratio M/X decreasing with increasing
trigonal prismatic sites at the twinning plane [49]. The smaller
M/X value for InSn2Bi3Se8 thus results in a greater concentration
of trigonal prismatic sites in the InSn2Bi3Se8 structure than in
In0.2Sn6Bi1.8Se9.
InSn2Bi3Se8 crystallizes in monoclinic space group C2/m with
four formula units per unit cell. The structure of InSn2Bi3Se8 is
isostructural with HgBi2S4 [39] and CuBi5S8 [40]. The structure
contains eight crystallographically inequivalent atoms—four cations
(In3+, Sn2+ and Bi3+) and four anions (Se2ꢀ). The InSn2Bi3Se8
structure viewed along the b-axis is shown in Fig. 1a. The
structure contains two slabs (I and II) near a NaCl (311) tilt
plane [41]. Each slab contains distorted rectangular parts that
expand along the b-axis to form ribbon-shaped units. These
NaCl (311) tilt planes are stitched together with M1–Se to form a
three-dimensional framework. Slab I contains sites rich in Bi
of M1 (73.2/26.8% Bi/Sn) and M2 (58.6/41.4% Bi/Sn) that exhibit
distorted octahedral environments with average M–Se distances
˚
2.9(1) and 2.880(3) A, respectively. Slab II consists of M3 (46.4/
53.6% Bi/Sn) and In4 (100% In) sites that are coordinated with five
and six Se atoms, respectively. The coordination geometry of
M3 atom is describable as a distorted square pyramid contain-
3.2. Electronic structure
˚
ing three near Se (ꢁ2.81(7) A) atoms and two remote Se
3.2.1. InSn2Bi3Se8
˚
(ꢁ3.1982(8) A) atoms at the corners with two farther Se atoms,
The electronic band structure of InSn2Bi3Se8 was calculated
with a tight-binding method and the LMTO program [33–37].
As discussed above, the original structure in C2/m contains only
four independent cation sites that limit its possible models. To
investigate the effect of site preference on the stability of the
InSn2Bi3Se8 structure, we applied a model with space group P2 of
low symmetry, which yields 33 possible models with varied
arrangements of Sn, Bi and In atoms in M1–M4 sites (Model
]1ꢀ33 in Table S1). The results of our LMTO calculations show
that the total energy of four models failed to converge, but the
other 29 models converged (Fig. 2, Table S1). The results shown in
Fig. 2 are divided into three regions—]1–13 are models with Sn
distributed over M1–M3 and In4 site (blue rhombus); ]14–21 are
models with Sn distributed over M3 and In4 (pink triangles),
and ]22–29 are models with Sn distributed over M1 and M2
(red circles). According to the total energy per formula, the most
stable model is ]14 with M1–M3 and In4 sites assigned to Bi, Bi,
Sn and In, respectively. This result is consistent with the refined
crystal data from our single X-ray diffraction experiment. The
calculations of band structure, density of states (DOS) and crystal
˚
43.5 A. The In4 site is 100% In that is located in a distorted
octahedral environment with In–Se distances in
a range
˚
2.656(1)–2.8463(6) A. The M-Se distance in M1–M3 sites is
comparable with some multinary selenides containing mixed
occupied Bi and Sn, such as K0.54Sn3.54Bi11.46Se21 [42], K0.66Sn4.28
Bi11.18Se22 [43], Cs0.46Sn2.46Bi11.54Se20 [44], K0.54Sn4.92Bi2.54Se9
[41], KSn5Bi5Se13 [41], etc. [17,18] The In–Se distances for In4
sites are also comparable with the In–Se distance in In2Se3 and
˚
other multinary phases of Pb7.12In18.88Se34 [45] (In–Se ꢁ2.8(1) A)
˚
and Fe0.47In17.37Pb8.04Se34 [46] (In–Se ꢁ2.8(2) A).
3.1.2. In0.2Sn6Bi1.8Se9
In0.2Sn6Bi1.8Se9 is isostructural with Bi2Pb6S9 [47] and
K0.54Sn4.92Bi2.54Se9 [41] that crystallizes in orthorhombic space
group Cmc21 (No. 36) with four formula units per unit cell. The
structure contains 17 crystallographically inequivalent sites—five
for mixed-occupancy cations (M1–M5), three for Sn2+(Sn6–Sn8)
and nine for Se2ꢀ anions. Fig. 1b shows a (100) projection of the
structure that consists of slabs derived from NaCl (311) tilt planes
Fig. 1. (a) Crystal structure of InSn2Bi3Se8 in a projection along the crystallographic b-axis [010], showing NaCl (311) tilt planes I and II. (b) Projection of In0.2Sn6Bi1.8Se9
viewed along the crystallographic a-axis [100].

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Fig. 4. Densities of states (DOS) of InSn6BiSe9 (left), In0.5Sn5.5Bi2Se9 (center) and
In0.5Sn6Bi1.5Se9 (right). The horizontal dashed lines indicate positions for 87 eꢀ
/
formula.
3.2.2. In0.2Sn6Bi1.8Se9
Fig. 2. Relative total energies of InSn2Bi3Se8, from LMTO calculations using varied
To understand the electronic structure of In0.2Sn6Bi1.8Se9 as
synthesized, we calculated the band structure of three mod-
els—InSn6BiSe9 (]1), In0.5Sn5.5Bi2Se9 (]2), and In0.5Sn6Bi1.5Se9 (]3)
(Table S2). The results of the DOS curves shown in Fig. 4 indicate
that all three models show a fully occupied band at a valence-
electron concentration (vec) ¼ 87 eꢀ/formula, which corresponds
to the charge-balanced formula In0.5Sn6Bi1.5Se9 and is near
the refined formula from single-crystal X-ray measurements.
The band gap for model ]3, In0.5Sn6Bi1.5Se9, is about 0.1 eV. The
large maximum below the Fermi level is dominated by filled Se
(4p) states, whereas the valence band has dominant Sn and Bi p
character.
assignments of In, Sn and Bi atoms over the M1–M3 and In4 sites. The model with
the least energy (]14) is set as zero and
DE ¼ Etot(]14)–Etot(Model]) (see Table S1).
3.3. Physical properties
We measured the electrical properties of InSn2Bi3Se8 and
In0.2Sn6Bi1.8Se9 between 30 and 700 K (Fig. 5a). The results
show that both compounds exhibit decreasing resistivity with
increasing temperature, hence conforming to a trend typical of a
semiconductor. The calculated activation energies for InSn2Bi3Se8
and In0.2Sn6Bi1.8Se9 are 0.006 and 0.029 eV, respectively [50].
The electrical conductivities of InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 at
300 K are 7859 and 1974 S m-1, respectively. The temperature
dependence of the Seebeck coefficient of InSn2Bi3Se8 and
In0.2Sn6Bi1.8Se9, only slight for both phases, is shown in Fig. 5b.
The Seebeck coefficients for both compounds at 300 K are –270
Fig. 3. Densities of states (left), partial densities of states, and crystal orbital-
Hamiltonian populations (COHP) for M–Se (M1-Se: red line, M2-Se: red dashed
line, M3-Se: blue line, and In4–Se: blue dashed line) interactions (right) curves of
InSn2Bi3Se8. The horizontal dashed line denotes the Fermi energy (EF). The
contributions from slab I (M1(Bi)+M2(Bi)) and slab II (M3(Sn)+In4(In)) are denoted
with red and blue lines. [For interpretation of the references to color in this figure
legend, the reader is referred to the webversion of this article.]
and –230 mV/K, respectively, indicating that electrons are the
major charge carriers.
orbital-Hamiltonian population curves based on model ]14 are
shown in Fig. 3. The band structure of InSn2Bi3Se8 shows an
indirect band gap with a calculated band gap ꢁ0.2 eV, indicative
of a semiconducting property. The DOS curve (black) plus the
PDOS curves for contributions of slabs I (red) and II (blue)
are shown in Fig. 2. At the bottom of the Fermi energy, the
contributions to the DOS stem largely from Bi atoms (M1, M2) of
slab I. The large maximum below the Fermi level arises
predominantly from Sn and In atoms (M3, In4) in slab II. The
sharp maxima at –6 and –10 eV are dominated by filled Sn and Bi s
states, respectively, whereas two maxima centered about –5ꢁ0 eV
and 0ꢁ+6 eV have dominant Sn and Bi p characters. The Sn (5s)
and Bi (6s) states with narrow distributions might result from the
inert-pair effect commonly observed in heavy main-group
elements, but the partial DOS of In (5s, 5p) states show almost
no contribution about the Fermi level. According to the COHP
curves for InSn2Bi3Se8 presented in Fig. 3, the cumulative In–Se
interactions are essentially optimized. For Sn–Se and Bi–Se
contacts, antibonding states are filled below ꢀ1 eV for Sn–Se
and Bi–Se interactions, which might arise from the localization of
the lone-pair electrons.
3.4. Thermoanalysis
The TG–DSC curves versus temperature for InSn2Bi3Se8 (1) and
In0.2Sn6Bi1.8Se9 (2) are shown in Fig. 6. The DSC curve of 1 reveals
prominent endothermic transitions in a range 650–680 1C that are
attributed to the melting of 1. Thermogravimetric (TGA) curves
obtained on heating polycrystalline samples reveal a mass loss
beginning at ꢁ720 1C, which corresponds to the decomposition
of 1. These results were reproduced on heating the powder as
synthesized in a quartz ampoule under vacuum and subsequently
heated to 1000 1C. The PXRD pattern of the residue is indexed
as a combination of Bi2Se3, Bi2Se2, InSe, Sn0.571Bi0.286Se, Se and
an amorphous phase. The DSC measurements on
2 show
two exothermic transitions in the range 680–740 1C. The TGA
curve shows that a loss of mass occurs at Tꢁ760 1C for the
decomposition of 2. The X-ray powder-diffraction pattern of the
product found after heating compound 2 under vacuum to 1000 1C
indicates that the residue contains a mixture of SnSe, Bi2Se2, InSe

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1455
Fig. 6. Thermal analysis (TG/DSC scans) of InSn2Bi3Se8 (a) and In0.2Sn6Bi1.8Se9 (b).
Supporting information available
Fig. 5. Temperature dependence of electrical conductivity (a) and thermoelectric
power (b) of InSn2Bi3Se8 (red) and In0.2Sn6Bi1.8Se9 (black). [For interpretation of
the references to color in this figure legend, the reader is referred to the
webversion of this article.]
Experimental and simulated X-ray powder patterns for
InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9, models of InSn2Bi3Se8 and In0.2
Sn6Bi1.8Se9 for the LMTO calculations, and crystallographic data in
CIF format.
and Se. For both compounds, the mass decreased continuously
until T4 1280 1C because of evaporation of the products of
decomposition. For both compounds, additional decomposition of
the binary products from the title compounds was observed in the
DSC curve for temperature 4800 1C.
Acknowledgment
National Science Council (NSC94-2113-M-009-012, 94-2120-
M-009-014) supported this research.
4. Conclusions
Appendix A. Supplementary material
Quaternary chalcogenides InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9
have been synthesized and characterized. Both compounds
exhibit slabs of NaCl (311) tilt planes with disparate thickness,
and the structures are defined with topochemical cell-twinning
symbols L(3, 1) and L(7, 7), respectively. The thermopower and
electrical conductivity of InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 indicate
n-type semiconducting property with narrow band gaps. Electro-
nic structure calculations of InSn2Bi3Se8 indicate that the most
stable model has M1–M3 and In4 sites assigned to Bi, Bi, Sn and In
atoms, consistent with the results of measurements on a single
crystal. The calculations for the charge-balanced model In0.5
Sn6Bi1.5Se9 (vec ¼ 87 eꢀ/formula) show a semiconducting prop-
erty and the formula is near the refined formula from single-
crystal X-ray measurements.
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2009.03.013.
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