Homologous Series of 2D Chalcogenides Cs-Ag-Bi-Q (Q = S, Se) with Ion-Exchange Properties

Article abstract of DOI:10.1021/jacs.7b06373

Four new layered chalcogenides Cs_1.2Ag_0.6Bi_3.4S₆, Cs_1.2Ag_0.6Bi_3.4Se₆, Cs_0.6Ag_0.8Bi_2.2S₄, and Cs₂Ag_2.5Bi_8.5Se₁₅ are described. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_1.2Ag_0.6Bi_3.4Se₆ are isostructural and have a hexagonal P6₃/mmc space group; their structures consist of [Ag/Bi]₂Q₃ (Q = S, Se) quintuple layers intercalated with disordered Cs cations. Cs_0.6Ag_0.8Bi_2.2S₄ also adopts a structure with the hexagonal P6₃/mmc space group and its structure has an [Ag/Bi]₃S₄ layer intercalated with a Cs layer. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ can be ascribed to a new homologous family A_x[M_mS_1+m] (m = 1, 2, 3···). Cs₂Ag_2.5Bi_7.5Se₁₅ is orthorhombic with Pnnm space group, and it is a new member of the A₂[M_5+nSe_9+n] homology with n = 6. The Cs ions in Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ can be exchanged with other cations, such as Ag⁺, Cd²⁺, Co²⁺, Pb²⁺, and Zn²⁺ forming new phases with tunable band gaps between 0.66 and 1.20 eV. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ possess extremely low thermal conductivity (<0.6 W·m^-1·K^-1).

Full text of DOI:10.1021/jacs.7b06373

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Article
Homologous 2D Chalcogenides Cs-Ag-Bi-
Q (Q = S, Se) with Ion-exchange Properties
Jing Zhao, Saiful M. Islam, Shiqiang Hao, Gangjian Tan, Constantinos C. Stoumpos,
Chris Wolverton, Haijie Chen, Zhongzhen Luo, Rukang Li, and Mercouri G. Kanatzidis
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06373 • Publication Date (Web): 15 Aug 2017
Downloaded from http://pubs.acs.org on August 15, 2017
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Journal of the American Chemical Society
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Homologous 2D Chalcogenides Cs-Ag-Bi-Q (Q = S, Se) with
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Jing Zhao,1, 2 Saiful M. Islam,2 Shiqiang Hao,3 Gangjian Tan,2 Constantinos C. Stoumpos,2 Chris
Wolverton,3 Haijie Chen,2 Zhongzhen Luo,2 Rukang Li,1 Mercouri G. Kanatzidis2, *
1Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing 100190, P. R. China
2Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
3Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
ABSTRACT: Four new layered chalcogenides Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4 and Cs2Ag2.5Bi8.5Se15 are deꢀ
scribed. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are isostructural and have a hexagonal P63/mmc space group; their structures consist
of [Ag/Bi]2Q3 (Q = S, Se) quintuple layers intercalated with disordered Cs cations. Cs0.6Ag0.8Bi2.2S4 also adopts a structure with the
hexagonal P63/mmc space group and its structure has [Ag/Bi]3S4 layer intercalated with a Cs layer. Cs1.2Ag0.6Bi3.4S6 and
Cs0.6Ag0.8Bi2.2S4 can be ascribed to a new homologous family Ax[MmS1+m] (m = 1, 2, 3 …). Cs2Ag2.5Bi7.5Se15 is orthorhombic with
Pnnm space group and it is a new member of the A2[M5+nSe9+n] homology with n = 6. The Cs ions in Cs1.2Ag0.6Bi3.4S6 and
Cs0.6Ag0.8Bi2.2S4 can be exchanged with other cations, such as Ag+, Cd2+, Co2+, Pb2+ and Zn2+ forming new phases with tunable band
gaps between 0.66 and 1.20 eV. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 possess extremely low thermal conductivity (< 0.6 W·mꢀ1·Kꢀ
1).
Introduction
nides.29ꢀ31,32 Ternary or quaternary alkali metal chalcogenides
A/M/Bi/Q (A = Li, K, Rb, Cs; M = Sn, Pb, Ag, Cd; Q = S, Se
Te)28 form readily and tend to possess tunneled or layered
A layered structure is one where intralayer bonding is more
significant than interlayer bonding. When anionic covalent
frameworks are charge balanced by alkali metal ions,1ꢀ3 the
presence of alkali metals force the covalent frameworks to
form around them. Thus, alkali metal chalcogenides tend to
form chain, layered or tunneled structures depending on the
A2Q/MQ ratios (Q= S, Se, Te).4 The highly anisotropic nature
of the materials may lead to the possibilities of tailoring the
band gaps5,6 and can give rise to attractive and unusual physiꢀ
cal properties that makes them outstanding for applications in
the fields of topological quantum science,7ꢀ13 thermoelectriciꢀ
ty,14ꢀ16 γꢀray detection17,18 secondꢀharmonic generation,19 optiꢀ
cal data storage,20 superconductivity,21,22 charge density wave
formation and photovoltaics.23,24
structures. For example, the Am[M1+lSe2+l]2m[M1+2l+nSe3+3l +n
]
(A = alkali metal, M = Sn and Bi) homologous suꢀ
perseries33,34,29 features compounds with tunneled structures,
while
CsMmBi3Te5+m
the
members
of
the
Cs4[Bi2n+4Te3n+6],4
35,36
,
[MTe]n[Bi2Te3]m (M = Ge, Sn, Pb)37ꢀ45 and
A2[M5+nSe9+n] (A = Rb, Cs; M = Bi, Ag, Cd)30 series possess
layered structures. Recently, we reported the family AxCdxBi4ꢀ
xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se) of new comꢀ
pounds which also possess a layered structure.46
The discovery and classification of homologous series
makes structure design and prediction possible and they are
part of a growing toolbox for the creation of new
materials.26,47,48 Herein, we present the synthesis, structure and
characterization of four new compounds Cs1.2Ag0.6Bi3.4S6,
Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4 and Cs2Ag2.5Bi8.5Se15. They
crystallize in three different structure types. All compounds
have layered structures with intercalated Cs+ cations. We show
that Cs cations reside between the layers of Cs1.2Ag0.6Bi3.4S6
and Cs0.6Ag0.8Bi2.2S4 and can be ionꢀexchanged with other
cations without structural change of the frameworks (topotacꢀ
tically). Thus, they provide a “soft chemistry” route to generꢀ
ate new materials.46,49ꢀ51 Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4
Bismuth chalcogenide compounds are a special class beꢀ
cause they exhibit amazingly diverse compositions and strucꢀ
tures.25,26 The trivalent Bi atoms in the structures can have
coordination numbers varying from 3 to 9 and the inert 6s2
lone pair of electrons defines the electronic character of the
compounds. In these compounds the BiQ6 (Q = S, Se, Te)
octahedra can combine by edge or face sharing to form blocks
or modules that are fragments excised from the NaClꢀ, Bi2Te3ꢀ
, CdI2ꢀ and Sb2Se3ꢀtype structures.4,25,27,28 Furthermore, Bi
atoms can share sites with similar sized cations, such as alkali
or alkaline earth metals, Cu+, Pb2+, Sn2+, Ag+, Cd2+or lanthaꢀ
are members of a new homologous family Ax[MmS1+m
]
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Table 1. Crystal Data and Structural Refinement Statistics for Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4 and
Cs2Ag2.5Bi8.5Se15 at 293(2) Ka
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3
4
5
6
7
8
9
empirical formula
formula weight
crystal system
space group
a (Å)
Cs1.2Ag0.6Bi3.4S6
1130.08
Cs1.2Ag0.6Bi3.4Se6
1399.95
Cs0.6Ag0.8Bi2.2S4
754.05
Cs2Ag2.5Bi8.5Se15
3496.22
hexagonal
P63/mmc
hexagonal
P63/mmc
hexagonal
P63/mmc
orthorhombic
Pnnm
4.0300(14)
4.1456(8)
4.0331(6)
12.448(3)
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b (Å)
4.0300(14)
24.212(5)
340.5(2)
4.1456(14)
25.449(5)
378.77(16)
4.0331(6)
30.438(6)
428.77(15)
32.342(6)
4.1399(8)
1666.7(6)
c (Å)
volume (Å3)
Z
1
1
2
2
density (calculated) ( g/cm3)
absorption coefficient (mmꢀ1)
F(000)
5.510
48.635
474
6.137
57.705
577
5.840
50.229
634
6.967
64.656
2886
crystal size (mm3)
0.2252×0.1041×0.0123 0.2945×0.1289× 0.0013 0.1474×0.0945× 0.009 0.1434×0.1206×0.0055
θ range for data collection (deg.) 1.682ꢀ29.043
ꢀ4≤h≤5,
3.202ꢀ24.900
4.017ꢀ24.981
1.753 to 23.498°
ꢀ4≤h≤4,
ꢀ4≤k≤4,
ꢀ30≤l≤30
ꢀ4≤h≤4,
ꢀ4≤k≤4,
ꢀ36≤l≤35
ꢀ14≤h≤14,
ꢀ38≤k≤38,
ꢀ4≤l≤4
index ranges
ꢀ5≤k≤5,
ꢀ33≤l≤33
reflections collected
independent reflections
completeness to θ = 25.242°
refinement method
2994
1847
2177
9867
227 [Rint = 0.0920]
100%
167 [Rint = 0.1872]
98.8%
190 [Rint = 0.0683]
97.4%
1428 [Rint = 0.1722]
99.6%
fullꢀmatrix leastꢀsquares on F2
goodnessꢀofꢀfit
1.340
1.392
0.703
1.064
R1 = 0.0594,,
wR1 = 0.1096,
R2 = 0.0638,
wR2 = 0.1113
R1 = 0.0810,
wR1 = 0.1883
R2 = 0.0981,
wR2 = 0.1953
R1 = 0.0447,
wR1 = 0.1233
R2 = 0.0491,
wR2 = 0.1299
R1 = 0.0753,
wR1 = 0.1342
R2 = 0.1351,
wR2 = 0.1504
final R indices [> 2σ(I)]
R indices [all data]
largest diff. peak and hole (e·Åꢀ3) 2.527 and ꢀ1.862
3.468 and ꢀ2.622
2.295 and ꢀ1.240
2.414 and ꢀ2.277
aR = Σ||Fo|ꢀ|Fc||/Σ|Fo|, wR = 1/2 and calc w=1/[σ2(Fo2)+(0.0521P)2+3.9364P] where P=(Fo2+2Fc2)/3
(m = 1, 2, 3 …) while Cs2Ag2.5Bi7.5Se15 is a new member of
the A2[M5+nSe9+n] homology with n = 6.
Cs1.2Ag0.6Bi3.4S6. Single crystals of Cs1.2Ag0.6Bi3.4S6 were
obtained by combining 0.0894 g Cs2S (0.3 mmol), 0.0539 g
Ag (0.5 mmol), 0.3030 g Bi (1.45 mmol) and 0.0706 g S (2.2
mmol) together in a 9 mm carbon coated fused silica tube and
it was flame sealed at a vacuum of ~10−4 mbar. The tube was
put into a programmable furnace and heated to 300 ˚C in 6 h,
held at this temperature for 5 h for totally melt of sulfur, then
heated to 880 ˚C in 7 h and held at 880 ˚C for 72 h, then cooled
down to 450 ˚C in 96 h and held at this temperature for 1 h
followed by cooling to RT in 2 h. Black plateꢀlike crystals
with the longest edge up to 0.1 mm were obtained. Pure phase
(with no observable impurities from PXRD pattern) of
Cs1.2Ag0.6Bi3.4S6 can be obtained with the same starting ratio
but with a faster cooling rate of cooling from 880 ˚C to 450 ˚C
in 24 h.
Experimental Section
Reagents. All chemicals were used as obtained, silver metal
(99.99%, Sigma Aldrich), bismuth metal (99.9 %, Strem
Chemicals), sulfur pellets (99.99%, Sigma Aldrich) and seleꢀ
nium pellets (99.99%, Sigma Aldrich), AgNO3 (ACS Grade,
Sigma Aldrich), CdCl2 (ACS Grade, Mallinckrodt Baker,
Inc.), Co(NO3)2·6H2O (ACS Grade, Mallinckrodt Baker, Inc.),
ZnCl2 (ACS Grade, Columbus Chemical Industries, Inc.) and
Pb(NO3)2 (ACS Grade, Columbus Chemical Industries, Inc.).
Cs2S and Cs2Se were synthesized by reacting stoichiometric
amounts of the elements in liquid ammonia as described elseꢀ
where.52
Cs1.2Ag0.6Bi3.4Se6. Single crystals of Cs1.2Ag0.6Bi3.4Se6 were
obtained by the same staring molar ratio of Cs1.2Ag0.6Bi3.4S6
but changing sulfur to selenium by mixing 0.103 g Cs2Se (0.3
Synthesis.
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od and refined by SHELXTL program package.54 Table S1 to
mmol), 0.0539 g Ag (0.5 mmol), 0.3030 g Bi (1.45 mmol) and
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0.1737 g Se (2.2 mmol) together in a 9 mm carbon coated
fused silica tube and flame sealed at ~10−4 mbar. Using the
same heating profile as Cs1.2Ag0.6Bi3.4S6, this experiment
yielded black plateꢀlike crystals of Cs1.2Ag0.6Bi3.4Se6 large
enough for single crystal Xꢀray diffraction measurement. The
phase purity of the obtained sample is ~90%, Figure S1.
12 show the atomic coordinates and equivalent isotropic disꢀ
placement parameters, anisotropic displacement parameters
and bond lengths of the title compounds.
Scanning Electron Microscopy. Quantitative microprobe
analyses and crystal imaging of the compounds were perꢀ
formed by using Hitachi Sꢀ3400 scanning electron microscope
equipped with a PGT energyꢀdispersive Xꢀray analyzer. The
data were acquired by an accelerating voltage of 25 kV, a
probe current of 70 mA, and 60 s acquisition time. The comꢀ
positions reported here are an average of a large number of
independent measurements from given samples.
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Cs0.6Ag0.8Bi2.2S4. Cs0.6Ag0.8Bi2.2S4 crystals were obtained by
using the starting materials of 0.0745 g Cs2S (0.25 mmol),
0.0539 g Ag (0.5 mmol), 0.209 g Bi (1 mmol) and 0.0722 g S
(2.25 mmol). The mixture was loaded in a 9 mm carbon coatꢀ
ed fused silica tube in a dry, nitrogenꢀfilled glovebox. The
tube was then placed in a programmable furnace and heated to
350 ˚C in 10 h and held at 350 ˚C for 6 h for the totally melt of
S, then heated to 750 ˚C in 12 h and soaked for 18 h, followed
by slow cooling to 300 ˚C in 96 h and then held for 1 h, finally
cooled to RT in 3 h. Black plateꢀlike crystals were obtained.
Pure phase of Cs0.6Ag0.8Bi2.2S4 was obtained by combining
0.834 g Cs2S (2.8 mmol), 0.0895 g Ag (0.83 mmol), 0.3521 g
Bi (1.69 mmol) and 0.0926 g S (2.89 mmol) together in a carꢀ
bon coated fused silica tube. The tube was heated to 350 ˚C in
6 h and held for 6 h, then heated to 750˚C in 12 h and soaked
for 24 h, followed by slow cooling to 300 ˚C in 32 h and then
held for 1 h, finally cooled to RT in 2 h. The Cs0.6Ag0.8Bi2.2S4
pure phase was obtained with no observable extra peaks from
PXRD pattern.
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Differential Thermal Analysis. The ground single crystals
of Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 with a mass about 40
mg were sealed in silica ampoules under a vacuum of ~10−4
mbar. Differential Thermal Analysis (DTA) was performed
with a computerꢀcontrolled Shimadzu DTAꢀ50 thermal anaꢀ
lyzer. A silica ampoule containing alumina was placed on the
reference side of the detector. The Cs1.2Ag0.6Bi3.4S6 sample was
heated to 750 °C and Cs0.6Ag0.8Bi2.2S4 was heated to 620 °C at
10 °C/min followed by cooling at 10 °C/min to room temperaꢀ
ture and finally this cycle was repeated. The DTA products
were examined with PXRD after the experiment.
Ultraviolet-Visible Spectroscopy. The ultravioletꢀvisible
spectrums were recorded, in the infrared region 500ꢀ2000 nm,
with the use of UVꢀ3600 Shimadzu UVꢀ3600 PC doubleꢀ
beam, doubleꢀmonochromator spectrophotometer. Finely
ground samples were used to do the measurements at room
temperature. By using Kubelka−Munk equation absorption
(α/S) data was obtained: α/S = (1−R)2/2R, where R is reflecꢀ
tance, α is absorption coefficient, and S is scattering coeffiꢀ
cient.55 The fundamental absorption edge was obtained by
linearly fitting the absorbance of the converted data.
Cs2Ag2.5Bi8.5Se15. Synthesis was conducted to obtain
Cs0.6Ag0.8Bi2.2Se4 crystals with the same starting molar ratio
used to obtain Cs0.6Ag0.8Bi2.2S4. The starting materials are
0.0862 g Cs2Se (2.8 mmol), 0.539 g Ag (0.83 mmol), 0.209 g
Bi (1.69 mmol) and 0.1777 g Se (2.89 mmol). The starting
materials were combined together in a 9 mm fused silica tube
in a dry, nitrogenꢀfilled glovebox. The tube was evacuated to
~10−4 mbar and flame sealed. The tube was heated to 300 ˚C
in 6 h, held for 6 h, and then heated to 750 ˚C in 12 h and held
for 18 h, then cooled down to 300 ˚C in 96 h and held for 1 h
followed by cooling to room temperature (RT) in 3 h. Bar like
crystals were obtained for single crystal diffraction and the
purity of the obtained phase is ~90%, Figure S1.
Ion-Exchange Experiments. Ionꢀexchange experiments of
Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 were conducted. The
crystals of Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 (~10.0 mg)
were added into 0.2 mol/L water solution (10 mL) of AgNO3,
CdCl2, Co(NO3)2·6H2O, ZnCl2 and Pb(NO3)2, respectively.
The mixture kept shaking for 24 hours at RT and then isolated
by filtration (through filter paper, Whatman no. 1); washed
several times with water and acetone then dried in air. Crystals
after ionꢀexchange were handꢀpicked and analyzed with enerꢀ
gy dispersive spectroscopy (EDS) analysis. The ground powꢀ
der of Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 (~100 mg) were
added in 0.2 mol/L water solution (20 mL) of AgNO3, CdCl2,
Co(NO3)2·6H2O, ZnCl2 and Pb(NO3)2, respectively. The mixꢀ
ture kept under magnetic stirring for 24 hours at RT. The soluꢀ
tion was filtered and washed as previously mentioned and
characterized with PXRD and UVꢀvisible spectroscopy.
Finally, Cs2Ag2.5Bi8.5Se15 (with a nominal formula of
Cs0.53Ag0.67Bi2.26Se4) with Pnnm space group instead of
Cs0.6Ag0.8Bi2.2Se4 (P63/mmc space group) was obtained. When
trying to obtain better crystals of Cs2Ag2.5Bi8.5Se15 with differꢀ
ent starting material ratios,
a
reported compound
30
Cs2Ag1.5Bi7.5Se13 was also obtained but the structure of this
compound was reꢀrefined and the CIF file is given in the supꢀ
porting information.
Powder X-ray Diffraction. The purity of the samples was
characterized by Rigaku Miniflex powder Xꢀray diffractomeꢀ
ter with Niꢀfiltered Cu Kα radiation operating at 40 kV and 15
mA. The scanning width is 0.02o and a collecting rate of
10o/min was used. Simulated powder Xꢀray diffraction
(PXRD) pattern was obtained by using the CIF file of the reꢀ
fined structure by using the Visualizer software package of the
program FINDIT.
Spark Plasma Sintering. Phase pure samples of
Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 were pulverized and
hand grounded into fine powders in N2ꢀfilled glove box. The
powders were then loaded into graphite dies with a diameter
of 10 mm for SPS at 763 K for 10 min under an axial pressure
of 30 MPa. The relative mass density of the SPSed sample is
about 97% for Cs1.2Ag0.6Bi3.4S6 and 93% for Cs0.6Ag0.8Bi2.2S4.
Single Crystal X-ray Diffraction. The single crystals used
to conduct Xꢀray diffraction measurements were adhered to
the tips of glass fibers with glue. STOE IPDS II single crystal
diffractometer was operating at 50 kV and 40 mA with Mo Kα
radiation (λ = 0.71073 Å). Data collection were conducted by
using XꢀArea software,53 integration were operated in XꢀRED,
and numerical absorption corrections were applied with Xꢀ
SHAPE.53 The crystal structures were solved via direct methꢀ
Thermal Conductivity. The SPSed pellets were cut and
polished into desired geometry with dimensions of ~6 mm × 6
mm × 2 mm and coated with a thin layer of graphite to miniꢀ
mize the emissivity of the material. By using the laser flash
diffusivity method in Netzsch LFA457 thermal diffusivity (D)
coefficients were measured perpendicular to the SPS pressure
direction. The thermal diffusivity data were analyzed using a
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Cowan model with pulse correction. The total thermal conducꢀ Synthesis and Thermal Stability. Single crystals of
1
2
3
4
5
6
7
8
tivity was calculated from κtot = DCpd, density (d) is deterꢀ
mined by using the dimensions and mass of the SPSed sample.
The specific heat capacity (Cp) was estimated by DulongꢀPetit
Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4 and
Cs2Ag2.5Bi8.5Se15 were obtained with a relatively slow cooling
rate (~5 oC/h). Pure phases of Cs1.2Ag0.6Bi3.4S6 and
Cs0.6Ag0.8Bi2.2S4 can be obtained by heating a mixture of Cs2S,
Ag, Bi and S in sealed fused silica tubes at 750 oC as indicated
by the powder Xꢀray diffraction (PXRD) patterns, Figure 1(a).
All compounds are black and stable in air, water and acetone.
Energyꢀdispersive Xꢀray spectroscopy (EDS) was performed
as semiquantitative elemental analysis of Cs, Ag, Bi, S and Se.
The EDS results of all these compounds are in agreement with
the compositions obtained from the refinements of the crystal
structure, Figure S2. The Cs2Ag2.5Bi8.5Se15 was found in the
products of the reaction performed to obtain Cs0.6Ag0.8Bi2.2Se4
as an isostructural analog of Cs0.6Ag0.8Bi2.2S4.
law Cp = 3R/ꢀ, in which R is gas constant 8.314 J·mol−1K−1
and ꢀ is average molar mass. The uncertainty of the thermal
conductivity is estimated to be ~8%.56
Electrical Properties. SPSꢀprocessed pellet were cut into
bars (~3 mm × 3 mm × 8 mm) perpendicular to the sintering
pressure direction and the samples were sprayꢀcoated with
boron nitride to minimize outgassing, except where needed for
electrical contact. The Seebeck coefficients and electrical conꢀ
ductivities were measured simultaneously on an Ulvac Riko
ZEMꢀ3 instrument under a lowꢀpressure helium atmosphere.
The uncertainty of the Seebeck coefficients and electrical conꢀ
ductivity measurements is ~ 5%.56
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Density Functional Theory (DFT) Calculations. The total
energies and relaxed geometries were calculated by DFT using
the exchange correlation functional within the generalized
gradient approximation (GGA) of PerdewꢀBurkeꢀErnzerhof
the exchange correlation functional with Projector Augmented
Wave potentials.57 We use periodic boundary conditions, a
plane wave basis set, and Projector Augmented Wave potenꢀ
tials57 as implemented in the Vienna ab initio simulation packꢀ
age.58 The total energies were numerically converged to apꢀ
proximately 3 meV/cation using a basis set energy cutoff of
500 eV and dense kꢀmeshes corresponding to 4000 kꢀpoints
per reciprocal atom in the Brillouin zone. Our theoretically
relaxed CsAgBi2S4 lattice constants for the P63/mmc crystal
structure are a = 7.04 Å, b = 4.09 Å, and c =28.11 Å, which
are in reasonable agreement with the experimentally measured
lattice parameters, a = 4.0331(6) Å, b = 4.0331(6) Å, c =
30.438(6) Å. Building a smallꢀunitꢀcell periodic atomic strucꢀ
ture to represent Cs0.6Ag0.8Bi2.2S4 is difficult, since Cs sites are
partially occupied and certain Bi atoms occupy the Ag site.
We thus used a simplified CsAgBi2S4 stoichiometry, with no
partially occupied Wyckoff sites as a model to simplify the
problem.
Phonon Dispersion Calculations. To quantitatively exꢀ
plore the origin of lattice thermal conductivity at the atomic
level, we utilize the DebyeꢀCallaway model to quantitatively
evaluate the value of lattice thermal conductivity of CsAgꢀ
Bi2S4. It is known that the Grüneisen parameters, which charꢀ
acterize the relationship between phonon frequency and crysꢀ
tal volume change, serve as a useful measure of the lattice
anharmonicity and thus helpful to interpret the physical nature
of lattice thermal conductivity behavior.59,60 The phonon and
Grüneisen dispersions are calculated using firstꢀprinciples
DFT phonon calculations within the quasiꢀharmonic approxiꢀ
mation. The CsAgBi2S4 phonon dispersions are calculated on
a 128 atom cell at two volumes, one is the equilibrium volume
V0 and an isotropically compressed volume 0.98V0. Details of
Grüneisen parameter calculations are given in the supporting
information.
Figure 1. (a) Comparison of powder Xꢀray diffraction specꢀ
trum between synthesized and the simulated ones. A:
Cs1.2Ag0.6Bi3.4S6 simulated, B: Cs1.2Ag0.6Bi3.4S6 synthesized, C:
Cs0.6Ag0.8Bi2.2S4 simulated, D: Cs0.6Ag0.8Bi2.2S4 synthesized;
(b) The DTA curves of Cs0.94Ag0.56Bi2.24S4
。
Differential thermal analysis (DTA) of Cs0.6Ag0.8Bi2.2S4 was
conducted with a heating and cooling rate of 10 oC/min to 750
°C for two consecutive cycles. DTA revealed an endothermic
o
o
peak at 687 C and one exothermic peak at 688 C, Figure
1(b). PXRD of the DTAꢀderived products showed that the
compound melts congruently, Figure S3. Cs1.2Ag0.6Bi3.4S6 is
stable up to 620 °C as indicated by DTA, Figure S4.
Crystal Structures. Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6,
Cs0.6Ag0.8Bi2.2S4 and Cs2Ag2.5Bi8.5Se15 crystallize in three differ
ent structure types. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are
Results and Discussion
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Figure 2. The crystal image (left), (a) the unit cell of Cs1.2Ag0.6Bi3.4S6 and (b) the unit cell of Cs0.6Ag0.8Bi2.2S4; The scale bar is 100 ꢁm.
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isostructural with the space group of P63/mmc, see Table 1.
The unit cell of Cs1.2Ag0.6Bi3.4Se6 is slightly larger than that of
Cs1.2Ag0.6Bi3.4S6 because of the larger radius of Se.61 Herein,
we describe in detail only the representative crystal structure
of Cs1.2Ag0.6Bi3.ꢀ42Sx6. Cs1.2Ag0.6Bi3.4S6 consists of infinite anionꢀ
ic [AgxBi2ꢀxS3]n quintuple slabs separated by Cs+ cations.
SEM images show a stepꢀlike texture on the plateꢀlike crystals
of Cs1.2Ag0.6Bi3.4S6 which arise from the stacking of the indiꢀ
vidual layers, Figure 2. There is only one crystallographic
and 4f. The Ag/Bi1 (58% Bi and 42% Ag) site forms regular
octahedral coordination with Ag/Bi1ꢀS2 distances of 2.818(6)
Å. The Ag/Bi2 (82% Bi and 18% Ag) site has three long
Ag/Bi2ꢀS2 and three short Ag/Bi2ꢀS1 bonds with an average
bond distance of 2.891 Å. The Cs cation layer resembles that
observed in Cs1.2Ag0.6Bi3.4S6.
Cs2Ag2.5Bi8.5Se15 adopts the Pnnm space group and its strucꢀ
ture belongs to the A2[M5+nSe9+n] homology with n = 6.63 The
n = 1, 2, 3, 4 members of the series were reported previously.63
The discovery of Cs2Ag2.5Bi8.5Se15 validates the predictive
function of the homologous series. In terms of the homology it
can be expressed as Cs2[(Ag/Bi)11Se15] featuring the
[M11Se15]2ꢀ (M = Ag/Bi) modules, Figure 3(b). This module is
four ‘‘BiSe6’’ octahedra wide and three octahedra thick, and is
propagated by linking with identical neighboring modules
through sharing an edge of the Ag/Bi4–Se octahedron to form
a stepwise slab. The structure has six crystallographically difꢀ
ferent Ag/Bi atoms. Except for Bi1, all metal sites have mixed
occupancy by Bi and Ag. Ag/Bi6 forms a regular octahedron
and other Ag/Bi atoms are in distorted octahedral coordination
environments. The bond distances of Ag/BiꢀSe vary from
2.780(5) to 3.194(5) Å with an average Ag/BiꢀSe distance of
2.941 Å. The Cs+ cations are in a bicapped trigonal prismatic
coordination with Cs–Se distances range between 3.579(7)
and 3.579(7) Å.63
ꢀ2x
position for the Ag/Bi atoms. Each [AgxBi2ꢀxS3]n slab is two
octahedra thick and they are isostructural to the layers of
Bi2Se3, and has mixed occupancy of Ag/Bi1 (15% Ag and
85% Bi) octahedral sites bound to six sulfur atoms, Figure
2(a). The Ag/BiꢀS bond distances range from 2.62(2) to
3.10(3) Å. The average Ag/BiꢀS distance is 2.830 Å, which is
very similar to the BiꢀS distances reported in KBi6.33S10 and
K2Bi8S13.62 The Cs+ cations partially occupy two different
Wyckoff positions 2b and 2d making these ions highly disorꢀ
dered in the abꢀplane.
The Cs1.2Ag0.6Bi3.4S6 structure is similar to the AxCdxBi4ꢀxQ6
(A = Cs, Rb, K; Q = S and A = Cs; Q = Se) compounds.46 The
AxCdxBi4ꢀxQ6 structure consists of infinite anionic [CdxBi4ꢀ
xS6]nꢀx layers which adopt the hexagonal motif of the Bi2Se3
structure type, separated by disordered Cs cations.
Cs1.2Ag0.6Bi3.4S6 is derived from AxCdxBi4ꢀxQ6 by changing the
six coordinated Cd2+ to Ag+ with the extra negative charge
balanced by accommodating more alkali metals between the
layers i.e. A2xAgxBi4ꢀxQ6.
Homologous series of compounds are families of related
structures that are composed of similar building blocks whose
number increases by regular increments. Homologous series
provide an ideal playground to predict new materials.27,32 Theꢀ
se structural characteristics make it possible to manipulate
Cs0.6Ag0.8Bi2.2S4 also adopts the P63/mmc space group, see
Table 1. The structure is composed of [AgxBi3ꢀxS4]n slabs and
disordered Cs+ cations. These slabs are thicker than the
the physical properties through a controlled modification of
the crystal structure and/or chemical composition.64 These
compounds with flat layers intercalated with Cs+ cations can
ꢀ2x
[AgxBi2ꢀxS3]n layer described above. In comparison, the unit
cell parameters along a and b directions of Cs0.6Ag0.8Bi2.2S4
are the same as Cs1.2Ag0.6Bi3.4S6 but the cꢀaxis of
Cs0.6Ag0.8Bi2.2S4 is lengthened by 6.223 Å compared to
Cs1.2Ag0.6Bi3.4S6. Figure 2(b) shows how adding a layer of
[Ag/BiS]n atoms the [AgxBi2ꢀxS3]n slab leads to the thicker
[AgxBi3ꢀxS4]n slabs. The [AgxBi3ꢀxS4]n are three octahedra thick,
and feature two crystallographically different Ag/Bi sites 2a
be summarized into a new homologous series Ax[MmQ1+m] (Q
= S and Se). K2xMnxSn3ꢀxS6 (KMSꢀ1)65, K2xMgxSn3−xS6 (KMSꢀ
67
2)66 and RbBiS2 contain [MS2] monolayers. K2xMnxSn3ꢀ xS6
and K2xMgxSn3−xS6 can be denoted as K2x/3(M/Sn)S2 (M = Mn
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Figure 3.
The unit cell of (a) Cs2Ag1.5Bi7.5Se13 and (b) Cs2Ag2.5Bi8.5Se15 and the evolution of the layers (middle).
and Mg) with m = 1. The monolayer in KMSꢀ1 and KMSꢀ2
derives from the simplest of octahedral layers SnS2 by substiꢀ
tuting some Sn4+ atoms for divalent Mg2+ and Mn2+ atoms with
the resulting negative charge being balanced by K+ atoms.
Cs1.2Ag0.6Bi3.4Q6 and CsxCdxBi4ꢀxQ6 (Q = S, Se) can be denoted
as Ax(M/Bi)2Q3 (m = 2) with structure possessing [M2Q3] douꢀ
ble octahedral layers.67 Cs0.6Ag0.8Bi2.2S4 possesses [M3S4]n triꢀ
ple octahedral layer with m = 3. [M3S4]n layers are also found
in the three dimensional PbBi2Se4 compound which is a memꢀ
ber of the galenobismuthite homologous series.38
Cs0.6Ag0.8Bi2.2S4 can be derived from a [PbBi2Se4] slab by subꢀ
stituting Pb2+ atoms with Ag+. This renders the slab negatively
charged and the Cs+ balance the charge by residing between
the layers.
0.21 eV for Cs2AgBi3S6, see Figure 5. This underestimation of
the calculated bandgaps compared to experiment is a wellꢀ
known tendency of semiꢀlocal exchangeꢀcorrelation functionꢀ
als like PBE.68,69
Figure 4. The band gaps of A: Cs1.2Ag0.6Bi3.4S6 with Eg = 1.20
eV; B: Cs0.6Ag0.8Bi2.2S4 with Eg = 1.09 eV; C: Cs1.2Ag0.6Bi3.4Se6
with Eg = 0.67 eV; D: Cs2Ag2.5Bi7.5Se15 with Eg = 0.65 eV.
Figure 5. Electronic band structure of CsAgBi2S4 (a) and
Cs2AgBi3S6 (b).
Ion-Exchange Chemistry. The interlayer space available
for the disordered Cs+ cations in Cs1.2Ag0.6Bi3.4S6 and
Cs0.6Ag0.8Bi2.2S4 with their weak ionic interactions, gives amꢀ
ple room for ion mobility and this suggested to us that these
atoms could participate in ionꢀexchange reactions with other
cations. In fact, the study of metal sulfide as ionꢀexchangers is
an emerging field and promises materials with unique applicaꢀ
tions.70 We find that when these two compounds are immersed
in a water solution of Ag+, Cd2+, Co2+, Pb2+ and Zn2+ ions for
24 h the ionꢀexchange proceeds with apparent ease. Table 2
lists
Optical Absorption and Electronic Structure Calcula-
tions. The four title compounds are semiconductors as indicatꢀ
ed by the solid state electronic absorption spectra which reveal
the presence of bandgaps. Cs1.2Ag0.6Bi3.4S6 shows a sharp and
strong optical absorption edge at 1.20 eV, Figure 4.
Cs0.6Ag0.8Bi2.2S4 shows a smaller band gap (1.09 eV) than
Cs1.2Ag0.6Bi3.4S6 due to the thicker layer. Cs1.2Ag0.6Bi3.4Se6
possesses a band gap of 0.67 eV. Cs2Ag2.5Bi7.5Se15 has the
smallest band gap of 0.65 eV.
The DFT calculated electronic structures indicate an indirect
band gap of 0.51 eV for CsAgBi2S4 and a direct band gap of
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Figure 6. The ionꢀexchange mechanism and band gaps changes after ion exchange (a) C1.2Ag0.6Bi3.4S6 and (b) Cs0.6Ag0.8Bi2.2S4.
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the EDS elemental analysis results of the materials after ion
exchange. The Table shows that in Cs1.2Ag0.6Bi3.4S6 the Cs+
can be completely replaced by Cd2+, Pb2+ and Zn2+ and most
exchanged by Co2+ and the exchanged cations distribute hoꢀ
mogeneously in the layered crystal, Figure S5. In the case of
Cs0.6Ag0.8Bi2.2S4 the Cs+ cations can be completely exchanged
by Ag+, Cd2+, Co2+, Pb2+ and Zn2+, Figure S6. The ionꢀ
exchange process is illustrated in Figure 6 and the ion exꢀ
change was further confirmed by the shift of the PXRD peaks,
i.e. (002), which are shown in Figure 7. Taking Pb2+ as an
example, after ion exchange the (002) basal peak moved to
higher angles indicating a decrease of the interlayer distance
as a result of ion exchange which is consistent with the smaller
size of Pb2+ than Cs+.
Table 2. Energyꢀdispersive Xꢀray Spectroscopy (EDS)
Elemental Analyses Results of Ionꢀexchange Products
Cs
X * Ag
Bi
S
Cs1.2Ag0.6Bi3.4S6
Asꢀ
synthesized
0.92
ꢀ
ꢀ
0.70
3.20
6
Agꢀexchanged
Cdꢀexchanged
ꢀ
ꢀ
10.74 0.41
6
6
6
6
6
0.32 0.68
3.20
3.40
4.00
3.00
Coꢀexchanged 0.20 0.50 0.62
Figure 7. (a) The PXRD of C1.2Ag0.6Bi3.4S6 before and after ion
exchange; (b) The PXRD of Cs0.6Ag0.8Bi2.2S4 before and after ion
exchange.
Pbꢀexchanged
Znꢀexchanged
Cs0.6Ag0.8Bi2.2S4
ꢀ
ꢀ
0.50 0.62
0.40 0.70
The process of ion exchange can be described by the followꢀ
ing equations:
Asꢀsynthesizd 0.94
ꢀ
ꢀ
0.56
1.23
2.24
2.49
2.70
2.11
2.30
2.14
4
4
4
4
4
4
Agꢀexchanged
Cdꢀexchanged
ꢀ
ꢀ
0.30 0.60
Cs1.2Ag0.6Bi3.4S6 (s) + 0.6 Pb(NO3)2 (aq)→Pb0.6Ag0.6Bi3.4S6
(s) + 1.2 CsNO3 (aq)
Cs0.6Ag0.8Bi2.2S4 (s) + 0.3 Pb(NO3)2 (aq)→Pb0.3Ag0.8Bi2.2S4
(s) + 0.6 CsNO3 (aq)
Coꢀexchanged 0.02 0.12 0.64
Pbꢀexchanged
Znꢀexchanged
ꢀ
ꢀ
0.20 0.82
0.21 0.75
X* = Ag, Cd, Co, Pb and Zn cations, correspondingly.
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Figure 8. (a) Electrical conductivity, (b) Seebeck coefficient and (c) Thermal conductivity values as a function of temperature for
Cs1.2Ag0.6Bi3.4S6 (black) and Cs0.6Ag0.8Bi2.2S4 (red).
It is characteristic that the ionꢀexchange process results in
weak tempera ture dependence. This low value is comparable
materials with a redꢀshifted optical absorption. The Pb2+ exꢀ
to compounds known for their ultralow thermal conductivity
and excellent thermoelectric performance, i.e., Cu2ꢀxSe (0.66ꢀ
1.1 W·mꢀ1·Kꢀ1),72 Cu2ꢀxS (0.25ꢀ0.6 W·mꢀ1·Kꢀ1),73 AgSbSe2
(0.23ꢀ0.4 W·mꢀ1·Kꢀ1),74 CsAg5Te3 (0.18ꢀ0.14 W·mꢀ1·Kꢀ1),75
etc. Cs0.6Ag0.8Bi2.2S4 also possesses a lower thermal conductivꢀ
ity of 0.6 W·mꢀ1·Kꢀ1 at room temperature, Figure 8(c). It deꢀ
creases with increasing temperature with the lowest value of
0.49 W·mꢀ1·Kꢀ1 at 825K. The origin of this ultralow thermal
conductivity could be related to its layered structure and the
disordered Cs ion distribution that lends them to “rattle”. Ratꢀ
changed
materials
of
both
Cs1.2Ag0.6Bi3.4S6
and
Cs0.6Ag0.8Bi2.2S4 show the largest red shift and thus smallest
band gaps of 0.66 and 0.76 eV, respectively. This large band
gap narrowing is due to the inter layer bonding change from
ionic in Cs+···S2ꢀ to covalent in PbꢀS so the structure become
more threeꢀdimensional.71 When the Cs ions in
Cs1.2Ag0.6Bi3.4S6 are ion exchanged with Zn2+, Cd2+, and Co2+
the band gap also narrows from 1.20 eV of the pristine sample
to 1.05, 1.03 and 0.85 eV, respectively. When immersed in
solution of Ag+ PXRD measurements reveal that
Cs1.2Ag0.6Bi3.4S6 decomposes partially to Ag2S after ion exꢀ
change, Figure 7(a). This is attributed to the great thermodyꢀ
namic and kinetic stability of the Ag2S phase. The Cs+ in
Cs0.6Ag0.8Bi2.2S4 can be ion exchanged with Zn2+, Co2+, Cd2+
and Ag+ resulting in band gaps of 1.00, 0.96, 0.94, 0.85 and
0.76 eV, respectively. Unlike in Cs1.2Ag0.6Bi3.4S6 no Ag2S was
observed in the case of Cs0.6Ag0.8Bi2.2S4 under the same ion
exchange conditions, Figure 6(b). Both Cs1.2Ag0.6Bi3.4S6 and
Cs0.6Ag0.8Bi2.2S4 exhibit partial ion exchange in Co2+ solution
giving products with formula (derived by EDS) of
Cs0.20Co0.50Ag0.26Bi3.40S6 and Cs0.02Co0.12Ag0.64Bi2.11S4, respecꢀ
tively. The ion exchange process provides a direct “soft chemꢀ
istry” route to new compounds which are not readily accessiꢀ
ble by traditional solid state synthesis techniques.
tler atoms have been invoked as phonon scattering centers in
many compounds including Cs2Hg6S7,76 Ba8Au16P30
,
77
CsAg5Te375, Skutterudites78ꢀ82 and recently in a family of
Ba2AuBi and related Heusler compounds.83
To understand the origin of the low thermal conductivity of
CsAgBi2S4, the phonon dispersions and Grüneisen dispersions
were calculated by using DFT. Grüneisen parameters can
characterize the relationship between phonon frequency and
crystal volume change, thus provide an insight into the lattice
anharmonicity.60,75 The Grüneisen parameters for the acoustic
modes in CsAgBi2S4 are all large and give rise to large averꢀ
age acoustic Grüneisen parameters of γTA=2.87, γTA’=2.07,
γLA=1.61, Figures 9a and b. Thus, the Gruneisen parameters of
CsAgBi2S4 indicate strongly anharmonic vibrational properꢀ
ties. The low frequency vibration modes originate from the Cs,
Ag and Bi contributions which exhibit a rattlerꢀlike behavior.
These modes are identified as three low frequency peaks in
Figure 9c. Similar to CsAg5Te3, where the concerted rattling
modes account for the low lattice thermal conductivity, 75 ratꢀ
tling vibrations in CsAgBi2S4 contribute strongly to suppressꢀ
ing the lattice thermal conductivity. On the basis of the phoꢀ
non dispersions and Grüneisen parameters, the calculated latꢀ
tice thermal conductivities (calculated via a Debyeꢀ Grüneisen
model) of CsAgBi2S4, are shown in Figure 9d. The calculated
value is about 2.1 W·mꢀ1·Kꢀ1 at 300 K, which is significantly
larger than the measurements of total thermal conductivity
around 0.6 W·mꢀ1·Kꢀ1 for Cs0.6Ag0.8Bi2.2S4. On one hand, the
discrepancy could be due to the difference in composition
between the calculated ideal structure of CsAgBi2S4 and the
experimentally determined one at Cs0.6Ag0.8Bi2.2S4, which is
Electrical and Thermal Transport Properties.
Cs1.2Ag0.6Bi3.4S6 features a low electrical conductivity (< 1
S·cmꢀ1) and a relatively high Seebeck coefficient (ꢀ440 ꢁV·Kꢀ
1) at room temperature, which suggests a relatively low carrier
concentration, Figure 8 (a) and (b). With temperature increasꢀ
ing, the electrical conductivity rises rapidly while the absolute
value of Seebeck coefficient decreases, indicating semiconꢀ
ducting transport behavior, with the highest electrical conducꢀ
tivity of 11 S·cmꢀ1, and the Seebeck coefficient of ꢀ183 ꢁV·Kꢀ1
at 775K. The Seebeck coefficients are negative in the entire
temperature range, suggesting electrons as the majority carriꢀ
ers. The conductivity of Cs0.6Ag0.8Bi2.2S4 is 0.05 S·cmꢀ1 and the
Seebeck coefficient is relatively large with ꢀ432 ꢁV·Kꢀ1 at 373
K. When heating both electrical conductivity and Seebeck
coefficient have the same trend as that of Cs1.2Ag0.6Bi3.4S6 with
the highest conductivity of 11 S·cmꢀ1 and Seebeck coefficient
of ꢀ185 ꢁV·Kꢀ1 at 820 K.
really difficult to represent by a simple supercell (due to the
disorderꢀlinked mixed occupancy of Cs). This discrepancy
suggests the presence of extra phonon scattering induced by
the disorder in the Cs layers and the mixed occupancy disorder
of Bi in Ag which are not considered in our calculations.
Another noteworthy feature of Cs1.2Ag0.6Bi3.4S6 is its ulꢀ
tralow thermal conductivity of ~0.45 W·mꢀ1·Kꢀ1 at 300 K,
Figure 8(c). Moreover, the thermal conductivity shows very
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Figure 9. (a) Phonon and (b) Grüneisen dispersions for CsAgBi2S4. (c) The projected phonon density of states for CsAgBi2S4. (d) The
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temperature dependent lattice thermal conductivities of CsAgBi2S4, calculated by the DebyeꢀCallaway model.
Concluding Remarks
Author Contributions
Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6 and Cs0.6Ag0.8Bi2.2S4 toꢀ
All authors have given approval to the final version of the manuꢀ
gether with other reported compounds which possess flat
script.
MSe6 octahedral layers form a new homologous series
Ax[MmQ1+m] (Q = S and Se). In the context of this homology,
Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6 and Cs0.6Ag0.8Bi2.2S4 can be
denoted as Cs0.6(Ag/Bi)2S3 with m = 2, Cs0.6(Ag/Bi)2Se3 with
m = 2 and Cs0.6(Ag/Bi)3S4 with m = 3, respectively. The
Notes
The authors declare no competing financial interest.
Cs2Ag2.5Bi7.5Se15 is new member of the known A2[M5+nSe9+n
]
ACKNOWLEDGMENT
homology with n = 6 and its synthesis enlarges this homoloꢀ
gous family and inspires the pursuit of additional new memꢀ
bers. The Cs+ ions intercalated between the layers of
Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 structure can be topotacꢀ
tically exchanged with Ag+, Cd2+, Co2+, Pb2+ and Zn2+ at room
temperature to give new compound redꢀshifted band gaps tunꢀ
able from 1.20 to 0.66 eV. Thermal conductivity measureꢀ
ments show both Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 comꢀ
pounds possess extremely low thermal conductivity. Although
a full study of the thermoelectric properties is not the focus of
this paper future systematic doping studies should explore
their potential as high performance materials.
This work was supported in part by National Science Foundation
(Grant DMRꢀ1708254). This work made use of the EPIC facility
(NUANCE CenterꢀNorthwestern University), which has received
support under the State of Illinois, Northwestern University, and
the National Science Foundation with grants DMRꢀ1121262
through the MRSEC program at the Materials Research Center,
and EECꢀ0118025/003 through The Nanoscale Science and Engiꢀ
neering Center. S.H. and C.W. (DFT calculations) acknowledge
support from the Department of Energy, Office of Science Basic
Energy Sciences under grant DEꢀSC0014520.
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Xꢀray
crystallographic
data
of
Cs1.2Ag0.6Bi3.4S6,
Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4 Cs2Ag2.5Bi8.5Se15 and
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Corresponding Author
* Eꢀmail: mꢀkanatzidis@northwestern.edu
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