Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5

Article abstract of DOI:10.1021/acs.chemmater.6b02691

New quaternary thioiodides Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅ have been synthesized by isothermal heating as well as chemical vapor transport. Pb₂BiS₂I₃ and Sn₂BiS₂I₃ crystallize in the space group, Cmcm, with unit cell parameters a = 4.3214 (9), b = 14.258 (3), and c = 16.488 (3) ? a = 4.2890 (6), b = 14.121(2), and c = 16.414 (3) ?, respectively. Sn₂BiSI₅ adopts a unique crystal structure that crystallizes in C2/m with cell parameters a = 14.175 (3), b = 4.3985 (9), c = 21.625 (4) ?, and β = 98.90(3)°. The crystal structures of Pb₂BiS₂I₃ and Sn₂BiS₂I₃ are strongly anisotropic and can be described as three-dimensional networks that are composed of parallel infinite ribbons of [M₄S₂I₄] (M = Pb, Sn, Bi) running along the crystallographic c-axis. The crystal structure of Sn₂BiSI₅ is a homologue of the M₂BiS₂I₃ (M = Pb, Sn) which has two successive ribbons of [M₄S₂I₄] separated by two interstitial (Sn_1-xBi_xI₆) octahedral units. These compounds were characterized by scanning electron microscopy, differential thermal analysis, and X-ray photoelectron spectroscopy. Pb₂SbS₂I₃, Pb₂BiS₂I₃, Pb₂Sb_1-xBi_xS₂I₃ (x ～ 0.4), Sn₂BiS₂I₃ and Sn₂BiSI₅ are highly resistive and exhibit electrical resistivities of 3.0 G cm, 100 M cm, 65 M cm, 1.2 M cm, and 34 M cm, respectively, at room temperature. Pb₂BiS₂I₃, Sn₂BiS₂I₃, Pb₂SbS₂I₃, Pb₂Sb_1-xBi_xS₂I₃ (x ～ 0.4), and Sn₂BiSI₅ are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66, and 1.32 eV, respectively. The electronic band structures of Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅, calculated using density functional theory, show that all compounds are direct bandgap semiconductors.

Full text of DOI:10.1021/acs.chemmater.6b02691

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Article
Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5
Saiful M. Islam, Christos D. Malliakas, Debajit Sarma, David C Maloney, Constantinos
C. Stoumpos, Oleg Y. Kontsevoi, Arthur J. Freeman, and Mercouri G. Kanatzidis
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02691 • Publication Date (Web): 15 Sep 2016
Downloaded from http://pubs.acs.org on September 16, 2016
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Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3
and Sn2BiSI5
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Saiful M. Islam,† Christos D. Malliakas † Debajit Sarma,† David C. Maloney,† Constantinos C.
Stoumpos, † Oleg Y. Kontsevoi, § Arthur J. Freeman, §,& Mercouri G. Kanatzidis†,#
†Department of Chemistry, §Department of Physics and Astronomy,
Northwestern University, Evanston, Illinois 60208, United States
We dedicate this work to the deceased Arthur J. Freeman
ABSTRACT: New quaternary thioiodides Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 have been
synthesized by isothermal heating as well as chemical vapor transport. Pb2BiS2I3 and Sn2BiS2I3
crystallize in the space group, Cmcm, with unit cell parameters a = 4.3214 (9), b = 14.258 (3),
and c = 16.488 (3) Å; a = 4.2890 (6), b = 14.121(2), and c = 16.414 (3) Å, respectively.
Sn2BiSI5 adopts a unique crystal structure that crystallizes in C2/m with cell parameters a =
14.175 (3), b = 4.3985 (9), c = 21.625 (4) Å, and β = 98.90(3)o. The crystal structures of
Pb2BiS2I3 and Sn2BiS2I3 are strongly anisotropic and can be described as three dimensional
networks that are composed of parallel infinite ribbons of [M4S2I4] (M = Pb, Sn, Bi) running
along the crystallographic cꢀaxis. The crystal structure of Sn2BiSI5 is a homologue of the
M2BiS2I3 (M=Pb, Sn) which has two successive ribbons of [M4S2I4] separated by two interstitial
(Sn1ꢀxBixI6) octahedral units. These compounds were characterized by scanning electron
microscopy, differential thermal analysis, and Xꢀray photoelectron spectroscopy. Pb2SbS2I3,
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Pb2BiS2I3, “Pb2Sb1ꢀxBixS2I3” (x~0.4), Sn2BiS2I3 and Sn2BiSI5 are highly resistive that exhibit
electrical resistivity of 3.0 Gꢁ.cm, 100 Mꢁ.cm, 65 Mꢁ.cm, 1.2 Mꢁ.cm and 34 Mꢁ.cm,
respectively at room temperature. Pb2BiS2I3, Sn2BiS2I3, Pb2SbS2I3, “Pb2Sb1ꢀxBixS2I3” (x~0.4),
and Sn2BiSI5 are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66 and 1.32 eV,
respectively. The electronic band structures of Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5, calculated
using density functional theory, show that all compounds are direct bandgap semiconductors.
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-----------------------------------
Deceased: Arthur J. Freeman
Department of Physics and Astronomy,
Northwestern University, Evanston, Illinois 60208, United States
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INTRODUCTION
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Heteroꢀanionic inorganic compounds define a unique class of materials which contain at least
two different anionic species in their structures, and they can exhibit exceptional physical and
chemical properties.1ꢀ7 Their chemistry is not as well developed as that of homoꢀanionic
compounds. Because of the variance in electronegativity, ionic charge, charge densities,
polarizability, and other physicoꢀchemical properties different anions exhibit diverse bonding
propensities towards metal cations. When occurring in the same lattice these differences result in
different coordination preferences and diverse structural fragments to generate novel composite
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crystal structures with unique properties. For example, the heteroꢀanionic compounds Tl6SeI4
and SbSeI8 show promise in nuclear radiation detection, BiOI can be used as photoꢀcatalyst for
water purification,9ꢀ10 AgNa(VO2F2)2 is used as cathode material for electrochemical energy
12ꢀ16
storage,11 oxypicnides LnFeOPn (Pn = P, As, Ln = rareꢀearth)3,
evolved as high Tc
superconductors, BiOCuSe exhibits enhanced thermoelectric properties, LaCuOQ and BaCuQF
(Q = S, Se, Te)17ꢀ20 act as transparent conducing semiconductors, and Ba3AGa5Se10Cl2(A=Cs,
Rb, K),2 (Sb7S8Br2)(AlCl4)3,21 (Hg6P3)(In2Cl9)6 and (Hg8As4)(Bi3Cl13)6 are interesting due to
their strong second harmonic generation (SHG) properties.22
One of the subclasses of heteroꢀanionic inorganic compounds is chalcoꢀhalides. This class of
compounds exhibits unusual structures which are the results of the distinct bonding preferences
of chalcogenide and halide atoms as they compete for a stable site in the structure. Notable
examples include CuCr2Se4ꢀxBrx,1 MnSbS2Cl,23 MnSbSe2I,24 Ni8Bi8SI,25 CdSb6S8I4,26 LnSbS2Br2
(Ln = La, Ce),27 LaSbS5Br,28 LaCa2GeS4Cl3,29 Cu3Bi2S4Br,30 and AE2Sb2Q4F2 (AE = Sr, Ba; Q =
chalcogen),31 InBi2S4Cl,32 Cu2Cu2−x(Pb1−xBix)Bi2S5I2,33 CdSb6S8I4,26 SnSb2SI6,34 Sn2SbSe2I3,34
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Sn3SbSe2I5,34 Hg3AsSe4I,35 Sn2SbS2I3,36ꢀ37 Pb2SbS2I3,37ꢀ38 Bi2CuSe3I,39 Bi6Cu3S10I39 and the
mineral mutnovskite Pb2AsS3(I, Cl, Br).40
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In this contribution, we report the syntheses and crystal structures of three novel sulfoꢀiodides,
Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5. The materials are direct bandgap nꢀtype semiconductors with
bandgaps ranging between 1.2ꢀ1.6 eV and exhibit high electrical resistivity (> 1 Mꢁ.cm).
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EXPERIMENTAL SECTION
Synthesis
BiI3 and SbI3: Antimony and bismuth iodides were synthesized by the meltꢀquench technique
in a fast heating furnace. For SbI3, a mixture of antimony (363.0 mg; 3 mmol, 99.999% Alfa
Aeser), and iodine (1146 mg; 9.01 mmol; 99 +% Sigma Aldrich) were combined in an evacuated
sealed silica tube (l = 11 cm, 9 mm inner diameter). The reactants were melted in the furnace at
400oC and subsequently quenched in water. Initially during heating the interior of the tubes had a
dark color due to liberation of iodine gas. Subsequent meltingꢀquenching cycles (7ꢀ8 cycles in ~
½ h) allowed for the complete reaction of iodine evidenced by the disappearance of the dark gas
inside the tubes. Finally, the tubes were heated at 450oC for about an hour and quenched in air to
form SbI3 (red). To synthesize BiI3 (black), (bismuth (629.7 mg; 3 mmol; 99.999% Alfa Aeser),
and iodine (9.01 mmol, 99+% Sigma Aldrich) were loaded in an evacuated sealed silica tube (l =
11 cm, 9 mm inner diameter) and the same heat treatment was used as described for SbI3 (red).
SnS: 7.8736 g of tin (American Elements, 99.999%) and 2.1264 g sulfur (Sigma Aldrich,
99.99%) were combined in a fused silica tube (l = 30 cm, 8 mm inner diameter, ID). The first
tube was then sealed under a pressure of 5×10ꢀ4 Torr and sealed inside a second tube (15 mm ID)
under a pressure of 9×10ꢀ3 Torr. The system was then heated to 400oC in 4 hours, 450oC in 5
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hours, 1000oC in 11 hours, and held at that temperature for 5 hours. The tube was finally allowed
to cool to 610oC in 20.5 hours, 590oC in 20 hours, and then to room temperature in 11 hours.
Caution: It is important to follow this procedure closely to prevent the tubes from bursting
catastrophically. This is because sulfur has a high vapor pressure and if it fails to react with Sn
metal due to passivation of the surface it can burst the inner tube. The heating profile, tube sizes,
pressures, and reactant mass were chosen to minimize the effect of sulfur’s high vapor pressure
and to ensure that as much sulfur as possible had reacted before reaching its boiling point.
Furthermore, SnS has a phase transition at 600oC where it expands on cooling breaking the inner
tube. The cooling temperature program was chosen so that the inner tube breaks gently and does
not shatter the outer tube. If the outer tube breaks on heating or cooling while the reactants are at
high temperature the reactants will oxide violently.
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PbS. Was synthesized by heating a stoichiometric mixture of elemental lead (8288 mg,
40mmol) (99.999% Alfa Aeser) and sulfur (1280 mg, 40mmol) (99.98% 5N Plus Inc.) to 400oC
in 24h and holding for 24h and then the temperature was raised to 900oC in 24h followed by a
subsequent soaking for 48h and finally cooled down to room temperature in 12 h.
Pb2BiS2I3. Pb2BiS2I3 was obtained with a yield of ~ 90% (weight) by heating a stoichiometric
mixture of PbS (2.871mg, 12 mol) and BiI3 (3.538g, 6 mol) at 500oC for 48h, holding for 72h,
and cooling down to room temperature in 24h all in a sealed silica tube (l = 12 cm, width 9 mm
inner). This procedure led to black wellꢀformed needleꢀlike crystals Pb2BiS2I3 up to ~ 2 mm in
length (Figure 1). Attempting to synthesis from a stoichiometric mixture of elemental Pb (414.4
mg, 2 mmol), Bi (209.0 mg, 1.0 mmol), sulfur (64.0 mg, 2 mml) and iodine (~380 mg, 3 mmol)
resulted in a mixture of Pb2BiS2I3, BiSI41 and PbI2.
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Sn2BiS2I3 and Sn2BiSI5: Sn2BiS2I3 was synthesized from a melted mixture of elemental tin
(238.7 mg, 2 mmol; 99.999% Alfa Aeser), bismuth (209.0 mg, 1 mmol; 99.999% Alfa Aeser),
sulfur (64.0 mg, 2 mmol; 99.98% 5N Plus Inc.) and iodine (400 mg, 3.15 mmol; 99+%, Sigma
Aldrich). The reactants were heated in evacuated sealed silica tube (l = 12 cm, inner width 9
mm) for 72h to 500oC, held at that temperature for 24h, and finally cooled to room temperature
in 96 h. This procedure led to the formation of mainly Sn2BiS2I3 (black, Figure 1) along with
Sn2BiSI5 (black, Figure 1), SnS2 (yellow), and BiSI (needles). The sulfoꢀiodide, Sn2BiS2I3 was
also synthesized from a melted mixture of SnS (150.8 mg; 1 mmol), BiI3 (294.8 mg; 0.5 mmol)
in sealed silica tube (yield ~90% by weight). The reactants were melted and water quenched
repeatedly in a melting furnace at 420 and 520oC (seven to eight meltingꢀquenching cycles). The
tube was then kept in the furnace at 520oC for about 3 h and then the furnace was shut down for
cooldown to room temperature. This melting quenching cycles are required to capture the
dissociated iodine from BiI3 to react with SnS and to form Sn2BiS2I3.
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In order to synthesize Sn2BiSI5, a stoichiometric mixture of Sn (119.9 mg, 1 mmol), Bi (106.1
mg, 0.5 mmol), S (18.2 mg, 0.5mmol) and I (320.0 mg, ~2.5 mmol) was loaded in a silica glass
tube, vacuum sealed and melted at 400oC and then water quenched. This melting and quenching
processes continued several times until most of the iodine had reacted. This was determined by
observing the gradual disappearance of purple vapor in the tubes till they were almost colorless.
Afterwards, the tube was transferred into a furnace and heated to 500°C in 24h then held for 24 h
and cooled down to room temperature in 48 h. This procedure led to the formation of Sn2BiSI5,
Sn2BiS2I3, and an unknown impurity (identified Xꢀray powder diffraction) at the bottom of the
tube (reactant containing high temperature end) with a yield of about 70% while a small amount
of Sn2BiSI5 and “unknown impurity” (∑~50 mg) were transported to other side of the tube
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(cooler end) due to small temperature gradient (ꢂT~25oC) in the furnace. Sn2BiSI5 can be easily
separated from “unknown impurity” by washing the solid mixture with ethanol where the latter is
soluble.
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“Pb2Sb1-xBixS2I3”: “Pb2Sb1ꢀxBixS2I3” (x~0.4) was synthesized aiming for a stoichiometric
composition of Pb2Sb0.5Bi0.5S2I3. A mixture of PbS (478.6 mg; 2 mmol), Bi (105.0 mg; ½ mmol)
Sb (61.0 mg, ½ mmol), sulfur (64.0 mg, 2 mml) and iodine (~380 mg, 3 mmol) was heated to
500oC in 48h, holding for 72h, and cooling down to room temperature in 24h in an evacuated
”
sealed silica tube (l = 12 cm, inner width 9 mm). This procedure led mainly to “Pb2Sb1ꢀxBixS2I3
(x~0.4), along with other minor phases BiSI,41 SbSeI,8 and PbI2 which were identified by Xꢀray
powder diffraction as well EDS (Energy Dispersive Xꢀray Spectroscopy) analyses.
Powder X-ray Diffraction. Powder Xꢀray diffraction (PXRD) data were collected on ground
crystalline samples of Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 with a flat sample geometry using a
siliconꢀcalibrated CPS 120 INEL powder Xꢀray diffractometer (Cu Kα graphiteꢀ
monochromatized radiation) operating at 40 kV and 20 mA equipped with a positionꢀsensitive
detector. Simulated patterns were generated using the CIF of each refined structure and the
Visualizer program within FindIt.42
Scanning Electron Microscopy. Images and semiꢀquantitative energy dispersive Xꢀray
spectroscopy (EDS) analyses of Pb2BiS2I3, Sn2BiS2I3, “Pb2Sb1ꢀxBixS2I3” (x~0.4) and Sn2BiSI5
were obtained using a Hitachi Sꢀ3400 scanning electron microscope equipped with a PGT
energyꢀdispersive Xꢀray analyzer. For the analysis singleꢀcrystals of the above thioiodides were
selected under the optical microscope and placed on the surface of the double sided carbon tape
which was already attached on the flat surface of the SEM stubs. For EDS analysis spectra were
collected using an accelerating voltage of 15 kV and a 90 s accumulation time. These analyses
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show the homogeneous distribution of the respective atoms with the atomic ratio close to their
corresponding chemical compositions (Figure S1ꢀS4). The compositions obtained as
Pb=26.94%, Bi=12.06%, S=23.92%, and I=37.08% % for Pb2BiS2I3 (Figure S1); Pb=26.75%,
Sb= 6.89%, Bi=5.92%, S=23.63%, and I=36.81% for“Pb2Sb1ꢀxBixS2I3” (Figure S2); Sn=23.47%,
Bi=14.54%, S=27.5%, and I=34.49% for Sn2BiS2I3 (Figure S3); and Sn=25.97%, Bi=13.22%,
S=11.23%, I=49.58% (Figure S4) for Sn2BiSI5.
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Single-Crystal X-ray Crystallography. X–ray single crystal data collection of Pb2BiS2I3,
Sn2BiS2I3, and Sn2BiSI5 were performed on a STOE IPDS II or a STOE IPDS 2T diffractometer
using Mo Kα radiation (λ = 0.71073 Å) and operating at 50 kV and 40 mA at 293 K. Integration
and numerical absorption corrections were performed using XꢀAREA, XꢀRED, and XꢀSHAPE.
All structures were solved using direct methods and refined by fullꢀmatrix least squares on F2
using the SHELXTL program package. The crystal structure refinement of Sn2BiS2I3 exhibits
two distinct crystallographic sites 8f and 4c for metal atoms where the 8f Wyckoff site is mixed
occupied (Sn1ꢀxBix) and the 4c site is exclusively occupied by Sn atom. In contrast to the
straightforward refinement of the crystal structure of Sn2BiS2I3 the refinement of the crystal
structure of Pb2BiS2I3 was indistinct due to the very similar Xꢀray scattering power of lead
(Z=82) and bismuth (Z=83). Pb2BiS2I3 is isostructural to Sn2BiS2I3 and thus the 4c site is
occupied by the divalent Pb atom and 8f site is shared by Pb and Bi atoms. Because of similar
atomic number Z of Pb and Bi, the occupancy of the mixed 8f site was fixed to 50% of each
element in the final step of the refinement. This gives a composition Pb2BiS2I3 which is
consistent with the EDS analysis. According to bond valence sum calculations43 on Pb2BiS2I3 the
unique Pb site exhibits BVS ~ 2.04 v.u. and for the mixed (Pb0.5Bi0.5) sites BVS is 2.54 v.u.
These values are in good agreement with the respective charge of Pb2+ and mixed
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(Pb0.5+2Bi0.5+3 +2.5. A complete list of crystallographic information, data collections, structure
)
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refinements, atomic coordinates, thermal displacement parameters, and selected inter atomic
distances and angles are given in Table 1 and 3.
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X-ray Photoelectron Spectroscopy (XPS). Xꢀray photoelectron studies were performed using
a Thermo Scientific ESCALAB 250 Xi spectrometer equipped with a monochromatic Al Kα Xꢀ
ray source (1486.6 eV) and operated at 300 W. Samples were analyzed under vacuum (P < 10−8
mbar), whereas survey scans and highꢀresolution scans were collected using pass energy of 50
eV. Binding energies were referred to the C 1s binding energy at 284.6 eV. A lowꢀenergy
electron flood gun was employed for charge neutralization and ion beam etching for surface
cleaning. Prior to XPS measurements, ground crystals of each sample attached on copper foil and
mounted on stubs and successively put into the entryꢀload chamber to pump.
Resistivity and Seebeck Measurements. Resistivity measurements were conducted using a
homemade apparatus equipped with a nanovoltmeter (Keithley 2182A), an electrometer
(Keithley 6517), and a highꢀtemperature vacuum chamber governed by a temperature controller
(MMR Technologies Kꢀ20). For the Seebeck coefficient measurement, needleꢀlike crystals were
mounted on the stage and attached to the thermocouples with colloidal graphite liquid. The
Seebeck voltage V(T) was measured by the integral method, in which one end of the sample is
held at a fixed temperature T0, and the other end is varied through the temperature T range of
interest using a commercial MMR Technologies SBꢀ100 Seebeck measurement system. The
Seebeck coefficient S is obtained from the slope of the V(T) vs T curve, i.e., S = dV(T)/dT.44
Band Structure Calculations. In order to investigate the electronic structure of Pb2BiS2I3,
Sn2BiS2I3 and Sn2BiSI5, firstꢀprinciples calculations were carried out within the density
functional theory formalism using the Projector Augmented Wave method45 implemented in
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Vienna Abꢀinitio Simulation Package46ꢀ47. The energy cut off for planewave basis was set to 350
eV. The generalized gradient approximation (GGA), functional of Perdew−Burke−Ernzerhof
(PBE)48, was employed. The crystal structures and the lattice parameters were fixed to their
experimentally observed values, but the positions of atoms in the cells were relaxed until the
atomic forces on each atom are less than 0.01 eV/Å. The MonkhorstꢀPack kꢀpoint grids of
12×12×3, 12×4×3, and 12×12×2 were used for Brillouin zone (BZ) sampling for Pb2BiS2I3,
Sn2BiS2I3, and Sn2BiSI5, respectively.
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RESULTS AND DISCUSSION
Syntheses, purification and thermal behavior. Pb2BiS2I3 and Sn2BiS2I3 were synthesized by
slowly cooling a melted mixture of PbS with BiI3, and SnS with BiI3, in a molar ratio of 2:1 in
sealed silica tubes at 500oC. The synthesis of Pb2BiS2I3 and Sn2BiS2I3 from their direct elemental
combination led to the formation of mixtures of the binary compounds PbI2, SnI4, and SnS2, or
ternary compounds such as BiSI, alongside the dominant desired products. The formation of the
binary and ternary impurities along with the desired product could be due to the loss of iodine
gas from the reaction mixture that eventually results in a change in the stoichiometry of the
reaction. Attempts to synthesize a single phase product of Sn2BiSI5 resulted in the formation of
black needles of Sn2BiS2I3, Sn2BiSI5, and unknown impurities at the bottom (high temperature
end) of the silica tube while Sn2BiSI5 is transported in the other end (low temperature end).
Pb2BiS2I3, Sn2BiS2I3, Sn2BiSI5 and “Pb2Sb1ꢀxBixS2I3” (x~0.4) are stable in air and insoluble in
water, acetone, ethanol, formamide, DMF, and hydrazine. Differential thermal analysis (DTA)
was performed on ground crystals of Pb2BiS2I3 (Figure 2A) which showed that the compound
melts incongruently at 455oC and crystallizes at 410oC. Careful inspection of the DTA tube
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revealed the existence of yellow PbI2. This provides evidence of the incongruent melting
behavior of Pb2BiS2I3. Thermogravimetric analysis (TGA) under nitrogen atmosphere also
showed that Pb2BiS2I3 starts to decompose at about 460oC (Figure 2B) and thus in agreement
with its incongruent melting behavior. Due to the high volatility of iodine we anticipate other
thiohalides behave similarly to the Pb2BiS2I3.
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Crystal structures. Pb2BiS2I3 and Sn2BiS2I3 crystallize in centrosymmetric space group,
Cmcm, and are isostructural to Pb2SbS2I3.37, 49 The crystal structure refinement was forthright for
Sn2BiS2I3, however, because of very similar atomic scattering power of lead and bismuth
assignment of their positions in the crystal structure of Pb2BiS2I3 was supported by chemical
analyses of its isostructural thiohalides, bond valence sum calculation43, EDS analyses (discussed
in detail in experiment section, Figure S1) as well as the minimal energy at different distribution
of metals at different crystallographic sites during electronic band structure calculations.
Pb2BiS2I3 and Sn2BiS2I3 exhibit a statistically disordered structure where one M2+ (M=Pb or Sn)
cations exclusively occupy 4c crystallographic site while a mixed occupancy exists
3+ +2.5
(M0.52+Bi0.5
)
at the 8f Wyckoff site (Figure 3). This distribution of cations was also
observed in the crystal structure of Sn2SbS2I3 where Sn and Sb also have very similar atomic
number.37
The crystal structure of Pb2BiS2I3 can be simply described as parallel infinite chains of
[(Pb0.5Bi0.5)2S2] extending along the crystallographic aꢀaxis (Figure 3B). These chains are
interconnected by trigonal prismatic [PbI6] polyhedra through faceꢀsharing to give a rigid twoꢀ
dimensional network along cꢀaxis. A weak (Pb0.5Bi0.5)ꢀI ~ 3.80 Å interaction (as compared to
d(PbꢀI)=3.21 Å in PbI2) extend the crystal structure along the crystallographic bꢀaxis and thus
provides a threeꢀdimensional feature of the crystal structure.
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Similar structural topographies are observed for Sn2BiS2I3 where an infinite chain of [(Sn1ꢀ
xBix)2S2] (x~0.5) running along the crystallographic aꢀaxis and are interconnected by trigonal
prismatic [SnI6] polyhedra (Figure 3D). An alternative description of the crystal structures of
Pb2BiS2I3 and Sn2BiS2I3 are as the assembly of infinite [((Pb0.5Bi0.5)2Pb2S2I4] or [(Sn1ꢀ
xBix)2Sn2S2I4] (x~0.5) ribbons ([M4S2I4]; M = Pb, Sn, Bi), respectively running along the
crystallographic cꢀaxis (Figure 3AꢀB with gray shaded regions). Interestingly, these ribbons are
arranged in such a way that essentially form a two dimensional crystal structure (Figures 3B and
3D) along ac plane of the crystal structure. A weak interaction between M and I, d(MꢀI) = ~ 3.80
Å (M = PbBi, SnBi) provides the 3Dꢀlike character of the crystal structures Pb2BiS2I3 and
Sn2BiS2I3.
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Like Sn2BiS2I3, Sn2BiSI5 exhibits a statistical disorder in its crystal structure (Figure 4A, B). In
case of Sn2BiSI5, the [(Sn1ꢀxBix)2S2] (x~0.5) chains are extended along the crystallographic bꢀ
axis and two successive chains are inter connected by a trigonal prismatic [SnI6] and two (Sn1ꢀ
xBixI6) octahedral unit (Figure 4E, F) along the cꢀaxis. The second approach is to describe the
structure as an infinite ribbon [M4S2I4] (M = Sn, Bi) running along the crystallographic cꢀaxis.
Two adjacent ribbons parallel to the aꢀaxis are separated by two (Sn1ꢀxBixI6) (x~0.5) octahedral
unit which reveals a void of ~18Å in length along the cꢀaxis. The accommodation of such (Sn1ꢀ
xBixI6) octahedra into the structure of Sn2BiS2I3 results in the increase of the crystallographic cꢀ
axis from ~16.4 to 21.6 Å for Sn2BiSI5. Like Sn2BiS2I3, a weak bonding interaction between
(Sn1ꢀxBix) and iodine (MꢀI=~3.8Å) is again indicative of the 3D character of the crystal structure
of Sn2BiSI5. It is noteworthy to mention that a very similar ribbon of [M4S2I4] (M=Pb) also exists
in the crystal structure of the Pb5S2I6 (Figure 4C).50 In Pb5S2I6 two adjacent ribbons [Pb4S2I4] are
interconnected along the cꢀaxis by two isolated [PbI6] octahedra that leave a void of ~7.5Å. Each
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of the two [PbI6] octahedra in Pb5S2I6 also connects [Pb4S2I4] ribbons along the [211] lattice
plane, however, this kind of building feature is completely missing in the crystal structure of
Sn2BiSI5. Finally, we conclude that despite the difference in connectivity of [M4S2I4], all
thiohalides of M2BiS2I3 (M = Sn, Pb), Sn2BiSI5, and Pb5S2I6 are structurally closely related. A
homologous series can be defined where the crystal structure of M2BiS2I3 consists of an infinite
ribbon [M4S2I4]m=α (M = Sn, Pb, Bi) while for Pb5S2I6 and Sn2BiSI5, the ribbon is separated by
one [MI6] and two [MI6] octahedra, respectively. If a homologous series does exist in this family
then several more members with predictable structure and compositions could be expected to
form.51ꢀ52
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The coordination environments, interatomic distances and angles Pb2BiS2I3, Sn2BiS2I3, and
Sn2BiS2I5 are given in Figure 5 and Tables 4ꢀ6. Because of the isostructural character of
Pb2BiS2I3 and Sn2BiS2I3, the metals atoms exhibit similar coordination environments. In
Pb2BiS2I3, (Pb0.5Bi0.5) forms 8ꢀfold coordination polyhedra as [Pb0.5Bi0.5S3I5] where the d(Pb1ꢀ
S1) shows one short bond 2.638 Å and two long bonds 2.737Å. The d[(Pb10.5Bi10.5) ꢀI)]
distances can be classified as long (d[(Pb0.5Bi0.5) ꢀI1)] = 3.770 Å), intermediate d[(Pb0.5Bi0.5)ꢀI2)]
= 3.670Å), and short (d[(Pb0.5Bi0.5)ꢀI2)] = 3.454Å). When all bonding interactions are considered
the polyhedron adopts a distorted bicappedꢀtrigonal prismatic coordination environment. The
unique Pb atom (at 4c site) also adopts a bicapped trigonal prismatic coordination with six iodine
atoms assembling the trigonal prism, two additional sulfur atoms capping the square faces of the
prism thus completing the 8ꢀfold coordination of the [PbI6S2] polyhedra. In this polyhedron,
d(Pb2ꢀI) inter atomic distances can be classified as short and long d(Pb2ꢀI) = 3.134 and 3.600Å,
respectively. In Sn2BiS2I3, the coordination polyhedra of (Sn21ꢀxBi1x) (x~0.5) appear as [(Sn21ꢀ
xBi1x)S3I5] (3×d((Sn2Bi)ꢀS1) = 2.562ꢀ2.726Å and 5×d((Sn2Bi1ꢀI) ranging from 3.353 to
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3.817Å) and coordination environment of Sn1 comprises of two sulfur (2×d(Sn1ꢀS1) = 2.921Å)
and six iodine atoms (2×d(Sn1ꢀI2) = 3.029 Å and 4×d(Sn1ꢀI1) = 3.649 Å)) atoms.
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The crystal structure of Sn2BiSI5 consists of [(Bi2ASn2A)S3I5] (Bi2A~Sn2A~0.5), [Sn3SI6]
and [(Bi1ASn1A)I6] (Bi1A~Sn1A~0.5). The coordination polyhedra of [(Bi2ASn2A)S3I5]
(3×d((Bi2ASn2A)ꢀS1) ~ 2.637 to 2.722Å) and 5×d(Bi2ASn2AꢀI) ~ 3.472 to 3.802 Å) and
[Sn3SI6] (d(Sn3ꢀS1) = 2.677Å and d(Sn3ꢀI) distances ranging from 3.079 to 3.715Å and the Sn3ꢀ
I1 distance is 4.152Å and the latter remains just at the boundary of the sum of their van der
Waals radii that represents very weak interactions.
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X-ray Photoelectron spectroscopy. Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5 were characterized by
Xꢀray photoelectron spectroscopy (Figure 6). Table 7 shows the binding energies of the
representative orbitals of each element in these thioiodides. The splitting of the binding energy of
4f orbitals in lead and bismuth, and the energies of the 3d orbitals for tin and iodine are attributed
to the strong spinꢀorbit coupling. The binding energy of lead 4f is assigned as 143.34 (4f5/2) and
138.43 (4f7/2) eV which is consistent with +2 oxidation states of lead in Pb2BiS2I3 and thus in
agreement with its crystal structure.53 In all the three thioiodides the binding energies of Bi 4f are
very similar (~159 (4f7/2), ~164 (4f5/2) eV) and are corresponding to their +3 oxidation states.53 In
addition, the very similar binding energies of bismuth in these thioiodides are indicative for their
very similar chemical environments. The binding energies at 486.73 (495.12) and 487.23
(495.76) eV are attributed to Sn 3d5/2 (3/2) energies of Sn2BiS2I3 and Sn2BiSI5, respectively.54
Optical and electronic properties. Pb2BiS2I3, Sn2BiS2I3, “Pb2Sb1ꢀxBixS2I3” (x~0.4), and
Sn2BiSI5 are black in color which is in agreement with their experimental optical band gaps 1.60,
1.22, 1.66 and 1.32 eV, respectively (Figure 7). The antimony analogue Pb2SbS2I3 exhibits a
bandgap of ~1.92 eV which is close to the value (Eg~2.0 eV) reported by Starosta et al.38 The
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bandgap of Pb2BiS2I3 is about ~0.32 eV lower than that of Pb2SbS2I3. This can be understood by
the greater degree of extension of the atomic orbitals and higher spin orbit coupling of the
bismuth atom. Interestingly, despite the smaller cationic size of tin the bandgap of Sn2BiS2I3 is
about 0.4 eV lower than that of its lead analogue Pb2BiS2I3. Similar findings are observed for
Sn2SbS2I3 and Pb2SbS2I3. The high energy of the delocalized lone 5s2 pair of electrons on the
Sn2+ cation could explain such lowering of bandgaps.55ꢀ56 In all cases we observe sharp
absorption edges in the spectra which are suggestive of a direct electronic transition from the
valence band to conduction band, a statement which is supported by the electronic structure
calculations (vide infra).
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The room temperature electrical resistivity of Pb2SbS2I3, Pb2BiS2I3, “Pb2Sb1ꢀxBixS2I3” (x~0.4),
Sn2BiS2I3, and Sn2BiSI5 measured by a twoꢀpoint contact along the crystallographic cꢀaxis which
is the longꢀaxis of the needleꢀshaped crystals. This thioiodides are highly resistive and exhibit
electrical resistivity as 3.0 Gꢁ.cm, 100 Mꢁ.cm, 65 Mꢁ.cm, 1.2 Mꢁ.cm, and 34 Mꢁ.cm,
respectively (Figure 8). These values of the resistivity of the isostructural compounds A2BS2I3 (A
= Sn, Pb, and B = Sb, Bi) are in agreement with the trend in their optical bandgaps, with the
highest resistivity sample exhibiting the highest optical bandgap. The Seebeck coefficients for all
samples were negative and very large, between ~ ꢀ1500 uV/K for Pb2BiS2I3 and ~ ꢀ3000 uV/K
for Sn2BiS2I3, indicating an intrinsic semiconducting behavior (Figure S5) with a very low
number of charge carriers. The thermal conductivity of Pb2BiS2I3 was measured from 100 to
400oC and the values were found to be very low ranging from 0.7 to 1.2 W/m K (See supporting
information, Figure S6).
Band structure calculations
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In order to better understand the nature of the semiconducting character and electronic
transitions in Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 we performed electronic band structure
calculations (Figure 9ꢀ11). With the PBE exchangeꢀcorrelation functional the bandgaps are
predicted to have values of 1.18, 0.90, and 0.85 eV for Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5,
respectively, which are underestimated compared to the experimental values of 1.60, 1.22, and
1.32 eV. This underestimation of bandgaps compared to the experimentally obtained values is a
wellꢀknown tendency of semiꢀlocal functionals like GGA.57ꢀ58
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Pb2BiS2I3 has mixed occupancy of Pb and Bi at the 8f crystallographic site. In order to perform
the electronic structure calculations, a structural model was created that approximates mixed
occupation of these sites by a periodic array of sites occupied by either Pb or Bi. An unlimited
number of such models can be proposed with different arrangements and shortꢀ and longꢀrange
order within the Pb/Bi sublattice. A model utilized in this work employs a conventional unit cell
which has double the number of atoms compared with the primitive unit cell, and has the
orthorhombic primitive lattice instead of orthorhombic Cꢀcentered lattice of the primitive unit
cell. Such cell is the smallest possible unit cell and therefore has the highest degree of order.
The calculated electronic structure of Pb2BiS2I3 is shown in Figure 9 in the form of (a) the
electronic band structure plotted along the lines between the highꢀsymmetry points in the BZ and
(b) the electronic density of states (DOS) projected onto the atomic sites. The valence band (VB)
maximum (VBM) and the conduction band (CB) minimum (CBM) are both located at the Γ
point revealing that Pb2BiS2I3 is a direct bandgap material. This result is in agreement with the
sharp nature of the absorption edges observed from its experimental spectrum. It should be noted
that the VB is very flat near the VBM and it extends toward the Y point, while, in contrast, the
CB near the CBM shows a significant dispersion. This will result in a large hole effective mass
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but a small electron effective mass. From the DOS picture it can be seen that the VBM is formed
almost exclusively by hybridized S p and I p states, and there is little contribution from Pb and
Bi. In contrast, the CBM is dominated by Bi p hybridized with I p and, to the lesser degree, S p
states.
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Similarly to Pb2BiS2I3, Sn2BiS2I3 has mixed occupancy of Sn and Bi on the 8f site, so the same
structural model was employed in the calculations. The calculated electronic structure is
presented in Figure 10 in the form of the electronic band structure and the DOS shows a number
of features that are very similar to the electronic structure of Pb2BiS2I3. Sn2BiS2I3 is also a directꢀ
gap material, with the band gap at the Γ point. The VB near VBM is extremely flat extending
almost all the way towards both Y and Z points, while CBM exhibits a significant dispersion.
The composition of VMB is more complex than in Pb2BiS2I3: the VBM is similarly consists of
hybridized S p and I p states, but in addition there is a significant contribution from Sn2 5s states.
This results from the fact that Sn 5s states have higher energy than the corresponding Pb states.
By hybridizing with S p and I p states near the VBM, Sn s states effectively push VBM up,
which in effect leads to the band gap suppression in Sn2BiS2I3 compared to Pb2BiS2I3.
Interestingly, there is no contribution to the VBM from Bi states, even though Bi atoms occupy
the same mixed positions as Sn2 atoms. Instead, Bi p states contribute to the CBM, where they
hybridize with I p states together with Sn1 p. This result emphasizes that interaction of Sn and Bi
atoms with their neighbors plays a critical role in formation of VBM and CBM, respectively.
Sn2BiSI5 is also a disordered phase, in which an intermixture of Bi and Sn exists on sites 1 and
2 (see Table 2). For the electronic structure calculations, a simplified crystal structure was
adopted in which site 1 was fully occupied with Bi and site 2 occupied with Sn. This model has
the monoclinic baseꢀcentered lattice. The calculated electronic band structure and the DOS of
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this model are presented in Figure 11. According to the results the electronic transitions from the
VB to the CB are essentially direct but there is no single maximum or minimum, instead several
flat bands between Y and L comprise the VBM and CBM. This is a peculiar band structure
suggesting a possibility of a Fermi surface nesting upon doping (also between Z and Γ).
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Despite a very different crystal structure, the electronic structure of Sn2BiSI5 maintains many
features found in Sn2BiS2I3. The VBM consist of strongly hybridized Sn s, S p and I p states,
while the CBM is mainly made of fully hybridized Bi p and I p states. Therefore, it can be
suggested that the BiI6 octahedra of the extended BiI3 ribbons have a significant contribution to
the electronic structure and are responsible for the widening of the bandgap.
CONCLUDING REMARKS
The novel Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5 can be synthesized by reacting a mixture of MS
(M=Pb or Sn) and BiI3, as well as their respective elements. Pb2BiS2I3 and Sn2BiS2I3 are
isostructural and the Sn2BiSI5 can be considered as their homologue. These thioidodies are
highly resistive nꢀtype direct band gap semiconductors. The band structure calculations of these
compounds show that the valence band mainly consist of iodine p orbital whereas conduction
band is attributed mainly by a combination of Bi and iodine orbitals. Pb2BiS2I3, Sn2BiS2I3 and
Sn2BiSI5 are stable in air, water and exhibit bandgaps between 1.2 to 1.6 eV making them
interesting for solar cell applications. Pb2BiS2I3, and Pb2SbS2I3, consist of atoms with high
atomic number, (Z ≥ 40), and possess high mass density, high electrical resistivity and wide
optical band gap (1.5 ≤ Eg ≤ 2.0), and therefore could be interesting for further research as room
temperature nuclear radiation detection.
NOTE: Authors declare no competing financial interest.
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ASSOCIATED CONTENT
*Supporting Information
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Details on the DTA and TGA as well as EDS analysis of the thioiodides, thermal conductivity
and Seebeck plots are given. Xꢀray crystallographic file (CIF), crystallographic refinement
details, atomic coordinates with equivalent isotropic displacement parameters, anisotropic
displacement parameters, and details bond distances and angles for Pb2BiS2I3, Sn2BiS2I3 and
Sn2BiSI5. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: mꢀkanatzidis@northwestern.edu
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ACKNOWLEDGEMENTS
DJ and MSI acknowledge partial support from NSF grant DMRꢀ1410169 and OYK and AJF
(electrical measurements) are supported by DHSꢀARI grant 2014ꢀDNꢀ077ꢀARI086ꢀ01. We also
MRSEC program (NSF DMRꢀ1121262) at the Materials Research Center for use of facilities.
SEM, EDS and XPS analyses were performed at the EPIC facility of the NUANCE Center at
Northwestern University, supported by NSFꢀNSEC, NSFꢀMRSEC, Keck Foundation, the State
of Illinois, and Northwestern University.
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Table 1 Details concerning data collection and structure refinement of Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5.
5
6
Crystal data
compounds
Molecular weight, Mr
Crystal system
space group
Temperature (K)
a (Å)
Pb2BiS2I3
1069.97
Orthorhombic
Cmcm
Sn2BiS2I3
891.18
Orthorhombic
Cmcm
Sn2BiSI5
1112.92
Monoclinic
C2/m
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
293
293
293
4.3214 (9)
14.258 (3)
16.488 (3)
90.0
1015.9 (4)
4
Mo Kα
60.54
0.02 × 0.01 × 0.01
4.2890 (6)
14.121 (2)
16.414 (3)
90.0
994.1 (3)
4
Mo Kα
32.30
0.02 × 0.02 × 0.01
14.175 (3)
4.3985 (9)
21.625 (4)
98.90 (3)
1332.1 (5)
4
Mo Kα
28.59
0.13 × 0.02 × 0.01
b (Å)
c (Å)
β (o)
V (Å3)
Z
Radiation type
µ (mm−1)
Crystal size (mm)
Data collection
Diffractometer
Θmax; θmin
STOE IPDS 2T
25.0°; 2.9°
h = −5→5
k = 0→16
l = 0→19
numerical
0.930, 0.941
883, 531, 474
STOE IPDS 2T
29.1°; 3.1°
h = −5→5
k = −19→19
l = −22→22
numerical
0.503, 0.657
4225, 763, 653
STOE IPDS 2
25.0°, 1.9°
h = −16→16
k = −5→5
l = 0→25
numerical
Measured hkl
Absorption correction
Tmin, Tmax
No. of measured,
independent
0.507, 0.757
2323, 1342, 1001
and observed reflections
Rint
0.020
0.594
0.094
0.685
0.040
0.594
(sin θ/λ)max (Å−1)
Refinement
R1
wR2
S
0.022
0.054
1.02
531
28
0.053
0.123
1.09
763
29
0.046
0.109
1.01
1342
57
No. of reflections
No. of parameters
ꢂρmax, ꢂρmin (e Å−3)
1.09, −1.43
2
5.93, −4.51
2.06, −1.92
a) R1 = Σ||Fo| ꢀ |Fc|| / |Fo|, Fo2 ≥ 2σ(Fo )
b)
2
2
wR2 = 1 / [σ2(Fo ) + (A⋅P)2 + B⋅P]; P = (Fo + 2Fc2) / 3
Table 2: Atomic coordinates and isotropic displacement parameters for M2BiS2I3 (M = Pb, Sn)
and Sn2BiSI5.
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2
3
U [Å2]
compounds
Atom Wyck. S.O.F
x/a
y/b
z/c
4
5
6
7
8
Pb1
Bi1
Pb2
I1
I2
S1
8f
8f
4c
4c
8f
8f
0.5
0.5
1.0
1.0
1.0
1.0
1/2
1/2
0
ꢀ1/2
1/2
0
0.36655(4)
0.36655(4)
0.19051(6)
0.34973(8)
0.05475(6)
0.2672(2)
0.02786(3)
0.02786(3)
1/4
1/4
0.12070(5)
0.0297(2)
0.0297(2)
0.0399(4)
0.0197(3)
0.0215(3)
Pb2BiS2I3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.08268(17) 0.0185(6)
Sn1
Bi1
Sn2
I1
I2
S1
4c
8f
8f
8f
4c
8f
1.0
0
0.2971 (2)
1/4 0.064 (1)
0.506 (13) 1/2
0.494 (13
1.0
1.0
1.0
0.13382 (7) 0.02054 (7) 0.0291 (4)
0.13382 (7) 0.02054 (7) 0.0291 (4)
0.44358 (9) 0.12166 (7) 0.0227 (4)
0.1456 (1)
0.2707 (3)
Sn2BiS2I3
1/2
1/2
1/2
1/2
1/4
0.0234 (5)
−0.0819 (2) 0.0179 (8)
4i
4i
4i
4i
4i
4i
4i
4i
4i
4i
4i
Bi1A
Sn1A
Bi2A
Sn2A
Sn3
S1
I1
I2
I3
I4
0.763 (14) 0.25718 (8)
0.237 (14) 0.25718 (8)
0.228 (14) 0.1274(1)
0.772 (14) 0.1274 (1)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0
0
0
0
0
0
−1/2
0
−1/2
−1/2
0
0.09971 (5) 0.0426 (5)
0.09971 (5) 0.0426 (5)
0.47820 (7) 0.0455 (7)
0.47820 (7) 0.0455 (7)
0.67635 (9) 0.0471 (5)
0.2379 (1)
0.2848 (4)
0.1257 (1)
0.4204 (1)
0.39160 (1)
0.3847 (1)
0.3792 (1)
Sn2BiSI5
0.5611 (3)
0.036 (1)
0.13935 (9) 0.0506 (5)
0.40632 (7) 0.0375 (4)
0.69695 (7) 0.0406 (4)
0.04602 (8) 0.0458 (5)
0.21971 (7) 0.0454 (4)
I5
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1
2
Table 3 : Anisotropic displacement parameters (Å2) for X, Y and Z…
3
4
5
6
7
8
compounds
atoms U11
U22
U33
U12
0
0
0
0
0
0
U13
0
0
0
0
0
0
U23
0.0068(2)
0.0068(2)
0
Pb1
Bi1
Pb2
I1
I2
S1
0.0244(3)
0.0330(3)
0.0330(3)
0.0339(5)
0.0230(6)
0.0218(4)
0.0316(3)
0.0316(3)
0.0590(6)
0.0172(5)
0.0216(4)
0.0244(3)
0.0269(5)
0.0185(6)
0.0210(4)
0.0219(15)
9
Pb2BiS2I3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
0.0019(3)
0.0002(12)
0.0194(15) 0.0140(13)
Sn1
Bi1
Sn2
I1
I2
S1
0.0311 (13) 0.035 (2)
0.125 (3)
0.0308 (6)
0.0308 (6)
0.0204 (6)
0.0198 (7)
0.012 (2)
0
0
0
0
0
0
0
0
0
0
0
0
0
0.0321 (6)
0.0321 (6)
0.0233 (6)
0.0212 (9)
0.020 (2)
0.0244 (6)
0.0244 (6)
0.0244 (7)
0.029 (1)
0.021 (2)
0.0012 (4)
0.0012 (4)
−0.0033 (4)
0
Sn2BiS2I3
−0.002 (2)
Bi1A 0.0520 (8)
Sn1A 0.0520 (8)
Bi2A 0.046 (1)
Sn2A 0.046 (1)
0.0399 (7)
0.0399 (7)
0.039 (1)
0.039 (1)
0.0369 (9)
0.029 (3)
0.043 (1)
0.0324 (8)
0.0308 (8)
0.0429 (9)
0.0463 (9)
0.0374 (7)
0.0374 (7)
0.054 (1)
0.054 (1)
0.048 (1)
0.034 (3)
0.064 (1)
0.0403 (8)
0.0454 (9)
0.0483 (9)
0.0384 (8)
0
0
0
0
0
0
0
0
0
0
0
0.0114 (5)
0.0114 (5)
0.0151 (7)
0.0151 (7)
0.0190 (9)
0.012 (2)
0.0242 (8)
0.0115 (7)
0.0080 (7)
0.0173 (7)
0.0090 (7)
0
0
0
0
0
0
0
0
0
0
0
Sn3
S1
I1
I2
I3
0.060 (1)
0.047 (3)
0.050 (1)
0.0414 (9)
0.0458 (9)
0.0494 (1)
0.0522 (1)
Sn2BiSI5
I4
I5
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2
3
4
Table 4: Selected interatomic distances (Å) and angles (°) in Pb2BiS2I3; estimated standard
5
6
deviation in parentheses.
7
8
9
Pb1|Bi1—S1i
Pb1|Bi1—S1
Pb1|Bi1—S1ii
Pb1|Bi1—I2i
Pb1|Bi1—I2iii
Pb1|Bi1—I1ii
Pb1|Bi1—I2iv
Pb1|Bi1—I2v
Pb2—S1vii
2.638(3)
I1—Pb2ix
3.1342(11)
3.6705(9)
3.6705(9)
3.6705(9)
3.6705(9)
3.4535(8)
3.4535(8)
3.4535(8)
3.7699(9)
2.638(3)
2.638(3)
2.7372(19)
3.7699(9)
2.638(3)
98.40(8)
98.40(8)
104.300
2.7372(19)
2.7372(19)
3.4535(8)
3.4535(8)
3.6705(9)
3.7699(9)
3.7699(9)
2.968(3)
I1—Pb1|Bi1x
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
I1—Pb1|Bi1x
I1—Pb1|Bi1ix
I1—Pb1|Bi1ix
I2—Pb1|Bi1i
I2—Pb1|Bi1i
I2—Pb1|Bi1iii
I2—Pb1|Bi1xii
Pb2—S1
2.968(3)
S1—Pb1|Bi1i
Pb2—I1ii
3.1341(11)
3.1342(11)
2.968(3)
2.968(3)
136.75(12)
74.52(4)
74.52(4)
74.52(4)
74.52(4)
87.17(4)
S1—Pb1|Bi1i
Pb2—I1
S1—Pb1|Bi1ix
Pb2—S1vii
I2—Pb1|Bi1xii
Pb2—S1
S1—Pb1|Bi1i
S1vii—Pb2—S1
S1vii—Pb2—I1ii
S1—Pb2—I1ii
S1vii—Pb2—I1
S1—Pb2—I1
I1ii—Pb2—I1
Pb1|Bi1i—S1—Pb1|Bi1ix
Pb1|Bi1i—S1—Pb1|Bi1ix
Pb1|Bi1—S1—Pb1|Bi1ix
Pb1|Bi1i—S1—Pb1|Bi1ix
Pb1|Bi1i—S1—Pb1|Bi1ix
Pb1|Bi1—S1—Pb1|Bi1ix
98.400
98.40(8)
104.26(10)
Symmetry codes: (i) −x+1/2, −y+1/2, −z; (ii) x+1, y, z; (iii) x, y, −z+1/2; (iv) x−1, y, z.
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1
2
3
4
Table 5: Selected interatomic distances (Å) and angles (°) in Sn2BiS2I3; estimated standard
5
6
deviation in parentheses.
7
8
9
Sn1—S1i
2.921(4)
2.921(4)
3.030(2)
3.030(2)
2.562(4)
2.726(3)
2.726(3)
3.3527(13)
3.3527(13)
S1—Bi1|Sn2iv
S1—Bi1|Sn2iv
S1—Bi1|Sn2i
S1—Bi1|Sn2i
S1—Sn1i
2.726(3)
2.726(3)
2.726(3)
2.726(3)
Sn1—S1ii
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Sn1—I2iii
Sn1—I2
Bi1|Sn2—S1
Bi1|Sn2—S1iv
Bi1|Sn2—S1i
Bi1|Sn2—I1i
Bi1|Sn2—I1iv
2.921(4)
Sn1—I1iii
3.6490(19)
2.9206(36)
3.8171(14)
3.3528(13)
Sn1—S1vi
Bi1|Sn2—I1vii
Bi1|Sn2—I1viii
S1i—Sn1—S1ii
S1i—Sn1—I2iii
S1ii—Sn1—I2iii
S1i—Sn1—I2
141.7(2)
76.61(7)
76.61(7)
76.61(7)
76.61(7)
90.11(9)
82.47(10)
82.47(10)
103.76(15)
77.83(8)
141.7(2)
76.61(7)
76.61(7)
76.61(7)
76.61(7)
Sn1v—I2—Sn1
90.11(9)
97.500
97.52(10)
Bi1|Sn2—S1—Bi1|Sn2iv
Bi1|Sn2—S1—Bi1|Sn2iv
Bi1|Sn2iv—S1—Bi1|Sn2iv 0.000
S1ii—Sn1—I2
Bi1|Sn2—S1—Bi1|Sn2i
Bi1|Sn2iv—S1—Bi1|Sn2i
Bi1|Sn2iv—S1—Bi1|Sn2i
Bi1|Sn2—S1—Bi1|Sn2i
Bi1|Sn2iv—S1—Bi1|Sn2i
Bi1|Sn2iv—S1—Bi1|Sn2i
Sn1v—I2—Sn1
97.500
I2iii—Sn1—I2
103.76(15)
103.76(15)
97.52(10)
103.800
103.76(15)
90.11(9)
97.500
S1—Bi1|Sn2—S1iv
S1—Bi1|Sn2—S1i
S1iv—Bi1|Sn2—S1i
S1—Bi1|Sn2—I1i
S1i—Sn1—S1ii
S1i—Sn1—I2iii
S1ii—Sn1—I2iii
S1i—Sn1—I2
Bi1|Sn2—S1—Bi1|Sn2iv
Bi1|Sn2—S1—Bi1|Sn2iv
97.52(10)
Bi1|Sn2iv—S1—Bi1|Sn2iv 0.000
S1ii—Sn1—I2
Bi1|Sn2—S1—Bi1|Sn2i
97.500
Symmetry codes: (i) –x+1/2, ꢀy+1/2, ꢀz; (ii) –x+1/2, ꢀy+1/2, z+1/2; (iii) xꢀ1, y, z; (iv) –x+3/2, ꢀy+1/2, ꢀz; (v) x+1, y,
z;
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2
3
4
Table 6: Selected interatomic distances (Å) and angles (°) in Sn2BiSI5; estimated standard
5
6
deviation in parentheses.
7
8
9
i
Bi1A—I5
Bi1A—I1
Bi1A—I1
Bi1A—I4
Bi1A—I4
Bi2A—S1
Bi2A—S1
Bi2A—S1
Sn3—S1
Sn3—I3
2.888 (2)
Sn3—I3
3.0791 (19)
2.727 (3)
2.727 (3)
2.727 (3)
2.727 (3)
3.0882 (15)
3.0882 (15)
3.0791 (19)
3.1780 (15)
3.1780 (15)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
i
ii
3.0881 (15)
3.0882 (15)
3.1780 (15)
3.1781 (15)
2.638 (7)
2.727 (3)
2.727 (3)
2.676 (6)
3.0791 (19)
S1—Sn2A
S1—Bi2A
S1—Bi2A
S1—Sn2A
I1—Sn1A
ii
iii
i
iii
iv
ii
iv
I1—Bi1A
iii
iv
I3—Sn3
iv
I4—Sn1A
I4—Bi1A
iv
i
ii
I5—Bi1A—I1
93.08 (5)
93.08 (5)
90.82 (5)
91.88 (5)
174.76 (6)
90.58 (3)
91.89 (5)
90.58 (4)
174.76 (6)
87.58 (5)
80.39 (15)
80.39 (15)
107.49 (19)
82.11 (11)
82.11 (11)
91.16 (7)
109.1 (2)
99.6(3)
Bi2A—S1—Bi2A
Sn3—S1—Bi2A
Bi2A—S1—Bi2A
Sn3—S1—Bi2A
Sn2A —S1—Bi2A
Bi2A —S1—Bi2A
Bi2A—S1—Sn2A
Sn3—S1—Sn2A
Sn2A —S1—Sn2A
Bi2A —S1—Sn2A
Bi2A —S1—Sn2A
Bi1A—I1—Sn1A
Bi1A—I1—Bi1A
Sn3—I3—Sn3
Sn1A —I4—Bi1A
Bi1A —I4—Bi1A
99.61 (15)
118.78 (13)
99.61 (15)
118.78 (13)
107.5
107.49 (19)
99.6(4)
118.78 (13)
107.49 (19)
107.49 (19)
0.00 (6)
ii
I5—Bi1A—I1
i
iii
I1—Bi1A—I1
I5—Bi1A—I4
iii
i
ii
iii
I1—Bi1A—I4
ii
iii
I1—Bi1A—I4
i
iii
I5—Bi1A—I4
i
i
iii
I1—Bi1A—I4
i
ii
iii
iii
I1—Bi1A—I4
I4—Bi1A—I4
S1—Bi2A—S1
i
ii
iii
ii
iii
iii
iv
S1—Bi2A—S1
90.8(2)
ii
iii
i
S1 —Bi2A—S1
S1—Sn3—I3
90.82 (6)
91.16 (7)
87.6(6)
87.58 (5)
118.78 (13)
iv
i
iv
S1—Sn3—I3
I3—Sn3—I3
Bi2A—S1—Sn3
Bi2A—S1—Sn2A
i
iv
ii
Sn3—S1—Sn2A
ii
Symmetry codes: (i) x, y+1, z; (ii) –x+1/2, ꢀy+1/2, ꢀz+1; (iii) –x+1/2, ꢀyꢀ1/2, ꢀz+1; (iv) x. yꢀ1, z.
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2
3
4
Table 7: Binding energies of Pb2BiS2I3, Sn2BiS2I3 and Sn2BiSI5 obtained from XPS spectra
5
6
7
8
Compounds
Binding energy
(eV)
assignment
Spinꢀorbit
components (eV)
References
9
53
53
Pb2BiS2I3
143.34
138.43
Pb 4f5/2
ꢂPb 4f = 4.91
ꢂBi 4f = 5.31
ꢂI 3d = 11.44
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Pb 4f7/2
Bi 4f7/2
Bi 4f5/2
158.90
164.21
53
619.63
631.07
161.84
225.64
I 3d5/2
I 3d3/2
S 2p
59ꢀ60
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
S 2s
53ꢀ54
Sn2BiS2I3
486.73
495.12
Sn 3d5/2
Sn 3d3/2
ꢂSn 3d = 8.39
ꢂBi 4f = 5.28
53
158.82
164.10
Bi 4f7/2
Bi 4f5/2
I 3d5/2
I 3d3/2
S 2p
53
619.32
630.78
ꢂI 3d = 11.46
53, 59
161.70
225.92
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
S 2s
54
53
Sn2BiSI5
487.34
495.76
159.12
164.41
619.68
631.16
Sn 3d3/2
Sn 3d5/2
Bi 4f7/2
Bi 4f5/2
I 3d5/2
I 3d3/2
S 2p
ꢂSn 3d = 8.42
ꢂBi 4f = 5.29
53
ꢂI 3d = 11.48
53, 59
162.10
225.97
ꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
S 2s
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Figure 1: SEM images of (A) Pb2BiS2I3, (B) “Pb2Sb1ꢀxBixS2I3” (x~0.4), (C) Sn2BiS2I3, and (D)
Sn2BiSI5 showing the characteristics growth of the crystal along cꢀaxis.
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Figure 2. (A) Differential thermal analysis (5deg/min) and (B) Thermogravimetric analysis a
sample of Pb2BiS2I3 (10 deg/min).
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Figure 3: Crystal structures of (A, C, D) Pb2BiS2I3, and (B, E, F) Sn2BiS2I3. Figures C and E
represent the infinite ribbon [M4S2I4] (M = Pb, Sn, Bi) running along cꢀaxis, red circles indicate
the isolated ribbon. D and F derived from grey shaded region of the ribbons (C and E) clearly
show the 2D sheet.
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