Mercury bismuth chalcohalides, Hg3Q2Bi 2Cl8 (Q = S, Se, Te): Syntheses, crystal structures, band structures, and optical properties

Article abstract of DOI:10.1021/ic3023826

Three quaternary mercury bismuth chalcohalides, Hg₃Q ₂Bi₂Cl₈ (Q = S, Se, Te), are reported along with their syntheses, crystal structures, electronic band structures, and optical properties. The compounds are structurally similar with a layer comprised of a hole perforated sheet network of [Hg₃Q ₂]²⁺ (Q = S and Te) that forms by fused cyclohexane, chairlike Hg₆Q₆ rings. The cationic charge in the network is balanced by edge-sharing monocapped trigonal-prismatic anions of [Bi ₂Cl₈]^2- that form a two-dimensional network located between layers. Compound 1, Hg₃S₂Bi ₂Cl₈, crystallizes in the monoclinic space group C12/m1 with a = 12.9381(9) A, b = 7.3828(6) A, c = 9.2606(6) A, and β = 116.641(5). Compound 2, Hg₃Te₂Bi ₂Cl₈, crystallizes in the monoclinic space group C12/c1 with a = 17.483(4) A, b = 7.684(2) A, c = 13.415(3) A, and β = 104.72(3). The crystals of the Hg₃Se₂Bi ₂Cl₈ analogue exhibit complex modulations and structural disorder, which complicated its structural refinement. Compounds 1 and 2 melt incongruently and show band gaps of 3.26 and 2.80 eV, respectively, which are in a good agreement with those from band-structure density functional theory calculations.

Full text of DOI:10.1021/ic3023826

Article
pubs.acs.org/IC
Mercury Bismuth Chalcohalides, Hg Q Bi Cl (Q = S, Se, Te):
3
2
2
8
Syntheses, Crystal Structures, Band Structures, and Optical
Properties
†
†,‡
†
§
§
Arief C. Wibowo, Christos D. Malliakas, Duck Young Chung, Jino Im, Arthur J. Freeman,
,†,‡
and Mercouri G. Kanatzidis*
†
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
§
Department of Chemistry and Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United
States
*
S Supporting Information
ABSTRACT: Three quaternary mercury bismuth chalcohalides, Hg Q Bi Cl (Q =
3
2
2
8
S, Se, Te), are reported along with their syntheses, crystal structures, electronic
band structures, and optical properties. The compounds are structurally similar with
2+
a layer comprised of a hole perforated sheet network of [Hg Q ] (Q = S and Te)
3
2
that forms by fused cyclohexane, chairlike Hg Q rings. The cationic charge in the
6
6
network is balanced by edge-sharing monocapped trigonal-prismatic anions of
2−
[
Bi Cl ] that form a two-dimensional network located between layers. Compound
2 8
1
1
2
=
, Hg S Bi Cl , crystallizes in the monoclinic space group C12/m1 with a =
2.9381(9) Å, b = 7.3828(6) Å, c = 9.2606(6) Å, and β = 116.641(5)°. Compound
3 2 2 8
, Hg Te Bi Cl , crystallizes in the monoclinic space group C12/c1 with a = 17.483(4) Å, b = 7.684(2) Å, c = 13.415(3) Å, and β
3
2
2
8
104.72(3)°. The crystals of the Hg Se Bi Cl analogue exhibit complex modulations and structural disorder, which complicated
3
2
2
8
its structural refinement. Compounds 1 and 2 melt incongruently and show band gaps of 3.26 and 2.80 eV, respectively, which
are in a good agreement with those from band-structure density functional theory calculations.
INTRODUCTION
of structural modulations and disorder, structural refinement
was not carried out.
■
Ternary metal chalcohalides are hybrid materials whose
compositions lie between those of binary chalcogenides and
1
EXPERIMENTAL SECTION
binary halides. Given the same elements, the energy band gaps
■
of the metal chalcohalides lie between those of binary
chalcogenides and binary halides. The former generally have
significantly lower energy gaps than the latter, while the ternary
metal chalcohalides have intermediate band gaps, as illustrated
Chemicals in this work were used without further purification: BiCl
(99.9% metal basis, Alfa Aesar), HgCl (99.9995% metal basis, Alfa
3
2
Aesar) ,elemental mercury (99.999%, electronic grade, Sigma-Aldrich),
sulfur chunks (99.999%, Spectrum Chemical Mfg. Corp.), selenium
shots (99.999%, Plasmaterials), and tellurium shots (99.999%,
Plasmaterials). All manipulations were done inside a nitrogen-filled
glovebox.
2
previously. Different potential applications have been reported
2
for hybrid chalcohalides such as X-ray and γ-ray detectors,
3
4
nonlinear optics, phase transitions, and upconversion
5
Synthesis of HgQ (Q = S, Se, and Te). HgS was prepared by
pipetting a stoichiometric amount of liquid mercury into a 15 mm o.d.
luminescence. We are interested in exploring highly dense,
heavy-element-containing chalcohalides, such as mercury and
bismuth, as part of our search to identify potential candidates
for X-ray and γ-ray semiconductor detectors. Both mercury(II)
and bismuth(III) not only adopt closed-shell electronic
configurations but also have large ionic radii with flexible
coordination environments, giving rise to interesting molecular
×
12 mm i.d. fused-silica tube and then adding a stoichiometric
amount of ground sulfur. The tube was flame-sealed under dynamic
vacuum of ∼10 mbar and then put into a programmable furnace.
The furnace was heated to 400 °C over 12 h and held isothermally for
−4
1
day, followed by cooling slowly to 50 °C over 6 h. The obtained
ingot was ground to powder before use. The purity of the product was
checked by powder X-ray diffraction (PXRD). A similar preparation
procedure was adopted for HgSe and HgTe, but the furnace was
heated to 550 °C over 16 h, held isothermally for 1 day, and then
cooled slowly to 50 °C over 6 h. Caution! Handling of mercury should
be carried out in a fume hood or a glovebox, and the temperature was
raised slowly to ensure that liquid mercury reacts without evaporating.
6
architectures. Recent reports on quaternary mercury-based
7
chalcohalides include [Hg Te ][UCl ], [Hg Q ][MX ] (Q =
3
2
6
3
2
6
8
S, Se; M = Zr, Hf; X = Cl, Br), Hg AsQ X (Q = S, Se; X = Cl,
3
4
9
10
11
12
Br, I), Hg ZnS Cl , [Hg As ][Bi Cl ], (Hg Cd S Br)Br,
3
2
4
8
4
3
13
2
2 2
1
3
and Hg InS Cl . Herein, we report two mercury bismuth
7
6
5
chalcohalides, Hg S Bi Cl (1) and Hg Te Bi Cl (2), along
3
2
2
8
3
2
2
8
with their syntheses, structural characterizations, electronic-
band-structure calculations, and optical properties. The
Hg Se Bi Cl (3) analogue was also synthesized, but because
Received: October 31, 2012
3
2
2
8
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic3023826 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of Hg S Bi Cl (1). Several attempts were made to
obtain a quantitative yield of 1 using a stoichiometric mixture of HgS,
3
2
2
8
Table 1. Crystal Data and Structural Refinement of
Compounds 1 and 2
a
HgCl , and BiCl by (a) melting and slow cooling, (b) melting and
2
3
quenching in air and ice water, (c) flame melting and ice water
quenching, and (d) low-temperature annealing at 300 °C for solid-
state diffusion. All of these conditions produced a mixture of 1 as the
major product and impurities of ternaries (BiSCl and β-Hg S Cl ). A
1
2
empirical formula Hg S Bi Cl
Hg Te Bi Cl
3 2 2 8
3
2
2
8
fw
1367.45
293(2)
0.71073
1558.53
293(2)
0.71073
3
2
2
temperature (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
quantitative yield of 1 was obtained after the addition of an excess of
BiCl3 (3-fold excess over what is required for a stoichiometric
reaction) using the following conditions. A mixture of HgS (2 mmol),
HgCl (1 mmol), and BiCl (6 mmol) was loaded into a 12 mm o.d. ×
monoclinic
monoclinic
C12/m1
C12/c1
2
3
1
0 mm i.d. fused-silica tube. Then the tube was flame-sealed under
−4
vacuum (∼10 mbar), put inside a programmable box furnace, heated
12.9381(9)
7.3828(6)
9.2606(6)
90.00
17.483(4)
7.684(2)
13.415(3)
90.00
to 300 °C over 24 h, and held isothermally for 12 h, followed by a
b (Å)
shutdown of the furnace. The excess of BiCl was washed away using
3
c (Å)
methanol. A white polycrystalline powder with a slight pale-purple tint
was obtained and amounted to a quantitative yield of 1.
α (deg)
β (deg)
116.641(5)
90.00
104.72(3)
90.00
In the course of these experiments, we found that reaction with a
γ (deg)
1
:1 mixture of HgS and BiCl provided single crystals of 1 suitable for
3
volume (Å3)
790.6(1)
2
1743.0(6)
4
structural characterization. This was done as follows. The thoroughly
Z
mixed powders of HgS and BiCl at a 1:1 ratio were loaded into a 12
3
density (calcd) (
5.744
5.939
mm o.d. × 10 mm i.d. fused-silica tube. The tube was flame-sealed
3
−4
g/cm )
under vacuum (∼10 mbar) and put inside a programmable furnace.
abs coeff (mm−1) 52.789
50.933
2584
The furnace was heated to 450 °C over 12 h and held isothermally for
F(000)
1148
1
day, followed by cooling slowly to 200 °C over 36 h and then to
3
room temperature within 1 h. White, platelike crystals (∼30% yield
based on HgS) were obtained. The presence of mercury, bismuth,
sulfur, and chlorine in the platelike crystals obtained was confirmed by
scanning electron microscopy−energy-dispersive spectrometry
cryst size (mm )
0.1 × 0.03 × 0.06
4.55−34.87
0.1 × 0.03 × 0.07
3.16−24.99
θ range for data
collection (deg)
index ranges
−20 ≤ h ≤ +20, −11 ≤ k ≤ −20 ≤ h ≤ +20, −9 ≤ k ≤
11, −14 ≤ l ≤ +14 +9, −15 ≤ l ≤ +15
+
(
SEM−EDS).
reflns collected
indep reflns
4685
5440
Synthesis of Hg Te Bi Cl (2). Unlike the sulfur analogue, a
3
2
2
8
stoichiometric reaction of HgTe, HgCl , and BiCl was successfully
1706 [Rint = 0.0526]
1531 [Rint = 0.0914]
2
3
performed by solid-state diffusion at 200 °C to give a quantitative yield
completeness to
93.1
99.4
θ = 32.49° (%)
of 2. A mixture of HgTe, HgCl , and BiCl at a stoichiometric ratio
2
3
was loaded into a 12 mm o.d. × 10 mm i.d. fused-silica tube. Then the
refinement
method
full-matrix least squares on full-matrix least squares on
2 2
−
4
F
F
tube was flame-sealed under vacuum (∼10 mbar), put inside a
programmable box furnace, heated to 200 °C over 14 h, and held
isothermally for 48 h, followed by a shutdown of the furnace. Dark-
green polycrystalline powder was obtained with a quantitative yield of
data/restraints/
param
1706/0/44
1531/0/71
GOF
1.038
0.946
final R indices [I > R1obs = 0.0299, wR2obs
=
R1obs = 0.0304, wR2obs
=
0.0690
2.
2σ(I)]
0.0695
Single crystals of 2 suitable for structural characterization were
R indices (all
data)
R1all = 0.0366, wR2all
0.0717
=
R1all = 0.0373, wR2all
=
0.0716
prepared by a reaction of HgTe and BiCl at a 1:1 ratio. The mixture
3
in a sealed quartz tube was heated to 600 °C over 16 h and held
isothermally for 24 h, followed by cooling slowly to 200 °C over 36 h
and then to room temperature within 1 h. Greenish, platelike crystals
were obtained at ∼30% yield based on HgTe. The composition of the
crystals was confirmed by SEM-EDS.
extinction coeff
0.00088(9)
0.00034(3)
largest diff peak 3 3.728 and −2.141
2.711 and −1.690
and hole (e/Å )
a
2
2 2
R1 = ∑||F | − |F ||/|∑|F |. wR2 = {∑[w(F − F ) ]/
o
c
o
o
c
2
2
1/2
∑
[w(F ) ]} .
o
Synthesis of Hg Se Bi Cl (3). The crystals were synthesized
3
2
2
8
analogously to compound 2. A ground 1:1 mixture of HgSe and BiCl
was sealed under vacuum (∼10 mbar) in a 12 mm o.d. × 10 mm i.d.
fused silica tube, heated to 600 °C over 16 h, and held isothermally for
3
−4
Supporting Information, SI. The selected bond lengths and angles are
given in Tables 3 and 4 for compounds 1 and 2, respectively.
PXRD. Ground powder of the sample was used to collect PXRD
patterns using a PanAlytical X’Pert Pro powder diffractometer (Cu Kα
radiation λ = 1.5418 Å) over the 2θ range of 10−70°, with a step size
of 0.02° and a scan speed of 0.25°/min.
UV−Vis Spectroscopy. Optical diffuse-reflectance measurements
were performed at room temperature using a Shimadzu UV-3600
spectrophotometer operating in the 200−2500 nm region. The
instrument was equipped with an integrating sphere and controlled by
24 h, followed by cooling slowly to 200 °C over 36 h and then to room
temperature within 1 h. Orange, platelike crystals (∼30% yield based
on HgSe) were obtained. The presence of mercury, bismuth, selenium,
and chlorine in the platelike crystals was confirmed by SEM-EDS.
These crystals were further examined by single-crystal X-ray diffraction
(
XRD) and PXRD.
Single-Crystal XRD. Single-crystal XRD data were collected at 293
K on a STOE 2T image-plate diffractometer with Mo Kα radiation (λ
0.71073 Å). An analytical absorption correction was applied to the
=
a personal computer. BaSO was used as a 100% reflectance standard.
4
data using the program X-Red on an optimized shape obtained with
the aid of X-Shape software. The structures were solved by direct
methods and refined with the SHELXTL software package. Thermal
displacement parameters were anisotropically refined for all atomic
positions. Data were collected on several single crystals, to check for
consistency in the lattice parameters. The complete data collection
parameters and details of the structure solution and refinement for the
compounds are given in Table 1. The fractional coordinates, thermal
The sample was prepared by grinding the powder and spreading it on
a compacted surface of the powdered standard material, preloaded into
a sample holder. The reflectance versus wavelength data generated
were used to estimate the band gap of the material by converting
reflectance to absorption data according to the Kubelka−Munk
2
equation α/S = (1 − R) /2R, where R is the reflectance and α and S
14
are the absorption and scattering coefficients, respectively.
Differential Thermal Analysis (DTA). Experiments were
performed on a Shimadzu DTA-50 thermal analyzer. A sample (∼30
mg) of ground crystalline material was sealed in a silica ampule under
displacement parameters (U ), and occupancies of all atoms with
eq
estimated standard deviations are given in Tables 2 and S1 in the
B
dx.doi.org/10.1021/ic3023826 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
4
2
3
Table 2. Atomic Coordinates (×10 ) and Equivalent Isotropic Displacement Parameters (Å × 10 ) at 293(2) K with Estimated
Standard Deviations in Parentheses
a
label
x
y
z
occupancy
Ueq
(
a) Hg S Bi Cl
3 2 2 8
Bi(1)
Hg(1)
Hg(3)
Cl(1)
Cl(2)
Cl(3)
S(1)
1271(1)
2500
0
4262(1)
0
1
1
1
1
1
1
1
17(1)
25(1)
27(1)
28(1)
27(1)
32(1)
19(1)
2500
0
5000
0
0
−2438(3)
2669(2)
0
5000
1930(2)
−492(2)
1249(2)
2870(2)
1494(3)
−1249(2)
5000
(
b) Hg Te Bi Cl
3 2 2 8
Bi(1)
Hg(1)
Hg(2)
Te(1)
Cl(1)
Cl(2)
Cl(3)
Cl(4)
409(1)
2500
2321(1)
7500
9374(1)
10000
1
1
1
1
1
1
1
1
24(1)
42(1)
47(1)
32(1)
36(1)
39(1)
40(1)
35(1)
2533(1)
1666(1)
1040(2)
1032(2)
1744(2)
10(2)
5145(1)
7511(1)
5241(4)
2702(4)
1076(4)
1089(4)
7432(1)
8059(1)
10366(2)
7758(2)
10244(2)
11134(2)
a
Ueq is defined as one-third of the trace of the orthogonalized U tensor.
ij
vacuum. A similar ampule of equal mass filled with Al O was sealed
Table 3. Representative Bond Lengths (Å) and Bond Angles
2
3
a
and placed on the reference side of the detector. The sample was
heated to 400 °C (compound 1) or to 450 °C (compound 2) at 5 °C/
min, and after 10 min, it was cooled at a rate of −5 °C/min to 50 °C.
Two cycles of heating and cooling were applied to the samples. The
residues of the DTA experiments were examined by PXRD.
(deg) of 1
Hg(1)−S(1)
Hg(1)−S(1)
Hg(3)−S(1)
Hg(3)−S(1)
Hg(3)−S(1)−Hg(1)
Hg(3)−S(1)−Hg(1)
Hg(1)−S(1)−Hg(1)
Bi(1)−Cl(3)
Bi(1)−Cl(2)
Bi(1)−Cl(2)
2.385(1)
2.385(1)
2.372(2)
2.372(2)
103.07(6)
103.07(6)
101.43(6)
2.556(2)
2.694(2)
2.694(2)
2.724(1)
2.724(1)
Band-Structure Calculations. To investigate the electronic
structures of both compounds, first-principles calculations were
performed within the density functional theory (DFT) formalism
15
using the full potential linearized augmented plane wave method.
The muffin tin radii for mercury, bismuth, chlorine, sulfur, and
tellurium were chosen to be 2.40, 2.60, 1.90, 2.00, and 2.20 Bohr,
respectively, and the energy cutoffs for the plane-wave basis and star
functions were set to 16 and 100 Ryd, respectively. For k-point
sampling, 8 × 10 × 8 and 6 × 6 × 4 regular meshes were chosen for
compounds 1 and 2, respectively. Within the local density
Bi(1)−Cl(1)
Bi(1)−Cl(1)
1
6
a
approximation (LDA), the Hedin−Lundqvist form was employed
Symmetry transformations used to generate equivalent atoms: (1) x,
1
1
1
for the exchange-correlation functional and spin−orbit coupling
−
y ,z; (2) −x, −y, −z + 1; (3) −x + / , −y + / , −z; (4) x + / , y +
17
2
2
2
1
1
1
(SOC) was treated by a second variation method. For a better
/2
, z; (5) −x, −y, −z; (6) −x, −y + 1, −z; (7) x − / , y − / , z; (8)
2
2
evaluation of the band gap, which is usually underestimated in LDA
formalism, we employed the nonlocal scheme of the screened-
1 1
−
x, y, −z; (9) −x + / , y + / , −z.
2
2
1
8
exchange LDA (sX-LDA) method, which leads to a good agreement
19
Table 4. Representative Bond Lengths (Å) and Bond Angles
with experimentally measured band gaps as well as band topology.
a
(deg) of 2
RESULTS AND DISCUSSION
Hg(1)−Te(1)
Hg(1)−Te(1)
Hg(2)−Te(1)
Hg(2)−Te(1)
2.642(1)
2.642(1)
2.6377(9)
2.6388(9)
2.6377(9)
93.70(3)
93.05(3)
95.40(3)
2.519(3)
2.678(2)
2.697(3)
2.759(3)
2.792(2)
■
Synthesis. The reactions of mercury chalcogenides with
mercury dichloride and bismuth trichloride in a closed ampule
under relatively low temperatures (200−300 °C) yield a series
of air-stable, isotypic compounds of the general formula
Hg Q Bi Cl (Q = S, Se, and Te). Synthesizing a single
Te(1)−Hg(2)
Hg(2)−Te(1)−Hg(2)
Hg(2)−Te(1)−Hg(1)
Hg(2)−Te(1)−Hg(1)
Bi(1)−Cl(3)
Bi(1)−Cl(2)
Bi(1)−Cl(1)
3
2
2
8
phase of 1 is challenging. In order to obtain a quantitative yield
of 1, we attempted several reactions using a stoichiometric
mixture of HgS, HgCl , and BiCl . These reactions involved a
2
3
variety of heat treatments such as (a) melting and slow cooling,
b) melting and quenching in air and ice water, (c) flame
melting and ice water quenching, and (d) low-temperature
annealing at 300 °C for solid state diffusion. None of these
synthetic attempts were successful in giving single-phase
samples of 1, and the ternaries BiSCl and β-Hg S Cl were
(
Bi(1)−Cl(4)
Bi(1)−Cl(4)
a
Symmetry transformations used to generate equivalent atoms: (1)
1
3
1
1
−
−
x, −y, −z + 2; (2) −x + / , −y + / , −z + 2; (3) −x + / , y − / ,
2
2
2
2
3
2
2
3 1 1 3
z + / ; (4) −x + / , y + / , −z + / .
2
2
2
2
always present as minor products.
A quantitative yield was finally obtained after the addition of
about a 3-fold excess of BiCl and running of the reaction at
3
C
dx.doi.org/10.1021/ic3023826 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
00 °C. The PXRD pattern of the product obtained is shown in
Investigations aimed at synthesizing other quaternary
chalcohalide analogues using HgQ (Q = S, Se, Te) with
different halides, BiX (X = Br, I) or SbX (X = Cl, Br, I), under
Figure 1. The excess of BiCl suppresses the formation of BiSCl
3
3
3
conditions similar to those described above were unsuccessful,
yielding the ternaries Hg Q X and BiQX or SbQX and BiX or
3
2
2
3
SbX3.
Structural Description. Compounds 1 and 2 have
7
structures similar to those of Hg Te UCl and Hg Q MX
3
2
6
3
2
6
8
(
Q = S, Se; M = Zr, Hf; X = Cl, Br), which are composed of
2+
[
Hg Q ] (Q = S, Te) layers that form from fused Hg Q rings
3
2
6
6
having a cyclohexane chair-type conformation. A two-dimen-
sional (2D) network of bismuth halide counteranions is
2+
intercalated between the [Hg Q ] interlayer, as shown in
3
2
Figures 3 and S2 in the SI. As shown in Figure 4, compounds 1
Figure 1. PXRD patterns of compound 1: (a) simulated; (b) observed.
and β-Hg S Cl phases. We also observed that an extended
3
2
2
period of heating 1 at temperatures between 350 and 450 °C
led to the formation of β-Hg S Cl and BiCl as separate major
3
2
2
3
entities, suggesting incongruent melting.
Unlike 1, the synthesis of 2 is less complicated. A quantitative
yield of a dark-green polycrystalline powder of 2 was obtained
using a stoichiometric ratio of HgTe, HgCl , and BiCl by a
2
3
low-temperature route at 200 °C, as shown in the PXRD
pattern of the product obtained (Figure 2). 2 also melts
incongruently, which is a common behavior in mercury
8
,13
chalcohalides.
Figure 3. Overall 2D structure of compound 1 viewed along
crystallographic (a) c and (b) b axes.
and 2 have dinuclear metal halides (Bi Cl ) instead of
2
8
mononuclear metal halides as observed in Hg Te UCl and
3
2
6
7
,8
Hg Q MX (Q = S, Se; M = Zr, Hf; X = Cl, Br). This leads
3
2
6
to the resulting formula Hg Q Bi Cl (Q = S, Se, Te), with a
3
2
2
8
correspondingly larger cell volume.
The mercury atoms are two-coordinated with a linear
geometry. The chalcogen atoms are three-coordinated, QHg3,
forming trigonal pyramids. The Hg S chair conformation is
6
6
distorted in compound 1, as indicated by its interior bond
angles and bond lengths, shown in Table 3. Normal
cyclohexane with a chair conformation has interior bond angles
of 109.5° with similar C−C bond distances. In compound 1,
the interior bond angles (Hg−S−Hg) are less than 109.5°
[ranging from 101.43(8) to 103.07(6)°] and the interior bond
distances (Hg−S) range from 2.372(2) to 2.385(1) Å. A more
distorted chair conformation of Hg Te is observed for
compound 2, shown in Table 4, in which the interior bond
angles are even smaller [ranging from 93.05(3) to 95.40(3)°],
with larger interior bond distances ranging from 2.6377(9) to
2.642(1) Å.
Figure 2. PXRD patterns of compound 2: (a) simulated; (b) observed.
Preparing good-quality single crystals of 3 was difficult, and
stoichiometric reactions did not produce large crystals suitable
for single-crystal XRD. Such crystals could be obtained only at
∼
30% yield from the 1:1 reaction of HgSe and BiCl at 600 °C.
The obtained average unit cell parameters are a = 6.971(1) Å, b
8.634(2) Å, c = 8.120(2) Å, and β = 106.87(3)°. Because of
3
6
6
=
modulation and structural disorder observed in the structural
model, it was difficult to obtain an acceptable structural
refinement for this compound. The structural refinements as
well as PXRD patterns, however, show that 3 probably has a
structure related to those of the other two analogues.
The bismuth atoms are seven-coordinated, forming mono-
capped trigonal prisms (Figure S3 in the SI) that edge share to
D
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7
Figure 4. Structural comparison of (a) the previously reported Hg Te UCl , (b) compound 1 with its thermal ellipsoid, and (c) compound 2 with
3
2
6
its thermal ellipsoid.
form readily recognizable dimeric units that form 2D sheets
along the crystallographic ab and bc planes for compounds 1
(
Figure 5) and 2 (Figure 6), respectively. The sheets of Bi Cl8
2
Figure 6. 2D network formation of Bi Cl of compound 2 (viewed
2
8
Figure 5. 2D network formation of Bi Cl of compound 1 (viewed
along the a axis), showing the formation of edge-sharing monocapped
2
8
along the c axis), showing the formation of edge-sharing monocapped
trigonal prisms. See the text for details.
trigonal prisms. See the text for details.
exothermic peaks upon cooling, which are slightly shifted
from cycle 1 to cycle 2 (Figure S4 in the SI). There are two new
endothermic peaks (labeled with arrows for clarity), in addition
to previous two endothermic peaks (also labeled with arrows),
observed upon a second heating for compound 2 (Figure S4b
in the SI). The shifting of the original DTA peaks and the
appearance of new peaks indicate that the compounds are
incongruently melting. This confirms our observations
described above regarding the synthetic chemistry of these
materials. The PXRD pattern of compound 1 after DTA
measurements shows the formation of β-Hg S Cl , in addition
in compound 1 possess a crystallographic 2-fold axis and a
mirror plane that contains all bismuth atoms consistent with
the space group C12/m1; see Figure 5. For compound 2, the
sheets of Bi Cl possess a crystallographic 2-fold axis and a
2
8
perpendicular c-glide plane; see Figure 6. Thus, the two sheets
of Bi Cl are very different in the two compounds.
2
8
Thermal Properties. The thermal behavior of compounds
and 2 was investigated by two consecutive cycles of heating
1
and cooling using DTA measurements. Both compounds
exhibit two endothermic peaks upon heating and two
3
2
2
E
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to the presence of 1 and BiSCl (Figure S5 in the SI). Similarly,
the PXRD pattern after DTA measurements showed the
formation of Hg Te Cl , which confirms the incongruent
3
2
2
melting behavior of compound 2. In this PXRD pattern, the
presence of compound 2, BiTeCl, and two minute peaks as
assigned to BiTe (marked as *) was also observed (Figure S6 in
the SI).
Optical Properties. Diffuse-reflectance UV−vis/near-IR
NIR) spectra were collected for both compounds 1 and 2 at
(
room temperature. The room temperature band gap of
compound 1 is ∼3.26 eV (Figure 7), in accordance with the
white platelike crystals obtained. The spectrum of compound 2
(
Figure 8) shows a band gap of 2.80 eV.
Figure 9. PDOS for 1 (a) and 2 (b) with the LDA including SOC. In
both panels, red, green, and orange solid lines correspond to Hg s, Bi
p, and Cl p orbital contributions, respectively. The blue solid line
means the S p (a) and Te p (b) orbitals. The insets in parts a and b
present LDA band structures without SOC near the CBM.
Figure 7. Diffuse-reflectance UV−vis/NIR spectrum of compound 1
measured at room temperature, showing a band gap of 3.26 eV.
due to SOC, as shown in Figure 9b. Because of the similarity in
the electronic structures of both compounds, compounds 1 and
2
have a similar band-gap character, whose VBM is from Cl p
orbitals in the Bi−Cl layer and CBM is from Bi p and Cl p
orbitals in the Bi−Cl layer.
In order to calculate the band gaps as well as the shape of
band edges of the two compounds accurately, we performed
sX-LDA band-structure calculations including SOC. As shown
in the previous PDOS in Figure 9, the spin−orbit splitting in
bismuth orbitals are quite large at the CBM; thus, it
significantly affects the band-gap sizes of the two compounds.
Furthermore, sX-LDA has shown a great improvement in the
excited electronic structure in terms of correctly determining
the band gap and band dispersion for wide-range semi-
conductors. The calculated band structures of compounds 1
and 2 are presented in parts a and b of Figure S7 in the SI,
respectively. While LDA calculations yield band gaps of 3.06 eV
without SOC and 2.26 eV with SOC in compound 1, the band-
structure calculations using sX-LDA including SOC predicts a
band gap of 3.09 eV. In addition, the result from the calculation
using sX-LDA with SOC shows that the band topology near the
CBM is significantly changed due to a larger spin−orbit
splitting in the Bi p orbitals at the Γ point. Moreover, the
indirect band gap indicated in the LDA calculation is changed
to a direct band gap between the VBM and CBM at Γ in the
SOC and sX-LDA+SOC calculations. Compound 2 also follows
a similar tendency shown in compound 1; LDA calculations of
compound 2 show an indirect band gap of 2.91 eV without
SOC and a direct band gap of 2.28 eV with SOC, while the
calculation using sX-LDA with SOC determines an indirect
band gap of 2.80 eV.
Figure 8. Diffuse-reflectance UV−vis/NIR spectrum of compound 2
measured at room temperature, showing a band gap of 2.80 eV.
Band Structure. To understand the electronic structures of
compounds 1 and 2, we performed the first-principles DFT
electronic-structure calculations. We used the experimental
lattice constants and atomic structures for these calculations.
First, to figure out atomic orbital contributions at band edges,
we calculated the projected density of states (PDOS) for both
compounds including SOC. As shown in Figure 9a, the valence
band maximum (VBM) of compound 1 is mainly composed of
the Cl p orbital and a relatively small contribution of the S p
orbital, while the conduction band minimum (CBM) consists
of Bi p and Cl p orbitals. Without SOC, the CBM are mainly
Hg s and S p orbitals (see the inset of Figure 9a); however, a
large spin−orbit splitting in Bi p orbitals leads to a change in
the atomic orbital contribution at the CBM when we include
SOC. In compound 2, the same situation occurs; the VBM
consists of the Cl p orbital and a relatively small contribution of
the Te p orbital, and the atomic orbital composition at the
CBM is changed from Hg s/Te p orbitals to Bi p/Cl p orbitals
CONCLUSIONS
■
The quaternary mercury and bismuth chalcohalides
Hg Q Bi Cl (Q = S, Se, Te) can be synthesized using
3
2
2
8
F
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Inorganic Chemistry
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synthetic conditions designed to suppress their tendency for
phase separation. The compounds combine two important
features needed for good X-ray and γ-ray detection applications,
namely, high specific densities, high atomic numbers, and wide
energy gaps of 3.26 and 2.80 eV for compounds 1 and 2,
respectively. Because of their incongruent melting nature,
however, it will be a challenge to grow large centimeter-sized
crystals of these compounds using conventional Bridgman
growth technique. Electronic-structure calculation using sX-
LDA including SOC shows that SOC of the bismuth atom
plays an important role in determining the CBM and predicts
that compound 1 has a direct band gap of 3.09 eV, while
compound 2 has an indirect band gap of 2.80 eV, in a good
agreement with the experimental results.
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ASSOCIATED CONTENT
■
*
S
Supporting Information
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AUTHOR INFORMATION
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Corresponding Author
E-mail: m-kanatzidis@northwestern.edu.
(
*
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
(
This work was supported by the Office of Nonproliferation and
Verification Research and Development under the National
Nuclear Security Administration of the U.S. Department of
Energy under Contract DE-AC02-06CH11357.
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