Thallium mercury chalcobromides, TlHg6Q4Br 5 (Q = S, Se)

Article abstract of DOI:10.1021/ic4014174

The new compounds TlHg₆Q₄Br₅ (Q = S, Se) are reported along with their syntheses, crystal structures, and thermal and optical properties, as well as electronic band structure calculations. Both compounds crystallize in the tetragonal I4/m space group with a = 14.145(1) ?, c = 8.803(1) ?, and d_calc = 7.299 g/cm³ for TlHg₆S₄Br₅ (compound 1) and a = 14.518(2) ?, c = 8.782(1) ?, and d_calc = 7.619 g/cm³ for TlHg₆Se₄Br₅ (compound 2). They consist of cuboid Hg₁₂Q₈ building units interconnected by trigonal pyramids of BrHg₃, forming a three-dimensional structure. The interstitial spaces are filled with thallium and bromide ions. Compounds 1 and 2 melt incongruently and show band gaps of 3.03 and 2.80 eV, respectively, which agree well with the calculated ones. First-principles electronic structure calculations at the density functional theory level reveal that both compounds have indirect band gaps, but there also exist direct transitions at energies similar to the indirect gaps.

Full text of DOI:10.1021/ic4014174

Article
pubs.acs.org/IC
Thallium Mercury Chalcobromides, TlHg Q Br (Q = S, Se)
6
4
†
5
†
†,‡
§
§
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: The new compounds TlHg Q Br (Q = S, Se) are reported
6
4
5
along with their syntheses, crystal structures, and thermal and optical
properties, as well as electronic band structure calculations. Both
compounds crystallize in the tetragonal I4/m space group with a =
3
1
4.145(1) Å, c = 8.803(1) Å, and dcalc = 7.299 g/cm for TlHg S Br
6
4
5
(
compound 1) and a = 14.518(2) Å, c = 8.782(1) Å, and d = 7.619 g/
calc
3
cm for TlHg Se Br (compound 2). They consist of cuboid Hg Q
6
4
5
12
8
building units interconnected by trigonal pyramids of BrHg , forming a
3
three-dimensional structure. The interstitial spaces are filled with thallium
and bromide ions. Compounds 1 and 2 melt incongruently and show
band gaps of 3.03 and 2.80 eV, respectively, which agree well with the calculated ones. First-principles electronic structure
calculations at the density functional theory level reveal that both compounds have indirect band gaps, but there also exist direct
transitions at energies similar to the indirect gaps.
7
c
INTRODUCTION
[Hg Q ][MX ] (Q = S, Se; M = Zr, Hf; X = Cl, Br),
3
2
6
■
7b
12
15
Hg AsQ X (Q = S, Se; X = Cl, Br, I), Hg ZnS Cl ,
3
4
3
2
4
Chalcohalide compounds are hybrids of a chalcogenide and a
halide, with energy band gaps generally between those of the
respective chalcogenide and halide. Earlier we have proposed
that chalcohalide may possess attractive physical properties, e.g.,
X-ray and γ-ray detection.
1
3
14
[
Hg
Hg As ][Bi Cl ],
(Hg Cd S Br)Br,
Hg InS Cl ,
7 6 5
7a
8
4
3
13
2
2
2
16
1
PbS
2
I
2
2
,
and Hg
3
Q
Bi
2
Cl
2
8
(Q = S, Se, Te). Herein, we
report new thallium−mercury-based chalcohalides, TlHg S Br
6
4
5
(
1) and TlHg Se Br (2), with wide band gaps and high mass
1
j,m,2
6 4 5
The chalcohalide class of
density. We describe their syntheses, structural characterization,
and thermal and optical properties, as well as electronic
structure, to evaluate their potentials to grow as large single
crystals for room temperature X-ray and γ-ray semiconductor
detector based on the criteria above.
compounds has already demonstrated strong optical non-
3
4
5
linearity, ferroelectricity, phase transitions, and upconversion
6
luminescence. We are interested in exploring highly dense,
heavy-element containing chalcohalides, such as mercury and
thallium, as part of a search for candidates for room
temperature X-ray and γ-ray semiconductor detectors. Gen-
erally, the density and energy band gap of potential candidate
EXPERIMENTAL SECTION
■
(
3
Chemicals in this work were used without further purification: HgBr
99.998% metal basis, Sigma-Aldrich), TlBr (99.999% metal basis,
2
materials for such purpose should be over 5 g/cm and 1.6 eV,
respectively, and preferably crystallizing in a three-dimensional
compact structure in a high symmetry. The work reported here
was motivated by the desire to identify new materials with
prerequisites for hard radiation detectors, as mentioned above.
Mercury(II) adopts a closed shell electronic configuration
and has a large ionic radius with flexible coordination
Strem Chemicals, Inc.), elemental Hg (99.999%, electronic grade,
Sigma-Aldrich), sulfur chunks (99.999%, Spectrum Chemical Mfg.
Corp.), and selenium shots (99.999%, Plasmaterials). All manipu-
lations were done inside a nitrogen-filled glovebox.
Synthesis of HgQ (Q = S, Se). HgS, a total of 10 g, was prepared
as a stoichiometric mixture of mercury metal and sulfur powder. Liquid
mercury was added by pipetting a stoichiometric amount into a fused
silica tube and then sulfur was loaded. The tube was flame-sealed
7
environments, giving rise to diverse molecular architectures.
Thallium(I), with its 6s electronic configuration, often
2
−4
under a dynamic vacuum of ∼10 mbar and then put into a
resembles the chemistry of alkali metal ions; hence, it is called
programmable furnace. The mixture was heated to 400 °C over 12 h
and held isothermally for 1 d, followed by cooling slowly to 50 °C over
6 h. The obtained dark red ingot was ground to powder before use.
The product’s purity was confirmed using powder X-ray diffraction
8
2
the pseudoalkali metal. Its so-called inert 6s electron pair can
8
,9
lead to a distorted local structure, and its high atomic number
can give high-density compounds.
conducting compounds that contain mercury and thallium are
of interest as potential materials for X-ray and γ-ray
semiconductor detectors. Recent reports on quaternary
1d,j,k,10
Therefore, semi-
(
PXRD). A similar preparation procedure was adopted for HgSe,
Received: June 4, 2013
Published: October 9, 2013
1
1
mercury-based chalcohalides include [Hg Te ][UCl ],
3
2
6
©
2013 American Chemical Society
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Inorganic Chemistry
Article
Table 1. Crystal Data and Structure Refinement details for 1 and 2 at 293(2) K
1
2
empirical formula
formula weight
wavelength (Å)
crystal system
space group
TlHg S Br
TlHg Se Br
5
6
4
5
6
4
1935.70
2123.30
0.71073 Å
tetragonal
I4/m
unit cell dimensions
volume (Å3)
Z
a = 14.145(1) Å, c = 8.803(1) Å
a = 14.518(2) Å, c = 8.782(1) Å
1761.5(5)
1851.0(5)
4
4
7.619
3
density (calcd) (g/cm )
abs coeff (mm−1)
F(000)
7.299
72.99
76.87
3200
3488
3
crystal size (mm )
0.05 × 0.03 × 0.02l
0.03 × 0.02 × 0.02
index ranges
−16 ≤ h ≤ 16, −16 ≤ k ≤ 16, −9 ≤ l ≤9
−17 ≤ h ≤ 17, −17 ≤ k ≤ 17, −9 ≤ l ≤10
reflections collected
independent reflections
completeness to θ = 24.97°
refinement method
data/restraints/parameters
GOF
5452
802 (Rint = 0.1225)
97.0%
5742
875 (Rint = 0.1702)
99.7%
full-matrix least-squares on F2
802/0/43
1.179
875/0/43
1.284
final R indices [I > 2σ(I)]
Robs = 0.0439, wRobs = 0.1045
Rall = 0.0653, wRall = 0.1481
Robs = 0.0505, wRobs = 0.1191
Rall = 0.0772, wRall = 0.1650
2.214 and −2.902
a
R indices (all data)
largest diff. peak and hole (eÅ−3)
2.452 and −2.487
a
R = ∑∥F | − |F ∥/∑∥F ∥. wR = {∑[w(F − F )]/∑[w(F )]}1/2.
2
o
2
c
2
o
o
c
0
2
4
which was heated to 550 °C over 16 h, held isothermally for 1 d, 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. Heating the mixture of
mercury and sulfur/selenium should be done slowly to ensure that the
liquid mercury reacts with sulfur or selenium without evaporating.
Synthesis of TlHg S Br (1) and TlHg Se Br (2). A ground
Table 2. Atomic Coordinates (×10 ) and Equivalent
2
3
Isotropic Displacement Parameters (Å × 10 ) at 293(2) K
with Estimated Standard Deviations in Parentheses
a
label
x
y
z
occupancy
Ueq
6
4
5
6
4
5
(
a) TlHg S Br
6
4
1/
5
mixture of HgQ (Q = S, Se) (4 mmol), HgBr (2 mmol), and TlBr (1
1/
2
Tl
0
1
1
1
1
1
1
1
1
54(1)
38(1)
43(1)
27(2)
32(2)
34(2)
29(1)
33(1)
2
4
mmol) totaling 0.5 g was loaded into a fused silica tube. The tube was
−4
Hg(1)
Hg(2)
S
0.1768(1)
0.1428(2)
0.1302(4)
0
0.0378(1)
0.2302(2)
0.2006(4)
0
0.2639(2)
0
flame-sealed under vacuum (∼10 mbar) and was heated in a furnace
to 200 °C over 6 h and held isothermally for 12 h for 1 or 48 h for 2,
followed by a shutdown of the furnace. Quantitative yields of
polycrystalline powders of 1 (white with slight yellow tint) and 2 (pale
yellow) were obtained.
0.2689(8)
Br(1)
Br(2)
Br(3)
Br(4)
1/
2
0
0
0
To obtain a suitable size of high-quality single crystals for X-ray
structural characterization, a separate synthetic reaction with a 1:1
mixture of HgQ and TlBr was carried out as follows. A thoroughly
mixed powder of HgQ and TlBr at a 1:1 ratio totaling 0.5 g was sealed
in a fused silica tube under vacuum, heated to 450 °C over 6 h and
held isothermally for 6 h, and then heating to 750 °C for 6 h and held
isothermally for 12 h. The tube was then cooled down slowly to 200
0.1724(2)
0.3153(2)
0.4293(2)
0.0711(2)
0
0
(
b) TlHg Se Br
6
4
1/
5
1/
Tl
0
1
1
1
1
1
1
1
1
52(1)
43(1)
45(1)
20(1)
25(2)
35(2)
25(1)
28(1)
2
4
Hg(1)
Hg(2)
Se
0.1769(1)
0.1380(2)
0.1309(2)
0
0.0395(1)
0.2383(2)
0.2048(2)
0
0.2640(2)
0
0.2800(3)
°
C over 36 h and finally to room temperature within 1 h. The well-
1/2
Br(1)
Br(2)
Br(3)
Br(4)
formed crystals of the quaternary compounds were produced
approximately at 40% yield based on HgQ. A quantitative analysis of
the crystals obtained was performed by scanning electron microscope-
energy dispersive spectrometry (SEM-EDS).
0
0
0
0.1682(2)
0.3145(2)
0.4314(2)
0.0692(3)
0
0
Single-Crystal X-ray Diffraction. Single crystal XRD data were
collected at 293 K on a STOE 2T image plate diffractometer with Mo
Kα radiation (λ = 0.710 73 Å). An analytical absorption correction was
applied to the 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.
The 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
a
Ueq is defined as one-third of the trace of the orthogonalized Uij
tensor.
Powder X-ray Diffraction. PXRD patterns on the ground powder
of the obtained products were collected 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−Visible Spectroscopy. Optical diffuse-reflectance spectra
were obtained at room temperature using a Shimadzu UV-3600
spectrophotometer operating in the 200−2500 nm region. The
instrument is equipped with an integrating sphere and controlled by a
coordinates, the displacement parameters (U ), and occupancies of all
eq
atoms with estimated standard deviations are given in Tables 2 and S1
(Supporting Information). The selected bond lengths and angles are
personal computer. BaSO was used as a 100% reflectance standard.
4
given in Table 3 for 1 and 2.
The finely ground powder sample was spread on a compacted surface
1
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Inorganic Chemistry
Article
Table 3. Representative Bond Lengths (Å) and Bond Angles
mercury chalcogenides with thallium bromide in a closed silica
ampule under a temperature of 750 °C. After discovery of these
new compounds, we attempted to optimize the synthetic
conditions to obtain quantitative yields using a stoichiometric
a
(deg)
TlHg S Br
TlHg Se Br
6 4 5
6
4
5
Bond Lengths
mixture of HgQ (Q = S, Se), HgBr , and TlBr. The
2
Hg(1)−Q
Hg(1)−Q
Hg(1)−Br(3)
Hg(1)−Br(4)
Hg(2)-Q
2.395(5)
2.401(5)
3.015(3)
3.075(3)
2.410(7)
2.410(7)
2.848(4)
2.401(5)
3.015(3)
3.015(3)
3.075(3)
2.495(3)
2.510(3)
3.088(3)
3.091(3)
2.509(3)
2.509(3)
2.837(4)
2.511(3)
3.088(3)
3.088(3)
3.091(3)
experiments involved two types of heat treatments such as
(
a) melting and slow cooling and (b) melting and quenching in
ice water. None of these synthetic attempts were successful in
giving a single phase product of either compound. Instead, the
ternary Hg S Br or Hg Se Br phases were always present as
3
2
2
3
2
2
Hg(2)-Q
minor products in addition to small amounts of Tl S or HgSe.
2
Hg(2)−Br(3)
Q−Hg(1)
Br(3)−Hg(1)
Br(3)−Hg(1)
Br(4)−Hg(1)
A quantitative yield of both compounds was finally obtained
by solid-state diffusion of each stoichiometric mixture at 200
°
C. The PXRD patterns of the products obtained are shown in
Figure 1 for 1 and 2, showing pure phase of both compounds.
The peak positions of these isostructural compounds are
slightly shifted from one another, as expected.
Bond Angles
171.8(2)
Q−Hg(1)−Q
171.0(1)
Q−Hg(1)−Br(3)
Q−Hg(1)−Br(3)
Q−Hg(1)−Br(4)
Q−-Hg(1)−Br(4)
Br(3)−Hg(1)−Br(4)
Q−Hg(2)−Q
Q−Hg(2)−Br(3)
Q−Hg(2)−Br(3)
Hg(1)−Q−Hg(1)
Hg(1)−Q−Hg(2)
Hg(1)−Q−Hg(2)
Hg(2)−Br(3)−Hg(1)
Hg(2)−Br(3)−Hg(1)
Hg(1)−Br(3)−Hg(1)
Hg(1)−Br(4)−Hg(1)
91.9(2)
92.3(2)
91.4(1)
88.9(1)
94.6(1)
94.3(1)
90.77(8)
157.1(1)
101.42(7)
101.42(7)
96.9(1)
96.1(1)
91.0(1)
104.3(1)
104.3(1)
84.3(1)
97.2(1)
92.4(2)
94.4(2)
92.66(7)
158.2(3)
100.5(1)
100.5(1)
97.9(2)
97.4(2)
94.6(2)
104.86(9)
104.86(9)
87.17(9)
98.1(1)
Figure 1. Powder X-ray diffraction patterns of (a) simulated and (b)
observed pattern of 1 and (c) simulated and (d) observed pattern of 2
(Cu Kα radiation).
a
Symmetry transformations used to generate equivalent atoms: (1) y,
1
1
1
−
−
x, z; (2) −x + / , −y + / , −z + / ; (3) x, y, −z; (4) −y, x, z; (5)
2
2
2
1 1 1
x + / , −y + / , z − / .
2
2
2
of the 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
Attempts to synthesize the tellurium analogue using a
stoichiometric reaction of HgTe, HgBr
, and TlBr produced
2
2
data according to Kubelka−Munk equation: α/S = (1 − R) /(2R),
Hg Te Br , HgTl Br , and HgTe. Investigations aimed at
3
2
2
4
6
where R is the reflectance and α and S are the absorption and
synthesizing other quaternary chalcohalide analogues with
different halides TlX (X = Cl, I) and different alkali bromides
ABr (A = K, Rb, Cs) under similar conditions described above
were also unsuccessful, yielding only the starting binaries and
Hg Q X .
17
scattering coefficients, respectively.
Differential Thermal Analysis (DTA). Thermal behavior of the
compounds was analyzed on a Shimadzu DTA-50. A sample (∼30 mg)
of ground powder was sealed in a silica ampule under vacuum. A silica
ampule of equal mass filled with Al O was sealed and placed on the
3
2 2
2
3
Structural Description. Both TlHg Q Br (Q = S, Se)
reference side of the detector. The sample was heated to 430 or 500
6
4
5
compounds are isostructural, consisting of Hg Q cuboids
°
C at 5 °C/min, and after 10 min it was cooled at a rate of −5 °C/min
12
8
to 50 °C. One or two cycles of heating and cooling were applied to the
sample. The residues from the DTA experiments were examined by
PXRD.
interconnected in three dimensions by BrHg trigonal
3
pyramids; see Figure 2. Conceptually, the structure is derived
7f,22
from α-Hg S Br
(Figure 2a) by incorporating 2 times the
amount of atoms in the unit cell, hence Hg Q Br , while
3
2
2
Band Structure Calculations. To investigate the electronic
structures of both compounds, first-principles calculations were
performed within the density functional theory (DFT) formalism
6
4
4
maintaining the cuboid units, Hg Q , as the main building
12
8
18
unit. Tl and Br ions fill the remaining interstitial space to
using the projector augmented wave (PAW) method implemented
1
9
complete the observed quaternary structures, TlHg Q Br (Q =
6
4
5
in the Vienna Ab-initio Simulation Package (VASP) code. The cutoff
energy of the planewave basis was set to 350 eV, and 5 × 5 × 5 regular
k-point meshes were used for the Brillouin zone sampling. In order to
evaluate the band gap accurately, we employed the hybrid functional
S, Se). There are significant differences between α-Hg S Br
3
2
2
and TlHg Q Br in terms of how the cuboid units are
6
4
5
connected. The former cuboid units and their surroundings
have 2/m symmetry (Figure 2a), whereas the latter have 4/m
symmetry (Figure 2b). These cuboids are linked into a three-
dimensional structure. There are three types of linkers in α-
20
within the Heyd−Scuseria−Ernzerhof (HSE) approach, which leads
2
1
to a good agreement with experimentally measured band gaps.
RESULTS AND DISCUSSION
■
Hg S Br , which consist of Br(3)Hg (orange color) and
3 2 2 3
Synthesis. Air stable, isotypic compounds of the formula
TlHg Q Br (Q = S, Se) were first identified by the reactions of
Br(4)Hg3 (turquoise color) trigonal pyramids and Br(5) atoms
(Figure 2a). In TlHg Q Br , however, only one linker,
6
4
5
6
4
5
1
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Inorganic Chemistry
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2
2b
Figure 2. (a) A representative 3D structure of α-Hg S Br showing 2/m symmetry. (b) A representative 3D structure of TlHg S Br along the c-
3
2
2
6
4
5
axis, showing 4/m symmetry. (c) A cuboid unit, Hg S , in TlHg Q Br with its detailed environments, and (d) view along the a-axis, showing Br(2)
12
8
6
4
5
located in between adjacent cuboid units similar to that in α-Hg S Br . Br(1) is located in the center of cuboid unit (not shown).
3
2
2
Br(3)Hg (blue color in Figure 2b) trigonal pyramids, is
observed.
°C (melting point) on heating and exothermic peaks at ∼436
°C (crystallization point) on cooling (Figure S2a, Supporting
Information). Similarly, compound 2 exhibits a broad
endothermic peak at ∼495 °C (melting point) and exothermic
peaks at ∼462 °C (crystallization point) (Figure S2b,
Supporting Information). In two cycles of heating/cooling,
there are slight shifts in melting point (for 1) and crystallization
point (for 2). This behavior indicates that the compounds 1
and 2 are incongruent-melting, as often observed in other
3
The cuboid units Hg Q (Q = S, Se) of TlHg Q Br shown
1
2
8
6
4
5
in Figure 2c are built from eight QHg trigonal pyramids with S
3
or Se atoms located in the cuboid corners, similar to the ones in
α-Hg S Br . Each S/Se forms the trigonal pyramid with two
3
2
2
Hg(1) and one Hg(2). S−Hg bond distances ranging from
2
.395(5) to 2.410(7) Å and Se−Hg bond distances ranging
from 2.495(3) to 2.511(3) Å are observed for 1 and 2,
respectively. In the same manner as the ones in α-Hg S Br ,
7a,c,15
mercury chalcohalides.
The incongruent-melting behavior
3
2
2
of both compounds is further confirmed by PXRD patterns on
the residues of DTA measurements (Figure S3 and S4
Br(1) atoms of TlHg Q Br are located in the center of Hg Q
6
4
5
12
8
cuboids, and Br−Hg bond distances ranging from 2.837(4) to
(
Supporting Information) for 1 and 2, respectively). In these
3
4
.091(3) Å and Br−Q bond distances ranging from 3.89(5) to
.301(3) Å are observed in compounds 1 and 2. In Figure 2d,
PXRD patterns, we observed the decomposition products
Hg Q Br , HgTl Br , and HgQ, as well as TlHg Q Br (Q = S,
3
2
2
4
6
6
4
5
Br(2) atoms are located in between adjacent cuboid units,
Se), which may be attributed to eq 1:
similar to that of α-Hg S Br , with bond distances of 3.455(1)
3
2
2
and 3.507(2) Å from neighboring Hg(1) atoms observed in 1
and 2, respectively. The blue trigonal pyramids (Figure 2b,c),
which connect the cuboid units in three dimensions, consist of
Br(3) surrounded by two Hg(1) and one Hg(2) with bond
distances ranging from 2.838(4) to 3.087(3) Å in both
compounds. Br(4) atoms bridge two Hg(1) atoms within a
cuboid unit with bond distance of 3.075(2) and 3.091(2) Å for
△
4
TlHg Q Br → 7Hg Q Br + Tl HgBr + 2HgQ
6
4
5
3
2
2
4
6
(
Q = S, Se)
(1)
In addition, there are minute endothermic and exothermic
peaks that appeared and/or disappeared during two cycles of
heating and cooling in compound 2 (Figure S4, Supporting
Information, labeled by arrows for clarity) that cannot be
attributed to the starting reagents nor be identified from the
PXRD pattern of the pristine compound, as indicated in Figure
2. There may be a possibility that this compound decomposes
before melting, which may correlate with the observed
decomposed products, eq 1 above, of the DTA residue. We
attempted to clarify these minute peaks by heating the sample
1
and 2, respectively. The Tl atoms are eight-coordinated, with
Br(3) and Br(4) atoms forming a slightly distorted cube with
Tl−Br bond distances ranging from 3.432(2) to 3.617(3) Å in
compounds 1 and 2 (Figure S1, Supporting Information).
Thermal Properties. The thermal behavior of 1 and 2 was
investigated by two consecutive cycles of heating and cooling in
DTA. Compound 1 exhibits a broad endothermic peak at ∼466
1
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Inorganic Chemistry
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to 430 °C and then cooling down to room temperature
followed by PXRD examination. Although the sample turns
dark after such heating, the PXRD pattern still shows pure
TlHg Se Br . Further investigation on this matter is underway
in order to optimize the conditions for a large-scale synthesis
and high-quality crystal growth required for a feasibility test for
hard radiation detectors.
implies that the sign of the indirect band gap is hidden by the
direct transition, which has much higher amplitude than that of
indirect transition in an optical spectrum measurement, and
thus, it can explain the sharp absorption edges in Figure 3. The
calculated direct band gaps agree well with the experimentally
measured ones (3.03 eV for 1 and 2.80 eV for 2).
6
4
5
From the projected density of states (PDOS) in Figure 5a,b,
we find that VBM of both compounds mainly consist of Br p
Optical Properties. Diffuse reflectance UV−vis/near-IR
NIR) spectra collected for both 1 and 2 at room temperature
(
show well-defined, sharp absorption edges. The band gap of 1 is
3.03 eV (Figure 3a), in accordance with the color (white with
∼
Figure 3. Optical absorption spectra of (a) 1 and (b) 2, measured at
room temperature.
slight yellow tint) of the plate crystals obtained. The spectrum
of compound 2 (Figure 3b) shows a band gap of 2.80 eV, as
expected from the yellow-colored plate crystals obtained. The
sharp characters of the absorption edges are suggestive of direct
transitions, supported by the electronic band structure
calculations below.
Figure 5. Calculated projected density of states (PDOS) of (a) 1 and
(
b) 2. In all plots, the E is set to the energy of the valence band
F
maximum. Red, green, blue, and orange curves correspond to Hg s, Hg
p, Br p, and Tl s orbitals, respectively; S p and Se p orbitals are plotted
by turquoise curves in parts a and b.
Band Structure. To understand the nature of the band gap
of compounds 1 and 2, we performed first-principles electronic
structure calculations using the HSE functional. As shown in
Figure 4a,b, the band structures of both compounds are similar
to each other. In both compounds, the band gaps are predicted
to be indirect; the conduction band minima (CBM) of both
compounds are located at the Γ points of the Brillouin zone,
while the valence band maxima (VBM) occur at the X points.
However, the direct transitions between the valence band and
the conduction band at Γ show band gaps (3.08 eV for 1 and
orbitals and S/Se p orbitals, while Hg s orbitals and S/Se p
orbitals mostly contribute to CBM. Compared to the VBM, the
CBM has s-orbital symmetry due to the strong contribution of
the Hg s orbital, resulting in a more dispersive conduction band
of both compounds, as shown in Figure 4a,b. In addition, the
bandwidth of the conduction band located at E−E = 3−5 eV
F
increases from 1.81 eV in 1 to 1.97 eV in 2 due to the more
extended character of the Se p orbital than the S p orbital (see
Figure 5). This result can explain the band gap difference of
2
.89 eV for 2) that are very close in energy to the calculated
indirect band gaps (3.00 eV for 1 and 2.74 eV for 2). This result
Figure 4. Calculated electronic band structures of (a) 1 and (b) 2. In all plots, the E is set to the energy of the valence band maximum. Indirect gaps
F
and direct gaps are denoted by red and black arrows, respectively.
1
1879
dx.doi.org/10.1021/ic4014174 | Inorg. Chem. 2013, 52, 11875−11880

Inorganic Chemistry
Article
compounds 1 and 2 found in the optical measurements (Figure
Sebastian, M.; Jin, H.; Chung, D.-Y.; Freeman, A. J.; Wessels, B. W.;
Kanatzidis, M. G. Proc. SPIE 2012, 8507, 14.
3).
(2) Androulakis, J.; Peter, S. C.; Li, H.; Malliakas, C. D.; Peters, J. A.;
Liu, Z.; Wessels, B. W.; Song, J.-H.; Jin, H.; Freeman, A. J.; Kanatzidis,
CONCLUSIONS
Two new quaternary mercury- and thallium-based chalcoha-
lides with general formula TlHg Q Br (Q = S, Se) were
synthesized at low temperatures by solid-state diffusion. Both
compounds exhibit important features, displaying three-dimen-
sional structures with high symmetry (I4/m space group), high
specific density (7.299 g/cm for 1 and 7.619 g/cm for 2), high
atomic numbers, and wide energy gaps of 3.03 and 2.80 eV for
and 2, respectively, which meet the prerequisites for X-ray
and γ-ray detection applications. First-principles electronic
structure calculations reveal that both compounds have an
indirect band, but with near-lying direct transitions, which are
responsible for the sharp absorption edges observed in the
optical spectrum. The calculated direct band gaps predict 3.08
eV for 1 and 2.89 eV for 2, which agrees well with the
experimental results. Growing large-size single crystals of these
compounds using the Bridgman technique, which is challenging
due to the incongruent melting nature of both compounds, is
currently underway for further investigation in X-ray and γ-ray
detection.
■
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ASSOCIATED CONTENT
Supporting Information
Further details are given in Table S1 and Figures S1−S4, as
noted in the text, and crystallographic data are given in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
(d) Axtell, E. A., III; Park, Y.; Chondroudis, K.; Kanatzidis, M. G. J.
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AUTHOR INFORMATION
Corresponding Author
E-mail: m-kanatzidis@northwestern.edu.
Notes
■
*
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The authors declare no competing financial interest.
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Kanatzidis, M. G. Chem. Mater. 2011, 23, 3120. (b) Huan, G.;
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(11) Bugaris, D. E.; Ibers, J. A. J. Solid State Chem. 2008, 181, 3189−
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ACKNOWLEDGMENTS
■
This work was supported by the Office of Nonproliferation and
Verification Research and Development under National
Nuclear Security Administration of the U.S. Department of
Energy under Contract No. DE-AC02-06CH11357.
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