Syntheses, structures, and nonlinear-optical properties of metal sulfides Ba2Ga8MS16 (M = Si, Ge)

Article abstract of DOI:10.1021/ic502362f

Two new metal sulfides, Ba₂Ga₈MS₁₆ (M = Si, Ge), have been synthesized by high-temperature solid-state reactions. They are isostructural and crystallize in the noncentrosymmetric space group P6₃mc (No. 186) with a = 10.866(5) ?, c = 11.919(8) ?, and z = 2 for Ba₂Ga₈SiS₁₆ (1) and a = 10.886(8) ?, c = 11.915(3) ?, and z = 2 for Ba₂Ga₈GeS₁₆ (2). Their three-dimensional frameworks are constructed by corner-sharing mixed (Ga/M)S₄ (M = Si, Ge) and pure GaS₄ tetrahedra, with Ba²⁺ cations filling in the tunnels. Compounds 1 and 2 are transparent over 0.42-20 μm and have wide band gaps of around 3.4 and 3.0 eV, respectively. Polycrystalline 2 displays strong nonlinear second-harmonic-generation (SHG) intensities that are comparable to that of the benchmark AgGaS₂ (AGS) with phase-matching behavior at a laser irradiation of 1950 nm. Of particular interest, compound 2 also possesses a high powder laser-induced damage threshold of ～22 times that of AGS. The alternate stacking of the mixed (Ga/M)S₄ (M = Si, Ge) tetrahedral layer with the pure GaS₄ tetrahedral layer along the c axis and the alignment of these two types of tetrahedra in the same direction may be responsible for the large SHG signals observed.

Full text of DOI:10.1021/ic502362f

Article
pubs.acs.org/IC
Syntheses, Structures, and Nonlinear-Optical Properties of Metal
Sulfides Ba2Ga8MS16 (M = Si, Ge)
Bin-Wen Liu,† Hui-Yi Zeng,† Ming-Jian Zhang,† Yu-Hang Fan,† Guo-Cong Guo,*,† Jin-Shun Huang,†
and Zhen-Chao Dong*,‡
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, P. R. China
‡Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui
230026, P. R. China
S
* Supporting Information
ABSTRACT: Two new metal sulfides, Ba2Ga8MS16 (M = Si,
Ge), have been synthesized by high-temperature solid-state
reactions. They are isostructural and crystallize in the
noncentrosymmetric space group P63mc (No. 186) with a =
10.866(5) Å, c = 11.919(8) Å, and z = 2 for Ba2Ga8SiS16 (1)
and a = 10.886(8) Å, c = 11.915(3) Å, and z = 2 for
Ba2Ga8GeS16 (2). Their three-dimensional frameworks are
constructed by corner-sharing mixed (Ga/M)S4 (M = Si, Ge)
and pure GaS4 tetrahedra, with Ba2+ cations filling in the
tunnels. Compounds 1 and 2 are transparent over 0.42−20 μm
and have wide band gaps of around 3.4 and 3.0 eV, respectively. Polycrystalline 2 displays strong nonlinear second-harmonic-
generation (SHG) intensities that are comparable to that of the benchmark AgGaS2 (AGS) with phase-matching behavior at a
laser irradiation of 1950 nm. Of particular interest, compound 2 also possesses a high powder laser-induced damage threshold of
∼22 times that of AGS. The alternate stacking of the mixed (Ga/M)S4 (M = Si, Ge) tetrahedral layer with the pure GaS4
tetrahedral layer along the c axis and the alignment of these two types of tetrahedra in the same direction may be responsible for
the large SHG signals observed.
strategy have been investigated as new NLO materials with high
LIDTs for IR applications. Related examples include LiGaS2,5
LiInS2,6 Li2CdMS4 (M = Ge, Sn),7 BaGa4S7,8 Li2Ga2GeS6,9
Li2In2MS6 (M = Si, Ge),10 and BaGa2MS6 (M = Si, Ge).11 In
this paper, we report a new phase in this largely unexplored
AE−III−IV−Q system (AE = alkaline-earth metal; III = Al, Ga,
In; IV = Si, Ge, Sn; Q = S, Se), specifically the syntheses,
structures, and NLO properties of two metal sulfides,
Ba2Ga8SiS16 (1) and Ba2Ga8GeS16 (2).
INTRODUCTION
■
Nonlinear-optical (NLO) materials are important in modern
optical science and technology for their applications in laser
sources based on the second-harmonic-generation (SHG) and
optical parametric oscillation processes.1 The well-known
oxide-based NLO materials such as KH2PO4 (KDP), KTiOPO4
(KTP), β-BaB2O4 (BBO), and LiB3O5 (LBO)2 are widely used
in high-power applications because of their high laser damage
thresholds. Nevertheless, their spectral range of good trans-
mission is limited to the UV−vis region, not suitable for the
mid- and far-IR region due to strong absorption there. On the
other hand, the commercial NLO materials used in the IR
region are chalcopyrite semiconductors such as AgGaS2 (AGS),
AgGaSe2 (AGSe), and ZnGeP2 (ZGP),3 which feature high
NLO coefficients, but they suffer from relatively low laser-
induced damage thresholds (LIDTs) because of their relatively
small band gaps, thus limiting their high-power applications in
the IR transmission region.4 Consequently, exploring new NLO
materials with both strong nonlinear responses and high LIDTs
is still a pressing and important topic in the IR−NLO material
field.
EXPERIMENTAL SECTION
■
Syntheses. All starting reactants were handled inside an argon-
filled glovebox with controlled oxygen and moisture levels below 0.1
ppm. Ba metal (99.9%), Ga2O3 powder (99.9%), Ge powder
(99.99%), Si powder (99.99%), S powder (99.99%), and B powder
(99.99%) were used as received without further purification. The
binary starting material, Ga2S3, was synthesized as a precursor, which
was obtained from a stoichiometric mixture of the elements B and S
and compound Ga2O3, then annealed at 950 °C for 2 days, and finally
cooled to room temperature (the use of B is to take away the O
component in the starting material). For the synthesis of target
The incorporation of an electropositive element such as an
alkaline or alkaline-earth element into the framework will widen
the band gap of the resulting material, leading to a higher
LIDT. Noncentrosymmetric chalcogenides following such a
Special Issue: To Honor the Memory of Prof. John D. Corbett
Received: September 28, 2014
© XXXX American Chemical Society
A
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compounds, a stoichiometric mixture with an overall weight of about
300 mg of the starting materials Ba, Ga2S3, Ge (Si), and S in a molar
ratio of 2:4:1:4 was loaded into a quartz tube and then flame-sealed
under vacuum (∼10−4 Torr). The tube was then placed in a
temperature-controlled muffle furnace, held at 300 °C for 5 h, then
heated to 650 °C for over 5 h and kept there for 10 h, subsequently
heated to 980 °C for over 10 h, dwelled for 5 days, and finally slowly
cooled to 400 °C before the furnace power was switched off.
Transparent yellow single crystals of the title compounds were
obtained in high purity and were stable in air for several months.
Single-Crystal X-ray Diffraction (XRD). Single-crystal XRD data
were performed on a Rigaku Mercury CCD diffractometer equipped
with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at
293 K. The intensity data sets were collected using a ω-scan technique
and reduced using CrystalClear software.12 The structures were solved
by direct methods and refined by full-matrix least-squares methods on
F2 with anisotropic thermal parameters for all atoms. All of the
calculations were performed with the Siemens SHELXTL, version 5,
package of crystallographic software.13 The space group was
determined to be P63mc, based on the intensity distribution and
systematic extinction of the observed structure factors. The final
structures were examined for additional symmetry with ADDSYM/
PLATON.14 The assignment of the Ba and S atoms in both structures
can be readily done assuming that these atoms take over the
corresponding crystallographic positions with full occupancy. How-
ever, the assignment of crystallographic sites M1 (12d) and M2 (6c)
to Ga or Si (Ge) or a mixture of them needs careful consideration.
Taking compound 2 as an example, on the basis of a molar ratio of
Ga:Ge close to 8:1 in terms of energy-dispersive X-ray (EDX)
measurement results and the charge-balance requirement for a neutral
compound, the M1 site was assigned to a mixed occupancy of Ga/Ge
with a partial occupancy of 0.833(3)/0.166(7) and the M2 site to pure
Ga, thus generating a consistent chemical formula of Ba2Ga8GeS16.
The above assignments can be further supported by calculating the
valence bond sums (VBSs)15 around each site, which suggests that that
the M1 site is partially filled by Ga and Ge (VBS = 3.17) and the M2
site is fully occupied by Ga (VBS = 3.01). Similar procedures were
applied for the structural determination of 1. Because Si is much
lighter (or smaller) than Ga, a judgment in terms of reasonable
amplitude of the isotropic thermal parameters for different assign-
ments of atoms to the M1 or M2 sites also favors the mixed-occupancy
model proposed above for compound 2. The program STRUCTURE
TIDY16 was used to put the positional parameters in a standard setting.
Additional crystallographic details are given in Table 1, the selected
bond distances and bond angles are given in Table 2, and the
positional coordinates and isotropic equivalent thermal parameters are
listed in Table S1 in the Supporting Information (SI). Further
information may be found in the SI.
Table 2. Selected Interatomic Distances (Å) and Bond
Angles (deg) in 1 (M = Si) and 2 (M = Ge)
1
2
Bond Lengths
3.454(2)
Ba1−S2 (×3)
Ba1−S3 (×3)
Ba1−S1 (×6)
Ba2−S1 (×6)
Ba2−S3 (×6)
Ba2−S2 (×3)
Ga/M1−S2
Ga/M1−S3
Ga/M1−S1
Ga/M1−S4
Ga2−S1 (×2)
Ga2−S5
3.442(1)
3.554(1)
3.606(7)
3.533(8)
3.603(1)
3.655(1)
2.248(8)
2.252(8)
2.254(1)
2.322(7)
2.244(8)
2.307(9)
2.320(1)
3.567(3)
3.597(2)
3.531(2)
3.606(3)
3.659(3)
2.230(2)
2.237(2)
2.253(2)
2.312(1)
2.241(2)
2.303(2)
Ga2−S4
2.324(2)
Bond Angles
114.13(7)
110.63(7)
104.59(7)
109.85(7)
107.68(6)
109.78(7)
110.35(7)
111.07(7)
111.07(7)
108.44(5)
108.44(5)
107.34(9)
S2−Ga/M1−S3
S2−Ga/M1−S1
S3−Ga/M1−S1
S2−Ga/M1−S4
S3−Ga/M1−S4
S1−Ga/M1−S4
S1−Ga2−S1
S1−Ga2−S5
S1−Ga2−S5
S1−Ga2−S4
S1−Ga2−S4
113.90(4)
110.62(4)
104.48(4)
109.86(4)
107.70(4)
110.12(4)
110.30(5)
111.25(4)
111.25(4)
108.41(4)
108.41(4)
107.09(6)
S5−Ga2−S4
Powder XRD. To make phase characterization and identification,
powder XRD data of samples 1 and 2 were recorded on an automated
Rigaku MiniFlex II X-ray diffractometer equipped with a diffracted
monochromator set for Cu Kα radiation (λ = 1.54057 Å), operating at
30 kV and 40 mA. The observed powder patterns were found to be in
good agreement with the simulated ones based on the single-crystal
structure refinements of 1 and 2, respectively (Figure S1 in the SI).
Such an agreement suggests that the crystals are quite pure,
particularly for phase 2, despite the possible existence of a tiny
amount of GaS (P63/mmc) in phase 1. Compound 2 also turns out to
be a high-yield phase (>80%). Therefore, the measurements of the
optical properties were mainly carried out for 2.
EDX Measurements. Semiquantitative EDX was performed with
an EDX-equipped Hitachi S-3500 scanning electron microscopy
spectrometer on the crystal that was used for single-crystal XRD.
The measurements confirmed the presence of Ba, Ga, Si (Ge), and S
in an approximate molar ratio of 2:8:1:16 (Figures S2 and S3 in the
SI). No other elements were detected.
IR and UV−Vis−Near-IR (NIR) Diffuse-Reflectance Spectros-
copy. The optical diffuse-reflectance spectra of powder samples 1 and
2 were measured at room temperature using a PerkinElmer Lambda
900 UV−vis spectrophotometer equipped with an integrating sphere
attachment and BaSO4 as the reference. The absorption spectra were
calculated from the reflection spectra using the Kubelka−Munk
formula: α/S = (1−R)2/2R,17 in which α is the absorption coefficient,
S is the scattering coefficient, and R is the reflectance. IR spectra were
recorded using a Nicolet Magana 750 Fourier transform IR
spectrophotometer in the range 4000−500 cm−1. Powdery samples
were diluted with dry KBr and pressed into pellets before
measurements.
Table 1. Crystal Data and Structure Refinements for 1 and 2
empirical formula
Ba2Ga8SiS16 (1)
1373.49
293(2)
Ba2Ga8GeS16 (2)
1417.99
293(2)
fw
temperature (K)
space group
a (Å)
P63mc
P63mc
10.866(5)
11.919(8)
120
10.886(8)
11.915(3)
120
c (Å)
γ (deg)
V (Å3)
Z
1218.8(1)
2
1223.0(1)
2
Dcalcd (g cm−3
)
3.743
3.851
μ (mm−1
)
13.279
14.389
GOF on F2
1.062
1.039
a
R1 [I > 2σ(I)]
0.0230
0.0150
b
wR2 [I > 2σ(I)]
0.0463
0.0272
a
R1 (all data)
0.0248
0.0164
b
wR2 (all data)
0.0469
0.0280
Flack parameter x
Δρmax/Δρmin (e Å−3
0.004(2)
0.766/−0.797
0.000(1)
0.457/−0.431
)
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = ∑[(w(Fo − Fc )2]/
2
∑[w(Fo )2)]1/2
.
B
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Figure 1. Crystal structure of 2. (a) 3D framework showing alternate stacking of two types of tetrahedral layers. (b) Tetrahedral layer constructed by
the pure GaS4 tetrahedra. (c) Tetrahedral layer made up of mixed (Ga/Ge)S4 tetrahedra. (d) 3D framework viewed along the c axis.
SHG Measurements. The powder SHG measurement of 2 was
investigated using a modified Kurtz−Perry powder technique under
laser irradiation at 1950 nm. Microcrystalline samples were sieved into
several distinct particle size ranges (30−50, 50−75, 75−100, 100−150,
150−200, and 200−300 μm) for the SHG phase-matching measure-
ments. The crystalline sample of AGS with similar particle sizes served
as the standard. The frequency-doubling signals (975 nm) were
detected by an Andor DU420A-BR-DD CCD.
atom, and one position partially filled by Ga and M (M = Si,
Ge) with an occupancy ratio of Ga:M = 5:1.
Both compounds adopt the same three-dimensional (3D)
framework structure that consists of alternate stacking of two
distinct tetrahedral layers (Figure 1a). Taking compound 2 as
an example, one layer is constructed by pure GaS4 tetrahedral
units, in which each Ga atom is coordinated by two S1, one S4,
and one S5 atoms in a slightly distorted tetrahedral
coordination geometry and every three neighboring GaS4
tetrahedra are condensed into a trimeric Ga3S16S43S5 unit
(Figure 1b). The other tetrahedral layer is built up from mixed
(Ga/Ge)S4 tetrahedral units, in which each Ga/Ge atom is
coordinated by one S1, one S2, one S3, and one S4 atoms in a
disordered tetrahedral environment. The (Ga/Ge)S4 tetrahedra
are interconnected by corner-sharing S2, S3, and S4 atoms in
LIDT Measurements.18 The powder LIDT measurement of 2 was
performed on a microcrystalline sample (200−300 μm), with a
focused high-power 1064 nm laser beam with a pulse width τp of 8 ns
and a repetition rate of 1 Hz. Microcrystalline AGS of a similar particle
size (200−300 μm) was used as the reference. The power of the laser
beam was measured by a Nova II sensor display with a PE50-DIF-C
energy sensor. An optical concave lens was used to adjust the diameter
of the laser beam to obtain different intensities. The measurements
were carried out by gradually increasing the laser power until the color
of the sample changed, at which time the laser power was defined as
the damage threshold. The area of the damage spot was then measured
to estimate the value of the LIDT.
Electronic Band Structure Calculation. Preliminary calculations
of electronic band structures are also performed for 1 and 2 based on
the structures determined by single-crystal XRD analysis. The
CASTEP code on the basis of density functional theory was used to
calculate band structures and densities of state (DOSs) using a plane-
wave expansion of the wave functions and an ultrasoft pseudopoten-
tial,19 in which the orbital electrons of Ba 4d105s25p66s2, Ga
3d104s24p1, Ge 4s24p2, Si 3s23p2, and S 3s23p4 were treated as valence
electrons. The number of plane waves included on this basis was
determined using a cutoff energy of 295 eV, and the numerical
integration of the Brillouin zone was performed using a 1 × 1 × 1
Monkhorst−Pack k-point sampling.
2
the ab plane, forming a ∞[(Ga/Ge)6S16S26/2S36/2S46/2] layer
(Figure 1c). The pure GaS4 and mixed (Ga/Ge)S4 layers are
stacked alternately along the [0 0 1] direction via sharing S1
and S4 atoms, with neighboring GaS4 and (Ga/Ge)S4 layers
revolving 60° against each other (Figure 1d). The above
condensation of tetrahedral units results in a 3D anion
framework of 3∞[Ga8GeS16]−4, which is charge-balanced by
two Ba2+ cations residing in the tunnels along the c direction
and at one of the interfaces between the pure GaS4 and mixed
(Ga/Ge)S4 layers (Figure 1a,d). Each Ba ion is coordinated to
12 S atoms to form a tetrakadecahedron with Ba−S distances
ranging over 3.44−3.66 Å, comparable to those observed in
BaGa4S78 and BaGa2GeS6.11 Nevertheless, both Ba2+ cations are
slightly off the interstitial center positions, as evidenced by the
asymmetry in the bond lengths (Figure 2).
Overall speaking, as listed in Table 2, the metal−sulfur
distances in both the pure GaS4 and mixed (Ga/Ge)S4
tetradedra vary over a similar range of 2.24−2.32 Å.
Nevertheless, in comparison with the pure GaS4 tetrahedron,
the mixed (Ga/Ge)S4 tetrahedron appears slightly more
distorted, judging from the slightly larger variations in the
RESULTS AND DISCUSSION
■
Crystal Structures. The metal sulfides 1 and 2 are
isostructural and crystallize in the noncentrosymmetric space
group P63mc (No. 186). In the asymmetric unit, there are two
crystallographically independent Ba atoms, five S atoms, one Ga
C
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Figure 2. View of the anisotropic thermal ellipsoids (95% probability
level) of Ba atoms in 2 with Ba−S distances marked.
bond angles, 107−111° for the pure S−Ga−S and 105−114°
for the mixed S−Ga/Ge−S. In addition, the Ga/Ge−S bond
distances in 2 are slightly larger than the corresponding Ga/Si−
S bond distances in 1, in agreement with the larger size of Ge
than Si. Both bond distances and bond angles observed here are
consistent with those reported in Ba4CuGa5S12,20 Li2CdGeS4,7
and Eu2Ga2GeS8,21 but the structural variations on the
tetrahedral distortion and bond lengths give further justification
to the successful assignment of Si or Ge to the crystallographic
M1 (12d) sites. Such an assignment leads to a crystal structure
that features alternate stacking of the mixed (Ga/Ge)S4 and
pure GaS4 tetrahedral layers along the c axis, which, in
combination with the alignment of these two types of
tetrahedra in the same direction and probably also the slight
off-center feature of alkaline-earth cations inside the interstitials,
may modify the electronic (or charge) distribution of the
structure. An enhanced capability of light polarization may thus
be obtained and may be responsible for the large nonlinear
SHG signals observed experimentally (see below for detailed
results).
Figure 3. UV−vis diffuse-reflectance spectra of 1 (a) and 2 (b).
IR and UV−Vis−NIR Diffuse-Reflectance Spectrosco-
py. The UV−vis−NIR diffuse-reflectance spectra in Figure 3
show strong absorption edges roughly at 3.4 and 3.0 eV
respectively for 1 (Figure 3a) and 2 (Figure 3b). In comparison
to the values of AGS (2.64 eV) and ZGP (1.75 eV),
compounds 1 and 2 have relatively wide band gaps, indicating
that they may have high LIDTs. The present compounds
exhibit high transparency in a broad spectral range from 0.42 to
20 μm (Figures 3 and S4 in the SI), which covers the important
band ranges of 3−5 and 8−14 μm of atmospheric transparent
windows. All of these observations suggest that they may be
suitable for a variety of NLO applications in longer-wavelength
(mid/far-IR) regions.
SHG. The SHG signals as a function of the particle size from
the measurements made on ground crystals for 2 are shown in
Figure 4; the SHG intensities increase with increasing particle
size and then reach a plateau at the maximum value after a
certain particle size. Such a correlation is consistent with a
phase-matching behavior for the nonlinear frequency-doubling
signals at 975 nm induced by the incident laser at 1950 nm
(Figure 4), according to the rule proposed by Kurtz and
Perry.22 A sample of AGS (200−300 μm) was also prepared as
a reference material. The measured SHG intensities of 2 are 0.9
times that of AGS (deff = ∼13.7 pm V−1;23 see the inset in
Figure 4). It is known that the measured SHG signal intensity
by the Kurtz and Perry powder method is proportional to the
Figure 4. Phase-matching results for 2. The curve is to guide the eye
and is not a fit to the data. Inset: SHG signals of 2 and AGS.
Powder LIDT. The powder LIDT data of 2 and AGS
measured by the single-pulse powder LIDT method are shown
in Table 3. The threshold of the laser power for the onset
Table 3. Powder LIDTs
damage energy spot diameter
τp
(ns)
damage threshold
square of the SHG coefficient deff, and the second-order
compound
(mJ)
(mm)
[MW cm−2
]
(2)
eff
susceptibility χ is twice that of the SHG coefficient deff.
Therefore, the estimated second-order susceptibility χ(e2ff) for 2 is
∼26 pm V−1.
2
19.0
4.8
1.9
4.5
8
8
83.8
3.8
AGS
D
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damage of sample 2 (19.0 mJ) is much larger than that of AGS
(4.8 mJ). So, a smaller spot diameter (1.9 mm) was selected for
2, in comparison with the 4.5 mm value for the AGS sample.
The damage thresholds of 2 and AGS were derived from the
equation Ithreshold = E/πr2τp, in which E is the laser energy of a
single pulse, r is the spot radius, and τp is the pulse width. The
sample color of compound 2 was changed from yellow to black
upon laser illumination for about 1 s. The LIDT thus estimated
for phase 2 was roughly 84 MW cm−2, ∼22 times of that of
AGS (3.8 MW cm−2), which indicates that compound 2 may be
a good candidate for high-power IR−NLO applications. The
laser damage mechanism is still unclear, but given the observed
band gap of 3.0 eV for compound 2, this phase should be able
to sustain laser illumination at 1064 nm without invoking two-
photon absorption problems.
band gap for phase 2 may be related to stronger state
hybridization between the Ge and S atoms in comparison with
the Si−S interaction for phase 1.
The band components of 1 and 2 are shown by the total and
partial DOSs in Figure 6. The conduction band is mostly
Electronic Band Structure Calculations. Figure 5 plots
the calculated electronic band structures of 1 and 2 along high
Figure 6. Total and partial DOSs of 1 (a) and 2 (b). The Fermi level is
set at 0 eV for all of the band structures and DOS.
composed of Ga 4s and S 3p states, mixing with small amounts
of Ba 5d, Ga 4p, Ge 4s (Si 3s), and Ge 4p (Si 3p) states, while
the valence band from −8.0 eV to the Fermi level originates
predominately from S 3p and Ga 4s states, mixing with small
amounts of Ga 4p, Ge 4s (Si 3s), and Ge 4p (Si 3p) states. The
band from −15.0 to −8.0 eV was mainly composed of Ga 3d
and S 3s states, as well as a small portion of Ba 6s and Ge 4s (Si
3s) states. Therefore, their optical absorptions can mainly be
ascribed to the charge transfer (or charge density oscillation)
from S 3p states to Ga 4s and Ga 4p states. Nevertheless, we
should note that the current calculation is still preliminary,
providing only a qualitative picture about their band structure
and DOS distribution. More sophisticated modeling and
calculations are still needed in order to figure out the
correlation between the crystal structure, electronic band
structure, and NLO properties observed.
Figure 5. Band structures of 1 (a) and 2 (b). The Fermi level is set at
0 eV for all of the band structures and DOS.
symmetry points of the first Brillouin zone. According to the
calculated band diagrams, they are direct-band-gap materials
with band gaps at ∼2.5 and ∼2.2 eV respectively for 1 and 2.
Note that these band gaps are smaller than the corresponding
experimental values of ∼3.4 and ∼3.0 eV derived from the
UV−vis diffuse-reflectance spectra presumably due to the
inaccurate description of the eigenvalues of the electronic states
in the generalized gradient approximation. A slightly smaller
E
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CONCLUSION
■
In summary, two new metal sufides, Ba2Ga8MS16 (1, M = Si; 2,
M = Ge), were obtained via high-temperature solid-state
reactions. They are isostructural and crystallize in the
noncentrosymetric hexagonal space group P63mc. Their 3D
frameworks are constructed by alternate layer stacking via
corner sharing in both the pure GaS4 and mixed (Ga/M)S4 (M
= Si, Ge) tetrahedral layers. Compounds 1 and 2 both have
wide-IR transparent regions and large band gaps. Furthermore,
phase 2 exhibits strong SHG signals that are comparable to
those of the benchmark AGS at laser irradiation of 1950 nm.
The powder LIDT measurements indicate a high LIDT for 2,
∼22 times that of AGS. The observed large nonlinear signals
are presumably related to the synergic action of the alternate
stacking of the mixed (Ga/Ge)S4 and the pure GaS4 tetrahedral
layers along the c axis, the alignment of these two types of
tetrahedra in the same direction, and the slight off-center
feature of alkaline-earth cations in the interstitial sites. These
structural factors may modify the charge distribution of the
structure and result in an enhanced performance in light
polarization. Our results indicate that 2 can be a good candidate
for high-power IR−NLO applications.
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Yin, W. L.; Feng, K.; Lin, Z. S.; Bai, L.; Yao, J. Y.; Wu, Y. C. Inorg.
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ASSOCIATED CONTENT
* Supporting Information
CIF data and additional tables and figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
́
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: gcguo@fjirsm.ac.cn.
*E-mail: zcdong@ustc.edu.cn.
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support by the NSF of
China (Grants 91222204, 21403231, 21101152, and
91021004), the Key Project from the CAS (Grant KJCX2-
EW-H03), and the NSF of Fujian Province (Grant
2014J05025).
(20) Kuo, S. M.; Chang, Y. M.; Chung, M.; Jang, J. I.; Her, B. H.;
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Mater. 2013, 25, 2427−2433.
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