A. Abudurusuli et al.
Journal of Solid State Chemistry 255 (2017) 133–138
structural features and optical properties (diffuse reflection and Raman
spectra). Moreover, first-principle theoretical studies are used to aid
the understanding of electronic structures and linear optical properties
and analyze the effect on optical bandgaps while introducing the Pb
atoms into the crystal structures.
2.4. Structure determination
Data collections were performed on a Bruker SMART APEX II 4K
CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 296(2) K.
The crystal structures were solved using direct methods and refined by
2
full-matrix least-squares on F using the SHELXTL program package
2
. Experimental section
[50]. The program XPREP was used for multi-scan absorption correc-
tion and no additional symmetry is found with PLATON [51]. For the
structure determination of BPSS, the first structural refinement
2
.1. Synthesis
7 3 1
generated a balanced formula of “Ba Sn S13” with R values of R =
Reactants in this work were used as obtained: Ba, PbS, PbSe, Sn, S
6.44% and wR = 20.08%, but the Ba1 and Ba2 atoms have abnormal
2
and Se were purchased from Aladdin Industrial Inc. All of the above
chemicals with purities higher than 99.9% were used without further
purification. The binary starting materials BaS and BaSe were synthe-
sized by the stoichiometric reactions of elements at high temperatures
anisotropy parameters. The EDX results suggested the existence of Pb,
and the molar ratio of Ba:Pb is very close to 5:2. In addition, the Ba1–S
[2.796(3)–3.485(2) Å] and Ba2–S [2.984(3)–3.284(3) Å] distances are
shorter than Ba3–S [3.157(5)–3.541(9) Å] and Ba4–S [3.248(6)–
3.435(0) Å] in the structure, respectively. Thus, the Ba and Pb atoms
were refined to occupy the same sites ((Pb/Ba)1 and (Pb/Ba)2) with a
−
3
in 10-mm-i.d. fused-silica tubes evacuated to 10 Pa.
2
.2. BPSS
ratio of ∼5:2 by the occupancy refinement, and R is converged to
3
better value of 3.94%. Therefore, the Ba Pb Sn S13 formula was
1
5
2
The mixtures of BaS, PbS, Sn, and S in the ratio of 1:1:1:2 were
planned to obtain the BaPbSnS crystal, which were ground and loaded
into 10 mm inner-diameter silica tubes under Ar atmosphere in a glove
box, then this tubes were flame-sealed under a high vacuum of 10 Pa.
The tubes were then placed in computer-controlled furnaces and
heated to 850 °C in 30 h, left for 100 h to ensure the mixture
completely melted, cooled to 300 °C at a rate of 3 °C/h, and finally
cooled to room temperature by switching off the furnace. Obtained
products were washed by the N, N-dimethylformamide (DMF) solvent
considered to be rational and agrees well with the results of EDX test
on this crystal. As for BPSSe, the structural refinement was treated by
the similar procedure as that for BPSS by combination with the EDX
results, thermal parameters, bond lengths, and charge balance. Finally,
4
−3
6 3
the formula was refined to be Ba PbSn Se13 and in agreement with the
EDX results on this crystal. Details of crystal parameters, data
collection and structure refinements are given in Table 1. The atomic
coordinates and the equivalent isotropic displacement parameters are
summarized in Tables S1, and selected bond lengths and angles are
listed in Tables S2 in the Supporting Information.
2 4
to remove the other byproducts (a small number of Ba SnS and other
unknown phases). Many block-shaped crystals with bright red color
were found in the ampules and the yield is about 80%. These crystals
were stable in air and moisture conditions. Note that the BPSS can be
also prepared by the stoichiometric mixture of the precursors.
2
.5. UV–Vis–NIR diffuse-reflectance spectroscopy
Optical diffuse reflectance spectra were measured in the wavelength
Moreover, we also attempted to prepare Ba
different ratios of reagents or reaction temperatures, but the main
products are Ba Sn 15 and Ba SnS [41,49], and no Ba Sn 13 phase
was found in the end.
7 3
Sn S13 by adjusting the
range from 190 to 2600 nm with Shimadzu SolidSpec-3700DUV
spectrophotometer.
7
5
S
2
4
7
3
S
2
.6. Raman spectroscopy
2
.3. BPSSe
Its preparation process was similar to that of BPSS and as follows: a
Raman spectra with hand-picked crystals were collected on a
LABRAM HR Evolution spectrometer equipped with a CCD detector
using 532 nm radiations from a diode laser using an integration time of
mixture of BaSe, PbSe, Sn, and Se in the ratio of 1:1:1:2 were loaded
into fused-silica tubes and flame-sealed under a high vacuum of
1
5
s.
−3
0
Pa. The tubes were heated to 800 °C in 30 h, left for 100 h, cooled
2
.7. Theoretical calculation
to 200 °C at a rate of 5 °C/h, and then cooled to room temperature
within a short time. Similarly, BPSSe can be also prepared by the
stoichiometric mixture of the precursors. After repeatedly washed with
DMF solvent, BPSSe crystals with the crimson color and other
unknown phases were found in the ampules. Besides, the yield of
BPSSe was about 85% and stable in air. In order to confirm the thermal
behavior of title compounds, the polycrystalline samples of title
compounds are placed into the silica tubes and heated to 850 °C, then
slowly cooled to room temperature.
To further investigate the structure-property relationship, density
functional theory (DFT) based ab initio calculations was used to
calculate the electronic structures [52]. By Perdew–Burke–Ernzerhof
(
PBE) functional within the generalized gradient approximation (GGA)
with the scheme, exchange-correlation potential was also calculated
53]. The following orbital electrons were treated as valence electrons,
[
2
6
2
10
2
2
2
2
2
4
2
4
Ba: 5s 5p 6s , Pb: 5d 6s 6p , Sn: 5s 5p , Se: 4s 4p , S: 3s 3p . To
achieve energy convergence, a plane-wave basis set energy cutoff was
Powder X-ray diffraction (XRD) analysis of the resultant powder
samples were used at room temperature in the angular range of 2θ =
1
9
10.0 eV within normal-conserving pseudo-potential (NCP) [54,55],
and the Monkhorst-Pack scheme was 3 × 1 × 4 in the Brillouin Zone
BZ) of the primitive cell are chosen [56]. The linear optical properties
of the title compounds were obtained through the dielectric function
0–70° with a scan step width of 0.02° using an automated Bruker D2
(
X-ray diffractometer equipped with a diffracted monochromator set for
Cu Kα (λ = 1.5418 Å) radiation. A comparison between the calculated
and experimental X-ray diffraction patterns are shown in Fig. 1.
Comparing the position of peaks, we found that the experimental
powder XRD patterns of BPSS and BPSSe are basically in agreement
with the calculated results based on their single-crystal data, respec-
tively. Moreover, analysis of the powder XRD pattern of the recrys-
tallized samples reveals that they exhibit the identical patterns to those
of the initial title compounds powder, further demonstrating that title
compounds are congruently melting compounds.
formula ε(ω) = ε
1
(ω) + iε
2
(ω). Among the formula, the imaginary part
2
ε can be calculated by the following Eq. (1):
2
2
e π
2
c
v
c
v
ε2 (q → O uˆ , hω) =
∑
φ u⋅r φ
δ [E − E − E]
k k
k
k
Ωεo
kcv
(1)
where e is the elementary charge, h is Planck’s constant, u is the vector
defining the polarization of the incident, r is the position operator, Ω is
the volume of the unit cell, ε is the dielectric constant, φ (c, v) denote
0 k
1
34