Synthesis, crystal structure, optical and electronic

Journal of Alloys and Compounds 762 (2018) 143e148

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BaM2As2S6 (M ¼ Cd, Hg): Synthesis, crystal structure, optical and electronic properties Yangwu Guo a, b, Fei Liang a, b, Molin Zhou a, b, Zheshuai Lin a, Jiyong Yao a, *, Yicheng Wu a, c a

Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c Institute of Functional Crystal Materials, Tianjin University of Technology, Tianjin 300384, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 November 2017 Received in revised form 18 May 2018 Accepted 19 May 2018 Available online 21 May 2018

The first two members in the quaternary A/M/M’/Q system (A ¼ alkaline-earth metals; M ¼ group 12 elements; M’ ¼ group 15 elements; Q ¼ chalcogen), namely, the BaCd2As2S6 and BaHg2As2S6 sulfides, were synthesized via solid-state reactions. These isostructural compounds exhibit a new connectivity type in space group Cmca. In the structure, the As atoms adopt trigonal-pyramidal AsS3 coordination owing to the stereochemically active 4s2 lone pair, while M (M ¼ Cd, Hg) atoms are connected with four S atoms to form seesaw-like MS4 units. The MS4 units are linked by sharing corner to form infinite 1 ∞[MS3] chains along the a axis, which are connected by AsS3 trigonal-pyramids alternately on either side to form an interesting two-dimensional 2 ∞[M2As2S6] layers. The optical direct band gap of BaCd2As2S6 was measured to be 2.60 eV. Interestingly, electronic structure calculations demonstrate the crucial role of AsS3, especially 4s lone pairs on As3þ, in determining optical transitions of title compounds. © 2018 Elsevier B.V. All rights reserved.

Keywords: Metal chalcogenides Syntheses Crystal structure Optical properties

1. Introduction In the past two decades, exploratory research has led to the discovery of many new metal chalcogenides with intriguing ferroelectric, superconducting, photocatalytic and nonlinear optical properties [1e8]. Metal chalcogenides show rich types of microscopic building units [9] including planar MQ3 units (such as the BS3 in Ba3(BS3) (SbS3) [10]), MQ4 tetrahedral units (such as the PS4 in AgGa2PS6 [11] and the GeS4 in K2Cu2GeS4 [12]) and MQ6 octahedral units (such as the TaS6 in CsTaS3 [13] and the CdS6 in Cd2P2S6 [14]). Especially, increasing attention has been focused on chalcogenides containing the heavy group 15 elements (As, Sb, Bi) due to their amazing structural and compositional diversity [15]. Their ns2 lone-pair electrons can influence the structure type, the electronic structure, and thus the properties of resultant compounds [16]. For instance, Ba2AsGaSe5 exhibits high visible-light-induced photocatalytic reactivity which is about 6.5 times higher than does P25 [17].

* Corresponding author. E-mail address: [email protected] (J. Yao). https://doi.org/10.1016/j.jallcom.2018.05.212 0925-8388/© 2018 Elsevier B.V. All rights reserved.

Besides, the d10 cations (Zn2þ, Cd2þ, Hg2þ) -containing metal chalcogenides have also been investigated intensively since these cations tend to exhibit flexible coordination numbers with chalcogens, and hence produce an efficient approach to design interesting crystal structures, as demonstrated by ACd4Ga5S12 (A ¼ K, Rb, Cs) [19], BaZnMSe4 (M ¼ Si, Ge) [20], Na2Hg3M2S8 (M ¼ Si, Ge, and Sn) [18]. In particular, Li2ZnSiS4 achieves the good balance between large SHG response (1.1 AGS) and high LDT (10 AGS) [21], and the planar [HgSe3] 4 p-conjugated anionic groups in BaHgSe2 provides a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability [22]. Despite the large number of quaternary chalcogenides reported so far, no compounds have been reported in the A/M/M’/Q system (A ¼ alkaline-earth metals; M ¼ Zn, Cd, Hg; M’ ¼ As, Sb, Bi; Q ¼ chalcogen). However, compounds in this system should be worthy of investigation as the cooperative interplay between the d10 cations and cations with ns2 electron lone pair may lead to novel structural types and interesting physical properties. Besides, the strong ionic bonding nature of the alkaline-earth metal may help to reduce the structural dimensionality, leading to lowdimensional structures and increasing the band gap of the

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Y. Guo et al. / Journal of Alloys and Compounds 762 (2018) 143e148

material. Our systematic exploration has led to the discovery of the first two members in this system, namely BaCd2As2S6 and BaHg2As2S6. They adopt a new connectivity type in space group Cmca, possessing an intriguing 2 ∞[M2As2S6] layers built from AsS3 trigonal pyramids and MS4 see-saw shaped units. Herein, we reported the synthesis, structure, optical property and band structure of these two compounds. 2. Experimental section 2.1. Syntheses High-purity raw materials, including BaS (3N), As2S3 (3N), CdS (4N), HgS (4N) were commercially purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. Mixtures of BaS, As2S3, and MS (M ¼ Cd, Hg) in the molar ratio of 1:1:2 were ground and loaded into fused-quartz tubes under an Ar atmosphere in a glove-box, which were sealed under 103 Pa atmosphere and then placed in a computer-controlled furnace. The samples were heated to 1273 K in 20 h and kept at that temperature for 72 h, then cooled at a slow rate of 3 K/h to 573 K, and finally cooled to room temperature. The resultant yellow crystals BaCd2As2S6 and black crystals BaHg2As2S6 were manually selected for structure characterization. Elemental analysis of the crystals was performed on an EDX-equipped Hitachi S-4800 SEM instrument, and the results showed that the crystals consisted of Ba, Cd, As, S and Ba, Hg, As, S in the approximate molar ratio of 1: 2: 2: 6. Polycrystalline samples of BaCd2As2S6 were successfully synthesized by heating stoichiometric mixtures of BaS, As2S3, and CdS at 1023 K for 120 h under high vacuum. However, numerous efforts to synthesize the pure polycrystalline samples of BaHg2As2S6, by using similar method or adjusting starting materials and temperatures, were unsuccessful. 2.2. X-ray powder diffraction X-ray powder diffraction analysis in the range 2q ¼ 5e70 with a 0.02 scan step width and a 0.1 s/step counting time was performed at room temperature with the use of an automated Bruker D8 X-ray diffractometer equipped with Cu Ka (l ¼ 1.5418 Å) radiation. Fig. 1 shows XRD pattern of the polycrystalline BaCd2As2S6 sample along

with the calculated one on the basis of the single crystal crystallographic data. The experimental pattern agrees with the calculated one except for a tiny extra peak belonging to CdS impurity. The Rietveld quantitative phase analysis result shows that the powder sample containing 94.73 wt % BaCd2As2S6 and 5.27 wt % CdS (Table S1). However, the major phase is still the quaternary compound BaCd2As2S6. As for the UVeviseNIR diffuse reflectance spectroscopy measurement, it is worth mentioning that the major optical absorption position should be unaffected by the tiny impurities. Meanwhile, considering there is no As-containing impurity existing in the polycrystalline sample of BaCd2As2S6, the AsS bonds identified by the Raman spectrum is also credible. 2.3. Structure determination Single-crystal X-ray diffraction data were collected at 153 K on a Rigaku AFC10 diffractometer (Mo Ka, l ¼ 0.71073 Å). The program CrystalClear [23] was used for the data collection, cell refinement, and data reduction. Face-indexed absorption corrections were performed with the program XPREP [24]. The structure was solved with the direct methods SHELXTLS program and refined with the least-squares program SHELXL of the SHELXTL.PC suite of programs [24]. The crystal data and structural refinement for the two compounds are given in Table 1. Selected bond distances are given in Table 2. Positional coordinates and equivalent isotropic displacement parameters for the title compounds are given in Tables 3 and 4. Further information may be found in Supplementary Information. 2.4. Diffuse reflectance spectroscopy A Cary 5000 UVeviseNIR spectrophotometer with a diffuse reflectance accessory was used to measure the spectrum of BaCd2As2S6 and BaSO4 as a reference in the range from 250 nm (5.0 eV) to 2500 nm (0.5 eV). 2.5. Raman spectroscopy The unpolarized Raman scattering spectrum of BaCd2As2S6 was recorded from ground powder sample at room temperature with an inVia-Reflex instrument under the solid state laser at 532 nm. The spectral resolution was 2 cm1 and the scanning range was from 100 cm1 to 500 cm1. 2.6. Computational methods The first-principles calculations at the atomic level for Table 1 Crystal data and structure refinement of BaCd2As2S6 and BaHg2As2S6. BaCd2As2S6

Fig. 1. Powder X-ray diffraction patterns of BaCd2As2S6; the peaks marked with * are from small amount of CdS impurity.

BaHg2As2S6

fw 704.3433 880.72 T (K) 153.15 153.15 a (Å) 7.9544 (16) 8.1304 (16) b (Å) 7.8649 (16) 7.9417 (16) c (Å) 16.979 (3) 16.328 (3) space group Cmca (No. 64) Cmca (No. 64) 3 V (Å ) 1062.2 (4) 1054.3 (4) Z 4 4 rc (g cm3) 4.404 5.549 m (mm1) 14.93 40.10 R(F)a 0.030 0.057 Rw(F2o )b 0.082 0.175 P P P P AR (F) ¼ jjFojjFcjj/ jFoj for F2o > 2s (F2o). bRw (F2o) ¼ { [w(F2o F2c )2]/ wF4o}½ for all data. w1 ¼ s2(F2o) þ (zP)2, where P ¼ (Max(F2o, 0)þ 2 F2c )/3.


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Table 2 Selected Bond Lengths (Å) and Bond Angles for of BaCd2As2S6 and BaHg2As2S6. BaCd2As2S6

BaHg2As2S6

BaS1 4 BaS1 4 CdS1 2 CdS2 2 AsS1 2 AsS2 S1CdS1 S2CdS2 S1CdS2 2 S1CdS2 2 S1AsS2 2 S1AsS1

BaS1 4 BaS1 4 CdS1 2 CdS2 2 AsS1 2 AsS2 S1HgS1 S2HgS2 S1HgS2 2 S1HgS2 2 S1AsS2 2 S1AsS1

3.2484 (12) 3.3735 (12) 2.5720 (12) 2.6619 (13) 2.2479 (15) 2.272 (2) 163.25 (6) 97.50 (6) 95.91 (5) 95.12 (5) 97.44 (5) 100.22 (7)

3.262 (4) 3.395 (4) 2.505 (4) 2.608 (5) 2.249 (4) 2.229 (7) 155.9 (2) 95.36 (18) 99.59 (18) 95.36 (18) 98.00 (17) 99.8 (2)

Table 3 Atomic Coordinates and Equivalent Isotropic Displacement Parameters in Å2 of BaCd2As2S6. Atom

Wyckoff

SOF

x

y

z

Ueq

Ba Cd As S1 S2

4a 8e 8f 16g 8f

1 1 1 1 1

0.5000 0.7500 0.5000 0.71683 (15) 1.0000

0.0000 0.24969 (5) 0.48045 (9) 0.29732 (17) 0.0265 (2)

0.5000 0.7500 0.60461 (5) 0.60095 (6) 0.73670 (12)

0.0104 0.0131 0.0088 0.0108 0.0095

(3) (3) (3) (4) (5)

Table 4 Atomic Coordinates and Equivalent Isotropic Displacement Parameters in Å2 of BaHg2As2S6. Atom

Wyckoff

SOF

x

y

z

Ueq

Ba Hg As S1 S2

4a 8e 8f 16g 8f

1 1 1 1 1

0.5000 0.7500 0.5000 0.7117 (5) 1.0000

0.0000 0.23849 (15) 0.4863 (3) 0.3042 (6) 0.0337 (10)

0.5000 0.7500 0.6055 (16) 0.6012 (3) 0.7401 (4)

0.0058 0.0187 0.0054 0.0079 0.0137

(7) (6) (7) (10) (15)

BaCd2As2S6 crystal are performed by the CASTEP package [25] based on density functional theory (DFT) [26], which has been successfully applied on many metal sulfides [27]. The exchangeecorrelation (XC) functionals are described by the generalized gradient approximation (GGA) PBE functional [28]. The ion-electron interactions are modeled by the norm-conserving pseudopotentials [29] for all elements. In this model, Ba 4d105s25p66s2, Cd 4d105s2, As 4s24p3 and S 3s23p4 electrons are treated as the valence electrons, respectively. The kinetic energy cutoff of 660 eV and Monkhorst-Pack k-point meshes [30] (4 4 1) spanning less than 0.04 Å3 in the Brillouin zone are chosen to ensure the sufficient accuracy of the calculated results. 3. Results and discussion

Fig. 2. (a) The coordination environment of As. (b) The coordination environment of Cd. (c) The coordination environment of Ba. (the unit of the bond lengths is Å).

3.1. Crystal structure These two compounds represent a new connectivity type in centrosymmetric space group Cmca of the orthorhombic system. As shown in Tables 3 and 4, there is one crystallographically independent Ba atom, one unique Cd (or Hg) atom, one unique As atom and two unique S atoms in the asymmetric unit. Without SS or metalmetal bonds in the structure, the oxidation states of 2þ, 2þ, 3þ, and 2 can be assigned to Ba, Cd (or Hg), As, and S, respectively. The two title compounds are isostructural, so we only present the structure of BaCd2As2S6 in Figs. 2 and 3. As a result of the stereochemical activity of the 4s2 electron lone pair, the As3þ cations are coordinated to a trigonal pyramid of three S atoms (Fig. 2a). The AsS distances within the AsS3 trigonal pyramid change from

2.2479(15) to 2.272(2) Å and the SAsS bond angles lie in range of 97.44(5) to 100.22(7) . Such values are close to those in NaAsS2 [31], which has AsS distances of 2.167e2.331 Å and SAsS bond angles of 95.45e103.61. As shown in Fig. 2b, the Cd atoms exhibit typical “see-saw” type coordination (distorted tetrahedron) with the CdS bonds ranging from 2.5720(12) to 2.6619(13) Å, similar to the reported values in Ba3CdSn2S8 [32] (CdS ¼ 2.564 Å). The Ba atoms are located in eight coordination environments forming the BaS8 bi-capped trigonal prism with BaS distances of 3.2484(12) 3.3735(12) Å (Fig. 2c), which are comparable to those of other reported compounds, such as Ba2GeSb4S10 [33] (3.251e3.412 Å), and Ba2AgInS4 [34] (3.128e3.314 Å).

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determines the type of optical transitions, n value is 1/2 and 2 for indirect and direct transitions, respectively. The absorption (F(R)) data are calculated from the following KubelkaMunk function (2) [39,40]:

FðRÞ ¼

K ð1 RÞ2 ¼ S 2R

(2)

In the above equation, R, K, and S represent the reflectance, absorption, and scattering, respectively. According to the Tauc method [41], (F(R) hn)2 and (F(R) hn)1/2 vs hn were plotted to determine the band gap being direct or indirect. As shown in Fig. 4, the band gap obtained from the plots of (F(R) hn)2 vs hn are much more consistent with the yellow color of the compound BaCd2As2S6. Therefore, the compound possesses a direct band gap of 2.60 eV with the corresponding absorption edge of about 477 nm. The Raman spectrum based on the polycrystalline sample of BaCd2As2S6 was shown in Fig. 5. Several obvious resonances at 367, 340, 298, 263, 208 and 191 cm1 can be seen from the spectrum.

Fig. 3. (a) View of one 2 ∞[Cd2As2S6] layer along c direction. The CdS4 tetrahedra are displayed as polyhedra for clarity. (b) Crystal structure of BaCd2As2S6 viewed along a direction.

As shown in Fig. 3, there exists an interesting linking mode between the CdS4 distorted tetrahedra and AsS3 trigonal pyramids to form the 2D layer: the CdS4 tetrahedra are linked with each other by sharing corner to form infinite 1 ∞[CdS3] chains along the a axis, then infinite 1 ∞[CdS3] chains are bridged via AsS3 trigonal pyramids alternately on either side to form 2 ∞[Cd2As2S6] layers, which are parallel to each other and stacked along the c direction at every c/2. The Ba cations are located between the layers to achieve the charge-balance. The AsS3 trigonal pyramids are the main functional groups in a number of nonlinear optical materials such as LiAsS2 [35] and Ag3AsS3 [36]. Even though the title compounds adopt a centrosymmetric structure and the contributions of [AsS3]3 anions to NLO properties are offset due to the symmetry center, there still a great chance to find the [AsS3]3 anions in other noncentrosymmetric structures with promising NLO properties. 3.2. Optical properties Based on the optical diffuse reflectance method, the optical band gap of BaCd2As2S6 can be determined by the following relation (1) [37,38]:

ðahnÞn ¼ A hn Eg

(1)

where a is the absorption coefficient and is proportional to F(R), h is Planck's constant, n is the light frequency, A is the absorption constant and Eg is the optical band gap, n is a constant exponent which

Fig. 4. Optical reflection spectrum of BaCd2As2S6: (a) the plot of (F(R) hn) 2 versus hn; (b) the plot of (F(R) hn) 1/2 versus hn. Obviously, the value (2.60 eV) obtained from Fig. 4a is much more consistent with the color of BaCd2As2S6 (yellow). Consequently, the compound possessed a direct band gap of 2.60 eV. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)


Fig. 5. Raman spectrum of BaCd2As2S6. The Raman shift at 367, 340, 298 and 263 cm1 can be attributed to the symmetric stretching of pyramidal [AsS3]3 groups and the absorption peak at 208 and 191 cm1 can be assigned to the CdS vibrations.

The Raman shift at 367, 340, 298 and 263 cm1 can be attributed to the symmetric stretching of pyramidal [AsS3]3 groups [42]. Compared to the values of KEuAsS3, the bands are shifted to slightly lower frequencies due to the weaker AsS interactions following from the longer AsS bonds [43]. In addition, the absorption peak at 208 and 191 cm1 can be assigned to the CdS vibrations [44]. 3.3. Band structure calculation The electronic band structure of BaCd2As2S6 calculated by PBE functional is shown in Fig. 6a. It has a direct bandgap of 1.77 eV with valance band maximum (VBM) and conduction band minimum (CBM) locating at G point. The calculated band gap is less than the

147

experimental value (2.60 eV) owing to the discontinuity of exchange-correlation energy. The corrected band gap by hybrid HSE06 functional (Fig. 7) is well consistent with measured band gap (2.53 eV v.s. 2.60 eV). Fig. 6b displays the density of states (DOS) and partial (PDOS) of the respective species in BaCd2As2S6. Clearly, the deep part of VB lower than 10 eV is mainly composed of Ba 5p, As 4s and S 3p orbitals. The upper part of the VB (10 to 5 eV) consists of S 3s, As 4s and Cd 4d orbitals. Notably, the Cd 4d orbitals are strongly located around 8 eV. The VB maximum is exclusively occupied by S 3p and As 4s orbitals, which is similar to that of LiAsS2 [45]. The bottom of the CB is mainly contributed from Ba 4d, As 4p and S 3p orbitals while the contribution of Cd atoms are nearly ignorable. Since the optical effects of a crystal are mainly determined by the optical transition between the electronic states close to the bandgap, accordingly, it is anticipated that they are dominantly contributed from the [AsS3]3- groups, while the contribution from the orbitals of the Cd2þ and Ba2þcations is negligibly small. In addition, the visualized HOMO and LUMO charge density map are shown in Fig. 6c and d. It is clearly that the HOMO are composed of umbrella-shaped As 4s lone pairs and spindle-shaped S 3p electrons while the LUMO consists of large amount of As 3p electrons and small amount of localized S 3p electrons. These also demonstrate that the crucial role of [AsS3]3-, especially 4s lone pairs on As3þ, in determining optical transitions of title compounds.

4. Conclusions In conclusion, two new quaternary As-based sulfides BaM2As2S6 (M ¼ Cd, Hg) have been synthesized and characterized. They crystallize in the orthorhombic space group Cmca and feature with infinite 2 ∞[M2As2S6] layers with charge balancing Ba2þ cations located inside the inter-spaces. The UVeviseNIR spectroscopy measurement indicates that BaCd2As2S6 has a direct optical band gap of 2.60 eV and theoretical calculations demonstrate [AsS3]3units play an important role in determining the optical properties of title compounds. The overall research results indicate that As-

Fig. 6. (a) Band structure of BaCd2As2S6; (b) Total DOS and partial DOS of BaCd2As2S6; The corresponding charge density distributions of (c) HOMO and (d) LUMO of BaCd2As2S6.

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[14] [15]

[16]

[17]

[18]

[19]

[20]

[21] Fig. 7. The electric band structure of BaCd2As2S6 calculated by hybrid HSE06 functionals.

based metal chalcogenide is a promising system to explore new functional materials in IR region. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51472251 and No. 91622123).

[22]

[23] [24] [25] [26] [27]

[28]

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.05.212.

[29] [30] [31]

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