(Cs6Cl)6Cs3[Ga53Se96]: A Unique Long Period-Stacking Structure of

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(Cs6Cl)6Cs3[Ga53Se96]: A Unique Long Period-Stacking Structure of Layers Made from Ga2Se6 Dimers via Cis or Trans Intralayer Linking Hua Lin,† Hong Chen,† Zi-Xiong Lin,† Hua-Jun Zhao,‡ Peng-Fei Liu,† Ju-Song Yu,† and Ling Chen*,† †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, Guizhou 563002, China S Supporting Information *

gave a stoichiometry of Cs39Ga53.3(6)Se96.3(3)Cl6.2(2), in good agreement with the single-crystal refinement (Figure S2). In this synthesis, adding of Mn seems necessary. Without Mn, the reaction only produces Ga2Se3 as a major phase, and (Cs6Cl)6Cs3[Ga53Se96] cannot be detected even as a minor phase. The role of Mn is not fully understood yet. One may speculate that Mn acts as a catalyst.16 Compound (Cs6Cl)6Cs3[Ga53Se96] with its own structure type crystallizes in the trigonal space group R3̅m (No. 166) with a = b = 11.990(5) Å, c = 50.012(4) Å, V = 6226.5(6) Å3, and Z = 1. The remarkably large c axis, about 4 times larger than the other two parameters, allows a unique long period stacking of the 2D [Ga53Se96]33− anionic layers (Figure 1a). There are three such layers in the unit cell. Each layer consists of two A slabs that are interconnected by a slab B (Figure 1a). The intralayer connection is very interesting. As shown in Figure 1b, within slab A, Ga2Se6 dimers (Ga1 and Ga2) are cis-linked into a so-called six-membered-ring (6-MR) motif. Slab B is also made of Ga2Se6 dimers (Ga3), which are trans-intralayer-linked instead. In spite of the different linking details, 6-MRs in both slabs are all about 12 × 12 Å in size, with each edge crossing a Ga2Se6 dimer. Further, the 6-MR, in slab B, catenates with six other neighboring rings via vertexes (i.e., the Se4 atom serving as a node). On the contrary, the 6-MR in slab A has no atom standing on the apex. A similar but different example is CsSnGaSe4,17 in which the M2Se6 dimer also serves as a building unit that constructs the layered structure via a sharing vertex with the help of a second building unit, a MSe4 tetrahedron (M = disordered Sn/Ga). Each Ga atom is 4-fold-coordinated in a distorted tetrahedral sphere with the Ga−Se distance varying from 2.367(2) to 2.448(2) Å (Figure 1b). These distances are consistent with those of 2.349−2.528 Å in Cs8Ga4Se10,18 2.363−2.451 Å in BaGa4Se7,10a and 2.386−2.411 Å in Ba3CsGa5Se10Cl2.14 The covalent matrix of [Ga53Se96]33− layers accommodates both a ClCs6 octahedron and an isolated Cs+ cation, as indicated in Figures 1a and S3a. There are three crystallographically independent Cs atomic sites, as shown in Figure S3. The 8-foldcoordinated Cs1 and Cs2 have Cs−Se distances of 3.605(2)− 4.012(2) and 3.685(2)−4.180(2) Å, respectively (Figure S3b). The Cs3 atom is surrounded by 12 Se atoms in a cuboctahedron, with Cs−Se distances ranging from 4.068(2) to 4.121(2) Å. These Cs−Se distances are similar to those of 3.578−4.086 Å in

ABSTRACT: The new compound (Cs6Cl)6Cs3[Ga53Se96] with its own structure type has been discovered by high-temperature solid-state reactions. The compound features a unique long period-stacking structure of layers that are built by the commonly observed dimeric Ga2Se6 unit extending in cis or trans intralayer linking. Single-crystal X-ray diffraction analyses show the trigonal space group R3̅m (No. 166) and a = 11.990(5) Å, c = 50.012(4) Å, and V = 6226.5(6) Å3. The UV−vis− near-IR spectrum reveals a wide band gap of 2.74 eV that agrees well with the electronic structure calculation.

S

emiconductor chalcogenides containing group 13 elements have received worldwide attention for their promising optoelectronic applications; examples are Sm 4 GaSbS 9 , 1 La4InSbS9,2 CuGaTe2,3 In4Se2.35,4 etc. From the point of view of crystallography, most of them adopt a MX4 tetrahedron as the building unit. With various connections, e.g., via sharing of common corners or edges, such units construct diverse structures of zero-dimensional (0D) discrete molecules,5 one-dimensional (1D) chains,1,2 two-dimensional (2D) layers,6 and threedimensional (3D) networks.7 The Ga-containing chalcogenides are of special interest. For example, some of them, e.g., LiGaQ2,8 AgGaQ2,9 BaGa4Q7 (Q = S, Se),10 and AgGaTe2,11 exhibit prospective nonlinear-optical properties. Also, some show rich structural chemistry and fascinating physical properties. For instance, (Cs6Cl)2Cs5[Ga15Ge9Se48], an unusual supercubooctahedron, exhibits both cation- and anion-exchange properties.12 (K3I)[SmB12(GaS4)3], a chiral honeycomb-like open framework, exhibits moderate second harmonic generation (SHG).13 Ba3AGa5Se10Cl2 (A = K−Cs) exhibits the coexistence of SHG and photoluminescence.14 Ba4F4CrGa2S6 shows an antiferromagnetic ordering at low temperature.15 However, no quaternary A/Ga/Q/X (A = alkali metal; Q = chalcogenide; X = halogen) compound is reported yet. Herein, we report (Cs6Cl)6Cs3[Ga53Se96], a unique long period-stacking structure of the anionic [Ga53Se96]33− layers. Typically, compound (Cs6Cl)6Cs3[Ga53Se96] was prepared from a mixture of 0.5 mmol of Ga, and 1.2 mmol of Se together with 0.6 mmol of a CsCl flux and 0.4 mmol of Mn at 1273 K (Supporting Information, SI). A pure phase could easily be obtained by manually picking crystals (light-yellow crystals; Figure 2, inset), and the corresponding powder X-ray diffraction patterns are shown in Figure S1. Energy-dispersive X-ray analyses © 2016 American Chemical Society

Received: December 8, 2015 Published: January 21, 2016 1014

DOI: 10.1021/acs.inorgchem.5b02846 Inorg. Chem. 2016, 55, 1014−1016

Communication

Inorganic Chemistry

Figure 1. (a) View of (Cs6Cl)6Cs3[Ga53Se96] projected slightly off the b direction with the unit cell marked. (b) Intralayer linking details of the Ga2Se6 dimer via cis linking in slab A (top) or trans linking in slab B (bottom). The asymmetric Ga2Se6 dimer is marked with atom numbers. Terminal S atoms and interlayer connections are omitted for clarity. A 6-MR is outlined by black lines to guide the eyes.

To understand the distribution of the states near the Fermi level, EF, the density of states (DOS) of (Cs6Cl)6Cs3[Ga53Se96] was calculated (Figure 3). The valence band (VB) near EF is

Figure 2. UV−vis diffuse-reflectance spectrum of (Cs6Cl)6Cs3[Ga53Se96]. Inset: Photograph of the as-prepared crystals.

CsBSe3,19 3.634−4.133 Å in CsLu7Se11,20 and 3.996−4.004 Å in CsTaSe3.21 The Cs3 atoms are more loosely bonded to Se atoms, as indicated by longer Cs−Se distances and larger atomic displacement parameters (Table S2). Such large values of thermal displacement are similar to those found in some known 3D frameworks with closed cavities.7a,b,20,22 In (Cs6Cl)6Cs3[Ga53Se96], the ClCs6 octahedron also serves as the center species of the closed cavity of (ClCs6)@Se31, as shown in Figure S3c. A similar but different situation [a closed cavity of (ClCs6)@Se32] was found in (ClCs6)[RE21Q34].20 These Cs−Cl distances of 3.412(2)−3.427(2) Å are consistent with those observed in CsAuCl3 (3.408−3.411 Å)23 and Cs4PbCl6 (3.414− 3.693 Å).24 According to diffuse reflectance at room temperature, the band gap of (Cs6Cl)6Cs3[Ga53Se96] is estimated to be 2.74 eV (Figure 2), which is in agreement with its light-yellow color and suggests semiconductor behavior. This value is smaller than those of Ba3AGa5Se10Cl2 (A = K−Cs; 3.22−3.25 eV),14 Ba4MGa4Se10Cl2 (M= Zn, Cd; 2.93−3.08 eV),25 and (Cs6Cl)2Cs5[Ga15Ge9Se48] (2.91 eV),12 but wider than those of Ba3GaSe4Cl (2.05 eV)26 and binary Ga2Se3 (1.88 eV).27 It is clear that the introduction of AX alkali-metal halides into MQn chalcogenides can effectively increase the band gap. This agrees with the concept of “dimensional reduction” found in other chalcogenides.28 Moreover, (Cs6Cl)6Cs3[Ga53Se96] has excellent thermal stability and shows no obvious weight loss up to 1101 K (Figure S4).

Figure 3. (a) Calculated band structure and (b) total and partial DOSs of (Cs6Cl)6Cs3[Ga53Se96]. The Fermi level EF is set at 0.0 eV.

mostly made up of Se 4p, whereas the conduction band (CB) has most of its contributions coming from Se 4p, Ga 4s, and Ga 4p. Note that the partial DOS of Cs 6s states almost locates above EF, which proves that the Cs atom acts primarily as an electron donor. Similarly, the Cl atom acts as an electron acceptor because its 3p states locate below EF. Consequently, the Cl atom contributes to stabilize the structure. Also, the covalent-bonding interactions between the Ga and Se atoms are pretty strong according to the DOS. The VB maximum and CB minimum locate at the same k point, indicating direct-band-gap semiconductor characteristics. Also, the calculated band gap of 2.25 eV is slightly smaller than the experimental value of 2.74 eV (Figure 2) because of the well-known underestimation of the band gap in the density functional theory framework. In summary, a new compound, (Cs6Cl)6Cs3[Ga53Se96], with its own structure type has been synthesized by a solid-state method. It illustrates a very large c axis that allows the unique stacking of the 2D [Ga53Se96]33− layer that is constructed by the commonly observed [Ga2Se6]6− dimers via cis or trans intralayer linking. Theoretical studies show that it is a direct-band-gap semiconductor, confirming the solid-state optical absorption 1015

DOI: 10.1021/acs.inorgchem.5b02846 Inorg. Chem. 2016, 55, 1014−1016

Communication

Inorganic Chemistry

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measurement. Further investment in the physical properties is worth trying.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02846. CIF data (CIF) Experimental and theoretical methods and additional tables and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (011)86-591-63173131. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Projects 21225104, 21233009, 21301175, 21571020, and 91422303) and the Natural Science Foundation of Fujian Province (Project 2015J01071).

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DOI: 10.1021/acs.inorgchem.5b02846 Inorg. Chem. 2016, 55, 1014−1016