YUAN Fng-Yu HUANG Yi-Zhi ZHANG Ho ZHOU An-Yi CHENG Wen-Dn LIN Chen-Sheng②

a (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

b (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

ABSTRACT A new quaternary selenide Ba4Sn3GeSe9 was synthesized by high temperature solid state reaction method and fully characterized by elemental analysis, UV-vis spectrum, and single-crystal X-ray diffraction. The title compound crystallizes in the orthorhombic space group Pnma with a = 12.463(3), b = 9.308(2) and c =17.892(5) Å. Ba4Sn3GeSe9 can be characterized by a zero-dimensional compound composed by special [GeSnSe5]4-units, [Sn2Se4]4- units and the adjacent cations Ba2+ ions. The [GeSnSe5]4- unit is composed of a SnSe3 trigonal pyramid formed by divalent Sn2+ and edge-sharing with a GeSe4 tetrahedron, and the [Sn2Se4]4- unit is composed of two SnSe3 trigonal pyramids. Ba4Sn3GeSe9 is an indirect semiconductor with a band gap of 1.21 eV.

Keywords: quaternary selenide, zero-dimensional structure, [GeSnSe5]4- unit, indirect band gap;

1 INTRODUCTION

The chalcogenides have a wide range of structure types,among which Sn/Ge-containing compounds are more prominent. Among chalcogenides, the common chemical states of Sn are positive tetravalent Sn(IV) and positive divalent Sn(II), but there are also compounds containing zero valence states, such as SnP2S6[1]. Many compounds of this system have important physical properties because of their rich structure types, such as nonlinear optics[2], ferroelectrics[3], and thermoelectricity[4]. The atom Sn and chalcogen atomic linkages can form abundant structural units, such as[Sn(IV)Q6]8-[5], [Sn(IV)Q4]4-[6], [Sn(II)Q3]4-[7], [Sn(II)Q6]10-[8],etc. These basic structural units are interconnected with each other or with other different types of structural units,resulting in a rich variety of structure types from zero- to three-dimension. The richness of the structural units forms the basis of a wide variety of structures, and different structures allow compounds to exhibit different physical properties, so it is interesting to explore new structural units.

In various structural types, compounds composed of new units built by two AQ4tetrahedra or BQ3pyramids (A = Zn,Cd, Ga, In, Ge, Sn; B = Ge, Sn, Sb, Bi; Q = S, Se) are rarely seen. For example, there is a unit [Ge2Se5]4-formed by edge-sharing GeSe4tetrahedron and GeSe3trigonal pyramid in Ba2Ge2Se5[9]and [SbGaSe5]4-unit formed by edge-sharing GaSe4tetrahedron and SbSe3trigonal pyramid in Ba2GaSbSe5[10]. These new units consisting of two units enrich the building units of the structure, which provides new ideas for structural design. In this work, we introduced the element Sn, which can form multiple units with chalcogen atomic, and the element Ge, which also exhibits two valence states in the compound, and synthesized them with elements Ba and Se by high temperature solid-state reaction method,resulting in a new compound Ba4Sn3GeSe9composed of[Sn2Se4]4-units and particular [GeSnSe5]4-units. The[GeSnSe5]4-unit is composed of edge-sharing SnSe3trigonal pyramid and GeSe4tetrahedron.

2 EXPERIMENTAL

2. 1 Reagents

The reagents were used as obtained: Ba rod (99%, Aladdin Chemistry Co. Ltd.), GeSe (99.999%, CNBM (Chengdu)Optoelectronic Materials Co. Ltd.), stannum powder (99.99%,Aladdin Chemistry Co. Ltd.), selenium powder (99.99%,Aladdin Chemistry Co. Ltd.). The binary compound BaSe was synthesized at 1123 K by elemental mixture in sealed silica tubes under vacuum of 10-2Pa.

2. 2 Syntheses

The crystal of Ba4Sn3GeSe9was synthesized by a solid-state reaction technique. A mixture of BaSe, Sn, GeSe and Se in the molar ratio of 4:3:1:4 was ground and loaded into a graphite crucible sealed in the evacuated silica tube under 10-2Pa atmosphere. The reactant was heated to 1173 K within 25 h (held for 50 h) under the operation by a computer controlled furnace, then the sample was slowly cooled to 1073 K in 50 h and finally to room temperature in 25 h,obtaining black crystals, but there are always small amounts of yellow crystals Ba2GeSe4present, and what we have shown is already the best solution for Ba2GeSe4crystal removal. Since crystal Ba2GeSe4is always associated with Ba4Sn3GeSe9, the pure phase of Ba4Sn3GeSe9is difficult to obtain. The following approach is the optimal way to obtain the pure phase of Ba4Sn3GeSe9. There is still Ba2GeSe4, but it is the best solution we have come up with after many experiments. The pure phase of Ba4Sn3GeSe9was produced by a stoichiometry mixture of BaSe, Sn, GeSe and Se in the molar ratio of 4:3:1:4. The mixture was heated to 1073 K in 15 h and kept for 50 h and then the furnace was turned off.

2. 3 Powder X-ray diffraction

The powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku MiniFlex II benchtop X-ray diffractometer. The 2θ scanning range was 10～70° in a step size of 0.02°. The experimental and simulated PXRD patterns of Ba4Sn3GeSe9are shown in Fig.1. The three peaks marked with * are due to very small amount of Ba2GeSe4, which crystallizes in space group Pnma.

Fig.1. Experimental and simulated powder X-ray diffraction patterns of Ba4Sn3GeSe9

2. 4 Single-crystal structure determination

Suitable single crystal of Ba4Sn3GeSe9was stuck to a glass fiber for single-crystal XRD analysis. The measurements were performed on a Mercury70 CCD detector equipped with graphite-monochromatic MoKα radiation (λ = 0.71073 Å) at 293 K. Absorption corrections based on the multi-scan method were executed. The crystal structure was resolved by direct methods using SHELXS-2018[11]and refined by full-matrix least-squares on F2using SHELXL-2018[12]in the Olex2 software[13]. The final refined structure was checked by PLATON[14], and no other missed or higher-symmetry element was found. The crystallographic data and structural refinement information are given in Table S1. The atomic coordinates and selected bond distances are listed in Tables S2 and S3 in the Supporting Information.

2. 5 Energy-dispersive X-ray spectroscopy (EDS)

One or several crystals were selected and washed twice with alcohol and dried to confirm that there were no other impurities on the surface of the crystals. A scanning electron microscope (FESEM, JSM6700F) equipped with energy dispersive X-ray spectroscopy (EDS, Oxford INCA) was used to carry out the semi-quantitative element analyses of single crystals.

2. 6 UV-Vis-NIR diffuse reflectance

The optical diffuse reflectance spectra of powdered sample Ba4Sn3GeSe9were measured using a PerkinElmer Lambda 950 UV-Vis-NIR spectrophotometer in the range of 200～2500 nm and BaSO4was used as a reference. Using Kubelka-Munk equation: α/S = (1 - R)2/2R (α is the absorption coefficient, S is the scattering coefficient, and R is the reflectance.)[15]to transform the reflection spectra to the absorption spectra.

2. 7 First-principles calculations

Electronic band structure and density of states (DOS) of the experimental structure were calculated using the density functional theory (DFT) method implemented in the CASTEP code[16,17]. Generalized gradient approximation(GGA) functional was employed. The cutoff energy for plane wave basis set was set at 400 eV, and the Monkhorst-Pack k-point grid size for Brillouin zone was 1 × 2 × 1.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

The title compound crystallizes in the orthorhombic space group Pnma with a = 12.463(3), b = 9.308(2) and c =17.892(5) Å. There are two Ba, three Sn, one Ge and seven crystallographically unique Se atoms in the crystal asymmetric structural unit of Ba4Sn3GeSe9. Ba4Sn3GeSe9displays a zero-dimensional structure. As shown in Fig.2a,the [Sn2Se4]4-and [GeSnSe5]4-units are distributed in the structure in a centrally symmetrical manner, and the Ba2+cations are located between [Sn2Se4]4-and [GeSnSe5]4-to balance the charge. The [GeSnSe5]4-unit is formed from edge-sharing connection of GeSe4tetrahedron with SnSe3trigonal pyramid (Fig.2b). The [Sn2Se4]4-unit is formed from edge-sharing connection of two SnSe3trigonal pyramids, as shown in Fig.2c. There is one coordination mode for Ba atoms, i.e., coordination with eight Se atoms, with the Ba-Se bond lengths ranging from 3.379(3) to 3.564(3) Å, which are close to those of BaGa4Se7[18]. There is one coordination mode for Sn atoms, i.e., coordination with three Se atoms to form the SnSe3pyramid, with the Sn-Se bond lengths falling in the 2.611(4)～2.858(3) Å region, which are close to those of Ba10Ga2Sn9Se22[19]. The Ge atom has one coordination mode, which forms a GeSe4tetrahedron with four Se atoms,and the Ge-Se bond lengths changing from 2.323(4) to 2.357(2) Å are close to those of BaGa2MQ6[20].

Fig.2. (a) Structure of the zero-dimensional compound Ba4Sn3GeSe9 viewed along the b-axis(neglecting the Ba-Se bond), (b) [GeSnSe5]4- unit formed by edge-sharing GeSe4 tetrahedron and SnSe3 trigonal pyramid, and (c) [Sn2Se4]4- unit formed by edge-sharing SnSe3 trigonal pyramids

3. 2 Optical properties

The UV-Vis-NIR diffuse-reflectance spectra in Fig.3 indicate that the band gap is 1.21 eV for Ba4Sn3GeSe9, which matches with the crystal color of black.

Fig.3. UV-Vis diffuse reflectance spectra of Ba4Sn3GeSe9

3. 3 Elemental analysis

The elemental analysis results confirm that the crystal contains four elements, and the molar ratio of each element displayed is close to the atomic ratio of Ba:Sn:Ge:Se =4:3:1:9 obtained by structural analysis. No other impurity elements were found, which also verified the single-crystal structure analysis results. The elemental analysis results are shown in Table S4 and Fig.S1 in the Supporting Information.

3. 4 Theoretical analyses

The calculated electronic band structure and DOS for Ba4Sn3GeSe9are shown in Fig.4. Near the Fermi level (EF),the highest valence band (VB) exhibits peaks and valleys,and the maximum located at Γ k-point. On the other hand,the lowest conduction band (CB) locates at the U k-points(Fig.4a). Therefore, the compound Ba4Sn3GeSe9is an indirect band gap semiconductor with a calculated band gap of 0.9 eV, which is smaller than the corresponding experimental value of 1.21 eV, estimated from the UV-Vis diffuse reflectance spectra. It is well-known that the band gap calculated by DFT is usually smaller than the experimental data because of the discontinuity of exchange correlation energy[21].

The density of states of compound Ba4Sn3GeSe9is shown in Fig.4b. Near the Fermi level energy, it is mainly composed of Sn(II) 5p, Sn(II) 5s and Se 4p states. The lowest unoccupied state from 0.9 to 2.5 eV is mainly composed of unoccupied Sn(II) 5p and a small amount of unoccupied Se 4p and Se 4s states. Therefore, linear spectral absorption is caused by the transition of electrons from the occupied Sn(II)5p, Sn(II) 5s and Se 4p states to the unoccupied Sn(II) 5p, Se 4p and Se 4s states.

Fig.4. (a) Band structure and (b) Density of states for Ba4Sn3GeSe9

4 CONCLUSION

In this work, we have synthesized a new compound Ba4Sn3GeSe9, which has a very interesting structure with[Sn2Se4]4-units and special [GeSnSe5]4-units. The[GeSnSe5]4-unit is formed from edge-sharing connection of GeSe4tetrahedron with SnSe3trigonal pyramid, the[Sn2Se4]4-unit is formed from edge-sharing connection of two SnSe3trigonal pyramids, and the Ba2+cations are used to balance the charge and form the zero-dimensional structure of the compound Ba4Sn3GeSe9. The compound is an indirect band gap semiconductor with band gap of 1.21 eV, and the presence of Sn2+ions in the structure is the main point of the band gap of the compound Ba4Sn3GeSe9. These new units,consisting of two units, enrich the building units of the structure and provide new ideas for crystal structure design.