Synthesis, Crystal Structure, and Thermoelectric Properties

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Quaternary Layered Semiconductor Ba2Cr4GeSe10: Synthesis, Crystal Structure, and Thermoelectric Properties Hong Chen,† Yu-Kun Chen,† Hua Lin,*,† Jin-Ni Shen,‡ Li-Ming Wu,*,†,§ and Xin-Tao Wu† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ College of Materials Science and Engineering, Fuzhou University, Fujian 350108, People’s Republic of China § Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China S Supporting Information *

formula unit.21 Here, we try to extend the above-mentioned regulation of the ternary system to the quaternary system and design a 2Ba:4Cr:M:10Se compound by replacing the Cr4+ ion in Ba2Cr5S10 with one M4+ ion with the expectation that the layered moiety in the parent structure is well kept while some new property will be introduced by the addition of the new M4+ component. More importantly, from sulfides to selenides, the band gap (Eg) will decrease; thus, the proposed quaternary chromium selenides may show the necessary small band gaps that are desired for good TE materials because small Eg in a material represents a good balance between the semiconductors with high Seebeck and the metals with low resistivity. In this Communication, we discover a new layered Ba2Cr4GeSe10 compound, of which the structure resembles that of many good TE materials showing a clear separation between the region of electronic conduction and the region of electronic insulation. The as-synthesized undoped hot-pressed polycrystalline Ba2Cr4GeSe10 shows a desirable low thermal conduction (κT) and a high Seebeck coefficient (S), while its electrical conduction (σ) is about 2 orders of magnitude lower than the known efficient TE materials. Furthermore, the optical Eg, thermal stability, and electronic structures are studied. The structure of Ba2Cr4GeSe10 represents a new structure type. Single-crystal X-ray diffraction (XRD) data reveal the triclinic P1̅ structure with unit parameters of a = 7.130(4) Å, b = 9.581(5) Å, c = 12.778(7) Å, α = 71.477(7)°, β = 81.940(9)°, γ = 89.962(1)°, V = 818.7(8) Å3, and Z = 2. A view of the structure of Ba2Cr4GeSe10 approximately along the a axis is shown in Figure 1a, and the major structure motif is the 2D anionic layers constructed of CrSe6 octahedra, between which locate the isolated Ba2+ cations and discrete GeSe4 tetrahedra in an alternately arranged manner. The interconnection between the Cr−Se layers of CrSe6 octahedra is shown in Figure 1b using a ball-and-stick representation. The CrSe6 octahedra share their edges and faces to form a layer that parallels the ab plane, and then the isolated GeSe4 tetrahedra are connected to the layer from two opposite sides by sharing Se apexes. Such undulated layers differ from, yet are related to, the flat layers in Ba2Cr5S1020 (Figure S1); also the two types of interlayer species are totally different: discrete GeSe4 tetrahedra versus discrete CrS 6

ABSTRACT: A quaternary narrow-band-gap semiconductor, Ba2Cr4GeSe10, has been discovered by solid-state reaction. It features a new structure type and crystallizes in the triclinic space group P1̅ (No. 2). The featured 2D anionic layers are constructed by condensed CrSe6 octahedra that are stacking along the c axis, with dispersed GeSe4 tetrahedra and located Ba2+ cations forming these layers. The energy-band structure shows a clear separation between the region of electronic conduction and the zone of electronic insulation. Significantly, an undoped Ba2Cr4GeSe10 sample shows a desirable low thermal conductivity κT (0.51−0.87 W/m·K) and a high Seebeck coefficient S (351−404 μV/K) and reaches a ZT ≈ 0.08 at 773 K.

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hermoelectric (TE) technology offers a potential for converting waste heat into useful electricity and solidstate cooling without emission. 1−6 Binary or multinary chalcogenides have attracted a lot of interest not only because of their diverse structure chemistry but also because of their potential application as TE materials. Among them, some layered or quasi-layered compounds show outstanding TE performance because of the charge-carrier confinement effect in the reduced dimension and the clear separation between the region of electronic conduction and the zone of electronic insulation (usually act as the zone of phonon scattering) in their structure, such as Bi2Te3,7 In4Se3,8 SnSe,9 CsBi4Te6,10 RECuTe2 (RE = Tb−Tm, Y), 11−13 AgCrSe 2 , 14 CuCrSe 2 , 15 Cr 2 Ge 2 Te 6 , 16 BaxCr5Se8,17 CsMBi3Te6 and CsM2Bi3Te7 (M = Pb, Sn),18 and BiCuSeO.19 The discovery motivates us to seek new layered or quasi-layered compounds and explore their potential TE properties. The ternary Ba/Cr/S system is interesting; five known examples, BaCr4S7,20 Ba2Cr5S10,20 BaCrS2,21 Ba3Cr2S6,22 and Ba3CrS5,22 show their own structure types, which are, however, all built by the same CrS6 tetrahedral primary building unit. More interestingly, as the Ba/Cr ratio increases, the dimensionality of these compounds decreases from 3D to 2D to 1D. We consider this to be a hint that the Ba/Cr ratio is an important key to determining the structure. For example, the average oxidation state of Cr in the layered compound Ba2Cr5S10 is calculated to be 3.2+, which indicates that there are four Cr3+ and one Cr4+ per © XXXX American Chemical Society

Received: November 27, 2017

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DOI: 10.1021/acs.inorgchem.7b03002 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Structure of Ba2Cr4GeSe10 approximately along the a axis. (b) Single 2D Cr−Se layer viewed along the ab plane. (c) Coordination environment of each metal site in Ba2Cr4GeSe10.

Figure 2. (a) UV−vis diffuse-reflectance spectra and (b) TGA−DTA data of Ba2Cr4GeSe10. (c) Calculated band structure (the Brillouin zone is provided in Figure S6). (d) DOS of Ba2Cr4GeSe10. The Fermi level (EF) is set at 0.0 eV.

octahedra and isolated Ba2+ cations versus chains of CrS6 octahedra along the [100] direction (Figure S1). There are 2 crystallographically independent Ba atoms, 4 Cr atoms, 1 Ge atom, and 10 Se atoms in the asymmetric unit (Figure 1c and Tables S1 and S2). Each Cr atom is coordinated with six Se atoms, forming a distorted octahedron with Cr−Se bonds ranging from 2.470(2) to 2.596(2) Å (Table S3). These values are close to those in CrSbSe3 (2.486−2.598 Å)23 and AgCrSe2 (2.523−2.546 Å).14 The isolated GeSe4 tetrahedra with Ge−Se bonds of 2.332(2)−2.356(2) Å and S−Ge−S angles of 100.38(3)−117.36(3)°, which is a common geometry in germanium selenides. Both Ba1 and Ba2 exhibit a normal tricapped trigonal-prismatic coordination sphere with nine Ba− Se bonds between 3.250(2) and 3.716(2) Å (as detailed in Figure 1c and Table S3); such a similar coordination geometry is also observed in some other Ba-based chalcogenides.24−26 As shown in Figure 2a, the diffuse-reflectance spectrum indicates that Ba2Cr4GeSe10 is a narrow-band gap semiconductor with an optical Eg of 0.64 eV. This value is consistent with its black color, somewhat larger than those of well-known TE materials, such as Bi2Te3 (0.14 eV),27 CoSb3 (0.22 eV),28 and PbTe (0.29 eV),29 but smaller than those of the wide-band-gap TE materials, such as BiCuSeO (0.8 eV),19 Cu2CdSnSe4 (0.98 eV),30 and CuInTe2 (1.02 eV).31 Moreover, Ba2Cr4GeSe10 has no melting behavior below 1273 K according to the thermogravimetric−differential thermal (TGA−DTA) analyses shown in Figure 2b. No obvious endo- or exothermal peaks are observed in the DTA curves, and there is no apparent weight loss in the TGA curve. This is in accordance with the powder XRD data analysis (detailed information is shown in Figures S4 and S5), and such an excellent thermal stability makes this compound promising as a high-temperature TE material candidate. The band structure of Ba2Cr4GeSe10 along the selected symmetry direction is plotted in Figure 2c. The top of the valence band (VB) is located at the Γ and Z points, while the bottom of the conduction band (CB) is located at the X point, indicating an indirect-semiconductor behavior with Eg ≈ 0.21 eV. In the highest filled band, the region between Γ and Z (i.e., along the crystallographic c axis) is extremely flat, whereas that between Y

and X (i.e., the crystallographic ab plane) is steep. The steep slope is indicative of fast charge carriers, while the flat slope corresponds to the high effective mass pointing toward the possibility of a low carrier mobility and a high Seebeck coefficient.32,33 The movement of the charge carriers is limited within the ab plane, which agrees well with the crystal structure motif shown in Figure 1. In order to understand the distribution of the orbitals near the Fermi level (EF), the density of states (DOS) for Ba2Cr4GeSe10 is shown in Figure 2d. The top of the VB contains significant contributions from Cr 3d and Se 4p states, and the bottom of the CB mainly originates from Cr 3d. Note that the contribution from Ge atoms and Ba ions near EF is almost negligible, which means that the Ba ions and isolated GeSe4 tetrahedra are electrically insulating and the CrSe6 octahedral layers are electrically conducting. The optical absorption for the title compound can be mainly ascribed to the electronic transitions from the Se 4p state to the Cr 3d state. The temperature dependence of TE properties for Ba2Cr4GeSe10 is shown in Figure 3. The obtained pellets had relative densities of no less than 96% of the theoretical value. The electrical conductivity (σ) increased with increasing temperature throughout the entire temperature range, indicating semiconductor behavior. The in-plane σ result (perpendicular to the hot-pressed direction) is about 50% higher than that in the other direction (Figure 3a). σ is comparable to that of undoped BiCuSeO19 and undoped In4Se3,34 but about 1−2 orders of magnitude lower than those of the most efficient TE materials. As shown in Figure 3b, the positive Seebeck coefficient (S) indicates that Ba2Cr4GeSe10 is a p-type semiconductor dominated by holes. S falls in the range of 350−410 μV/K, and a maximum value of S is usually a result of bipolar conduction. S is higher than those of some outstanding and optimized TE materials, such as PbTe (245 μV/K),35 CoSb3 (245 μV/K),36 and Bi2Te3 (212 μV/ K),7 and is comparable to those of the undoped BiCuSeO (350− 400 μV/K)37 and the undoped In4Se3(−300−450 μV/K).34 Such a high S might be related to its unique structure feature in which the conductive and insulating layers (acting as the charge reservoir) form a natural superlattice, resulting in 2D confineB

DOI: 10.1021/acs.inorgchem.7b03002 Inorg. Chem. XXXX, XXX, XXX−XXX

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tetrahedra in the EILs also can scatter the phonons effectively, which may be the reasons for the desirable low κL. We consider that further TE performance enhancement of this compound is possible via hole doping in the Ge4+ site, which creates not only more holes to enhance σ but also more scatter centers in the EILs to lower κL. Furthermore, Ba2Cr4GeSe10 is thermally stable up to 1273 K and shows good mechanical properties. These primary data suggest that Ba2Cr4GeSe10 is a new promising hightemperature TE material candidate, and further work is ongoing. In summary, a new quaternary selenide, Ba2Cr4GeSe10, is designed and obtained by solid-state reactions. It represents a novel structure type in which the layers of CrSe6 octahedra are conceptually inherited from the parent Ba2Cr5S10 compound. Such a structure feature shows clear separation between the regions of electronic conduction and electronic insulation that are well supported by band structures and DOS studies. The electrical insulating layer restricts movement of the charge carriers and phonons within the layers, which results in a high S. Such a structure motif scatters the phonons effectively, resulting in a desirable low κL, while σ is about 2 orders of magnitude lower than those of known efficient TE materials. Such a low σ may be attributed to the low concentration of carriers and relatively large Eg (ca. 0.64 eV) of the title compound. Therefore, further TE improvement is ongoing via optimization of the Eg and concentration of the carriers.

Figure 3. Temperature dependence of the (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) total thermal conductivity (κT), and (d) ZT value for the polycrystalline Ba2Cr4GeSe10 sample (Per/Par: perpendicular/parallel to the hot-pressed direction). The uncertainty of σ, S, and κT was estimated to be within 5%, 5%, and 7%, respectively.

ment of the charge carriers. It has been reported that carriers confined within the superlattice can yield unusually large S without degradation of the other TE values.38,39 Consequently, the power factor PF (=S2σ) of Ba2Cr4GeSe10 may be improved by hole doping in the insulating layers by partially substituting Ge4+ with cations with a lower oxidation state, such as Ga3+, Zn2+, and Cu+. Similar results were found in Bi1−xSrxCuSeO: σ is significantly enhanced from 470 S/m (BiCuSeO) to 4.8 × 104 S/m (Bi0.85Sr0.15CuSeO) at 293 K without significantly decreasing S.37 Figure 3c shows the total thermal conductivity (κT) of Ba2Cr4GeSe10. Similar to σ, the anisotropy is also seen in κT measured along different directions, and the Per direction is about 40% larger than the Par direction. The electric thermal conductivity (κe) was estimated according to κe = LσT,1 and the lattic thermal conductivity (κL) was calculated according to κL = κT − κe. As a result of the low σ, κe (0.0003−0.0046 W/m·K) is almost negligible, suggesting that κT is dominated by κL that falls in the ranges of 0.83−0.49 W/m·K in the Per direction and 0.58−0.34 W/m·K in the Par direction. Such low values of κL suggest that the structure of Ba2Cr4GeSe10 can scatter the phonon effectively, which is likely a consequence of its unique structure motif, of which the conducting layers alternate with the insulating layers of the Ba atoms and isolated GeSe4 tetrahedra that scatter phonons effectively.38,39 This is still an open issue and needs further clarification. To estimate the overall TE performance, the-figure-of-merit, ZT = S2σT/κ, needs to be calculated.1 As presented in Figure 3d, the ZT value increases with the temperature and reaches 0.08 at 773 K for Ba2Cr4GeSe10. In view of the rising trend of ZT with the temperature, we propose that even higher values of ZT could be realized at higher temperatures. The Ba2Cr4GeSe10 compound shows a desirable low κT, a high S, but a low σ. The layers of CrSe6 octahedra are the electrically conducting layers (ECLs), and the Ba ions and isolated GeSe4 tetrahedra are the electrically insulating layers (EILs). The EILs flatten the band perpendicular to the layers and restrict movement of the charge carriers and phonons within the ECLs, which results in a high S. The interfaces between the ECLs and EILs should scatter the phonons effectively along the c direction, and the isolated Ba atoms and discrete GeSe 4

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03002. Structural information tables and experimental details (PDF) Accession Codes

CCDC 1586494 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hua Lin: 0000-0002-7241-9623 Li-Ming Wu: 0000-0001-8390-2138 Author Contributions

H.C. and Y.K.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research was supported by the NSFC (Grants 21771179, 21571020, 21301175, and 91422303), and the “Chunmiao” Project of Haixi Institute of Chinese Academy of Sciences (Grant CMZX-2018-001). C

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