Fabrication and Thermoelectric Properties of Single-Crystal Argyrodite

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Fabrication and thermoelectric properties of single-crystal argyrodite Ag8SnSe6 Min Jin, Siqi Lin, Wen Li, Zhiwei Chen, Rongbin Li, Xianghu Wang, Yunxia Chen, and Yanzhong Pei Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00393 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Chemistry of Materials

Fabrication and thermoelectric properties of single-crystal argyrodite Ag8SnSe6 Min Jina, Siqi Linb,*, Wen Lib, Zhiwei Chenb, Rongbin Lia, Xianghu Wanga, Yunxia Chena, Yanzhong Peib,* a

College of materials, Shanghai Dianji University, Shanghai 201306, China Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China

b

*

Corresponding author: [email protected]; [email protected]

Abstract: Recently, poly-crystalline argyrodite Ag8SnSe6 has shown a promising thermoelectric performance, which largely relies on the ultralow lattice thermal conductivity. However, the thermoelectric properties of single crystalline Ag8SnSe6 have been unavailable so far. In this work, Ag8SnSe6 single crystals, with a highly preferred orientation of (110), are grown by a vertical Bridgman technique. Thermoelectric properties parallel and perpendicular to this direction are investigated, revealing a ~40% higher hall mobility but a very similar lattice thermal conductivity in single crystals as compared to that in polycrystals. Both transport and elastic properties are found to be nearly isotropic even for the low temperature phase with an anisotropic structure. This work not only successfully for the first time synthesizes high-quality single-crystal Ag8SnSe6, but also enables a fundamental understanding on the materials physics. 1. Introduction Without any moving parts or emission, thermoelectric (TE) technology has been considered as a sustainable green energy technology for waste heat recovery1. The limitation for a large-scale application is the low conversion efficiency, which is determined by the materials’ thermoelectric dimensionless figure of merit zT= S2T/ρ(E+L), where S, T, ρ, E and L are the Seebeck coefficient, absolute temperature, resistivity, electronic and lattice components of thermal conductivity (), respectively. To achieve a high thermoelectric performance, a high Seebeck coefficient (S), a low electrical resistivity (ρ) and a low thermal conductivity () should be satisfied simultaneously. However, these parameters are strongly correlated via carrier concentration, leading an individual optimization to be difficult for an effective zT-enhancement. Band engineering including band convergence2, band nestification3 and resonant states4, 5, has been proven effective for decoupling the electronic transport preparties at some degree. These strategies have been successful applied in various thermoelectric materials including group IV-VI compounds2, 6-9, CoSb310, 11, Mg2Si1-xSnx12, Half-heuslers13, 14 , Zintl compounds15, (Bi1-xSbx)2Se316 and diamond-like compounds17-19. Microstructure engineering is another effective approach for improving zT by reducing the only one independent parameter L. This can be realized through strengthening phonon scattering by introducing 0D point defects20, 21, 1D dislocations22-25, 2D nanostructures26-28, which contribute to the scattering of phonons with high-frequency, mid-frequency and low-frequency, respectively. Alternatively, the discovery of novel thermoelectrics with an ultra-low L is important as well. The concepts including a complex crystal structure, liquid-like ions, lattice anharmonicity and low sound velocity, have all successfully led to an intrinsically low L for exploring novel thermoelectrics, which are typified by Zn4Sn329, 30, MgAgSb31-33, Cu2Se34, 35 and SnSe36, 37. Recently, argyrodite compounds with a general formula of A(12-n)Bn+X62- (A=Ag/Cu, Bn+=Ga3+, Si4+, Ge4+, Sn4+, P5+ or As5+ and X=S/Se/Te) are particularly interesting for thermoelectric applications due to the extremely low L38-52. For example, Charoenphakdee et.al45 first reported the L of argyrodite Ag8GeTe6 to be as low as 0.25 W/m-K and a zT of 0.5. Li et.al50 found Ag8SnSe6 to show a low L of ~0.2 W/m-K and the peak zT can be as high as 1.2. Recently, Lin et.al49 revealed that Ag9GaSe6 exhibits a L of only ~0.15

W/m-K leading a zT to be 1.5. As can be seen, all these argyrodite materials share the commonness of an ultra-low L. This could be understood by their complex crystal structures and weakly bonded Ag atoms, which lead to a low sound velocity and a significantly reduced portion of acoustic phonons contributing to thermal conductivity49, 53, 54 . Despite of many interesting physical properties in argyrodite materials, such as extremely low sound velocity, low cut-off frequency of acoustic phonons, super strong lattice anharmonicity and high carrier mobility, all available literatures focused only on poly-crystalline samples prepared by a high-temperature melting and powder consolidation procedures. The thermoelectric properties of single crystal argyrodites have so far not yet been reported, to our best knowledge. This might result from the difficulty in single crystal fabrication, due to its richness in chemical composition, in phase transitions and in atomic ordering/occupancy55-57.Therefore, a successful synthesis of high-quality single-crystalline argyrodites might be important for an in-depth understanding on the fundamental materials physics. In this work, argyrodite Ag8SnSe6 single-crystal is successfully fabricated by a vertical Bridgman technique. X-ray diffraction measurement suggests a preferred orientation along the (110) plane. Physical properties, with a focus on the thermoelectric transport properties, are investigated in details with a comparison to those of poly-crystalline Ag8SnSe6 in the temperature range of 300-800 K. Single crystal shows a higher Hall mobility and a comparable lattice thermal conductivity of only 0.2 W/m-K. This work enables further insights into the fundamental physics of Ag8SnSe6 in addition to existing literature results on poly-crystalline materials. The technique for single-crystal grown here is believed to be equally applicable for many other argyrodites and similar semiconductors with a complex crystal structure. 2. Experimental details High purity Ag, Sn and Se elements (>99.99% purity) were weighted according to the stoichiometric ratio of Ag: Sn: Se= 8: 1: 6, and the total weight was about 334.2 g. The starting materials were loaded into a Ø25 mm quartz ampoule and then sealed by a hydrogen oxygen flame under vacuum (~10-3Pa). The sealed ampoule was placed into a 1273 K rocking furnace. After the raw materials were melted and soaked for half an hour, the rocking system was

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turned on with a rocking rate of 20r/min for 30 minutes to ensure the homogeneity of the melt. Finally, the furnace was cooled to room temperature naturally. Ag8SnSe6 crystal is grown by a vertical Bridgman technique, the furnace and the temperature distribution can be in our previous work58. Prior to the crystal growth, the volatility of Ag8SnSe6 is checked to guide the choice of growth temperature. Temperature dependent weight percentage was measured using thermogravimetry equipment (TG-DTA 8121, Japan). As shown in Fig. 1a, Ag8SnSe6 shows nearly no weight loss at temperatures up to ~1133K. The weight loss in Ag8SnSe at T>~1133K is presumably due to the volatilization of Se55,59. According to the Ag2Se-SnSe2 pseudo-binary phase diagram55, 60 as shown in Fig. 1b, the furnace temperature for the single crystal growth of Ag8SnSe6 is set at 1123 K. It should be noted that, to ensure a proper thermal stress releasing, the temperature gradient of 5-8 K/cm was used for the growth of Ag8SnSe6 crystals. The lowering rate of crucible during the growth is 1.5mm/hour. The as-grown crystals were cooled to room temperature with a rate of 20-25°C/hour. (b) 1273

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capacity determined by the Dulong-Petit limit. These properties, each comes with an uncertainty of ~5%, were measured from 300 K to 800 K. Results and discussion As-grown Ag8SnSe6 crystals show remarkable metallic color (Fig. 2a). Fig. 2b shows both the DSC heating and cooling curves from 30ºC to 200ºC, where an endothermic peak at 360 K and an exothermic peak at 352 K are observed. These correspond to a reversible phase transition between a room-temperature orthorhombic β-phase (Fig. 2c) to high-temperature cubic γ phase (Fig. 2d), and the phase transition temperature agrees well with literature results (~356 K)55, 56, 61. γ-Ag8SnSe6 (HT) consists of [SnSe4]4- and Se2- anion frameworks with a high symmetry. Ag atoms, sitting at three different crystallographic sites (96i, 48h and 48h), are all disordered and weakly bonded to the anion framework with a site occupancy of only 4.1%, 25% and 33%, respectively. This is very similar with that in cubic Ag9GaSe649, indicating the diffusive behavior of Ag particularly at high temperatures. As a comparison, the room-temperature orthorhombic structure (LT) comes with all atoms ordinarily ordered and occupied.

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SnSe2

Figure 1. Thermogravimetry curves for Ag8SnSe6 and Se (a), and the pseudo-binary phase diagram of Ag2Se-SnSe2 60.

The density d of Ag8SnSe6 crystal was measured by an Archimedes method (DM50, Switzerland). A small portion of the as-grown crystals was ground into fine powders for X-ray diffraction testing (Bruker D8, Germany) using the Cu Kα radiation (λ=1.5406 Å) at room temperature. The phase transition temperature was determined by a Differential Scanning Calorimetry (Flash DSC 2+, Switzerland). Microstructure and chemical composition were characterized by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) (Phenom Pro, Netherlands). Crystal orientation was determined using an X-ray orientation equipment (TYX-200/2H8, China). Optical reflectance measurements was carried out by a Bruker Tensor II equipped with a diffuse reflectance attachment. The Raman spectra were excited by the 532 nm lines of an argon laser in the back-scattering geometry, using a Jobin Yvon model U-1000 monochromator, with a conventional photo-counting system. Longitudinal and transverse sound velocities were measured using a pulse-receiver (Olympus-NDT) equipped with an oscilloscope (Keysight). Seebeck coefficient was obtained from the slope of the thermopower versus temperature difference within 0-5 K. Resistivity and Hall coefficient (with a reversible magnetic field of 1.5 T) was measured using the van der Pauw technique. All these electrical properties were measured under helium. Thermal diffusivity (λ) was measured using a laser flash technique under argon with the Netzsch LFA457 system. The thermal conductivity was determined via =dCpD, where d is the measured density, Cp is the heat

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The density of the obtained Ag8SnSe6 crystal is 7.08 g/cm-3, which is nearly identical to its theoretical density61. In this work, (110) facet of as-grown Ag8SnSe6 crystal is detected (Fig. 3a), according to the XRD results that only (330) diffraction peak is observed. The powder XRD pattern of Ag8SnSe6 can be well indexed to an orthorhombic structure with a Pmn21 space group61. The lattice parameters are estimated to be a=0.79235nm, b=0.78311nm and c=1.10876nm, which are coincided well with the literature results55. SEM observations and EDS composition mapping and line scan confirm the purity and homogeneity (Fig. 3b and 3c). Two square singlecrystalline wafers, both are ~1.0 mm in thickness and ~10 mm in width, are respectively sliced along directions parallel and perpendicular to this (110) plane.


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acoustic phonons63. Importantly, the Hall mobility for singlecrystalline Ag8SnSe6 is about ~40% higher than polycrystal in the entire temperature range, which could be understood by the low concentration of defects in single crystal.

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The optical band gap of Ag8SnSe6 is estimated from the diffusive reflectance measurements based on the Kubelka-munk equation62 F[R]=(1-R)2/2R, where R is diffuse reflection ratio. Fig. 4a shows the relationship of [F(R)hν]1/2 with hν at different temperatures and the optical band gap (Eg) decreases from 0.83 eV at 300 K to 0.74 eV at 450 K. It is further shown that the optical band gap for both β-Ag8SnSe6 and cubic γ-Ag8SnSe6 are weakly temperature-independent, while the β-to-γ phase transition leads to a much faster decrease in optical band gap (dashed line in Fig. 4b). (b)

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Figure 3. XRD patterns for the (110) facet of as-grown Ag8SnSe6 single-crystal and powders (a), and back-scattering SEM images with compositional mappings for as-grown Ag8SnSe6 single-crystal (b). The optical microscope image of the sample and the corresponding composition by an EDS line scan of a total length of ~4.6 mm (c).

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Figure 4. The normalized optical absorption versus photon energy (a) and optical band gap at different temperatures (b) for Ag8SnSe6.

Temperature dependent Hall carrier concentration for singlecrystalline Ag8SnSe6 sample parallel and perpendicular to (110) is shown in Fig 5a, with a comparison to that of literature poly-crystalline Ag8SnSe6 with a similar but slightly higher carrier concentration50. As can be expected, the Hall carrier concentration of Ag8SnSe6 single crystal is orientation-independent. It should be noted that the rapid increasing of carrier concentration between the temperature of 300-400 K can be ascribed to the phase transition. The n-type conduction observed in both poly-crystalline and singlecrystalline samples, presumably results from the Se-deficiency induced by its high vapor pressure. The Hall mobility decreases with increasing temperature via mH ~T−1.5 for both single and ploy-crystals (Fig 5a), indicating a dominant charge carrier scattering by

Figure 5. Temperature-dependent Hall carrier concentration and Hall mobility (a), density of state effective masses md* and deformation potential coefficient Edef (b) for singlecrystalline Ag8SnSe6, Hall carrier concentration dependent Seebeck coefficient (c), compared with literature results on polycrystalline Ag8SnSe650.

According to the literature band structure calculations64, a single parabolic band model (SPB) with acoustic phonon scattering is used here to understand the transport properties. The SPB model enables an estimation on the density of states effective mass (m*) and the deformation potential coefficient (Edef) for Ag8SnSe6, as shown in Fig. 5b. The effective mass md* (0.17 me) and Edef (22 eV) for singlecrystalline Ag8SnSe6 show a weak temperature dependence, suggesting a rigid conduction band for both low and high temperature phases. As compared to the case of polycrystals, which show a similar temperature dependence on md* and Edef, single-crystal might enable an in-depth understanding on the rigidity of the conduction band. When compared with poly-crystalline Ag8SnSe6, where the Seebeck coefficient can be well predicted by a SPB model, singlecrystaline Ag8SnSe6 shows no observable deviation on Hall carrier concentration-dependent Seebeck



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Figure 6. Temperature-dependent Seebeck coefficient (a), resistivity (b), power factor (c), total and lattice thermal conductivity (d) for singlecrystalline and poly-crystalline Ag8SnSe6, compared with literature results50.

Temperature dependent Seebeck coefficient, resistivity and power factor for singlecrystalline Ag8SnSe6 are shown in Fig. 6a, 6b and 6c, respectively. Significant changes in transport properties can be observed at temperature close to the phase transition temperature. The most distinct feature is the significant increase in Hall carrier concentration from ~31018 cm-3 in the low-T phase to ~11018 cm-3 in the high-T phase, which is presumably due to the increase of equilibrium concentration of anion vacancies. This increase reasonably leads to a dramatical decrease in both Seebeck coefficient and resistivity with increasing temperature at T350 K, Hall carrier concentration remains nearly unchanged, leading the material to show a typical degenerate semiconducting behavior. This corresponds to a continuous increase in both Seebeck coefficient and resistivity as shown in Figure 6. It is found that the power factor for singlecrystalline Ag8SnSe6 is higher than that of polycrystals. This is mainly because singlecrystalline Ag8SnSe6 might rule out the scattering of charges by defects or boundaries for a higher carrier mobility (Fig. 5a). Moreover, the electronic properties show no observable difference between orientation directions parallel and perpendicular to the (110) plane in the entire temperature range. At T>~350 K, this can be expected because the crystal structure for the high-temperature cubic phase is isotropic; while for T 1. Energy Environ. Sci. 2015, 8, (1), 216-220. 15. Zhang, J.; Song, L.; Pedersen, S. H.; Yin, H.; Hung, L. T.; Iversen, B. B., Discovery of high-performance low-cost n-type Mg3Sb2-based thermoelectric materials with multi-valley conduction bands. Nat Commun 2017, 8, 13901. 16. Wang, S.; Sun, Y.; Yang, J.; Duan, B.; Wu, L.; Zhang, W.; Yang, J., High thermoelectric performance in Te-free (Bi,Sb)2Se3 via structural transition induced band convergence and chemical bond softening. Energy Environ. Sci. 2016, 9, (11), 3436-3447. 17. Zhang, J.; Liu, R.; Cheng, N.; Zhang, Y.; Yang, J.; Uher, C.; Shi, X.; Chen, L.; Zhang, W., High-performance pseudocubic thermoelectric materials from non-cubic chalcopyrite compounds. Adv. Mater. 2014, 26, (23), 3848-53. 18. Qiu, P.; Qin, Y.; Zhang, Q.; Li, R.; Yang, J.; Song, Q.; Tang, Y.; Bai, S.; Shi, X.; Chen, L., Intrinsically High Thermoelectric Performance in AgInSe2 n-Type Diamond-Like Compounds. Adv Sci 2018, 5, (3), 1700727. 19. Zhang, J.; Song, L.; Madsen, G. K.; Fischer, K. F.; Zhang, W.; Shi, X.; Iversen, B. B., Designing high-performance layered thermoelectric materials through orbital engineering. Nat Commun 2016, 7, 10892. 20. B., V. C.; D.M., R., Silicon Germanium CRC Press: Florida, 1995. 21. Ioffe, A. F.; Ioffe, A. V., Thermal conductivity of solid solutions. Soviet Physics Solid State 1960, 2, (5), 719-728. 22. Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. J.; Shin, W. H.; Li, X. S.; Lee, Y. H., Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics. Science 2015, 348, (6230), 109-114. 23. Chen, Z.; Jian, Z.; Li, W.; Chang, Y.; Ge, B.; Hanus, R.; Yang, J.; Chen, Y.; Huang, M.; Snyder, G. J.; Pei, Y., Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence. Adv. Mater. 2017, 29, 1606768. 24. Chen, Z.; Ge, B.; Li, W.; Lin, S.; Shen, J.; Chang, Y.; Hanus, R.; Snyder, G. J.; Pei, Y., Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics. Nat Commun 2017, 8, 13828. 25. Xin, J.; Wu, H.; Liu, X.; Zhu, T.; Yu, G.; Zhao, X., Mg vacancy and dislocation strains as strong phonon scatterers in Mg2Si1−xSbx thermoelectric materials. Nano Energy 2017, 34, 428-436. 26. Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z., High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008, 320, (5876), 634-638. 27. Xie, W.; He, J.; Kang, H.; Tang, X.; Zhu, S.; Laver, M., Identifying the Specific Nanostructures Responsible for the High

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Table of Contents Graphic 1.0

 L ;

(110)

(330)

, L (W/m-K)

Ag8SnSe6single-crystal Ag8SnSe6 powder

0.6

0.4

0.2

0.0

30

40

50

60

; ;

ICSD #95093

20


;

0.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 Theta (°)

HT

LT 300

400

500 600 T (K)

8

ACS Paragon Plus Environment

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Fabrication and Thermoelectric Properties of Single-Crystal Argyrodite

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