Enhancement of Power Factor for Inherently Poor Thermal Conductor

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Enhancement of Power Factor for Inherently Poor Thermal Conductor Ag8GeSe6 by Replacing Ge with Sn Somnath Acharya, Juhi Pandey, and Ajay Soni* School of Basic Sciences, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh 175005, India

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S Supporting Information *

ABSTRACT: Superionic argyrodites have been identified as thermoelectric materials with inherent ultralow lattice thermal conductivity because of liquid-like behavior of cations in a relatively larger unit cell. The major drawback for these as potential thermoelectric materials is low carrier concentration and low electrical conductivity, which however can be improved by controlling the compositions while keeping the advantageous poor thermal conductivity intact. Here, we report ∼8 times enhancement of thermoelectric power factor of Ag8GeSe6 by replacing Ge with metallic Sn, at high temperatures. Thus, Ag8SnSe6 has been demonstrated as an efficient thermoelectric material with ZT ∼ 0.62 at 623 K, even at a very low carrier concentration (∼1016 cm−3). The extremely low thermal conductivity has been explained with weakly bonded Ag ions to rigid anion sublattice and presence of low-frequency Einstein optic modes, which provide the possibility of decoupling of charge and heat transport. KEYWORDS: thermoelectric materials, large unit cell materials, argyrodites, phonon liquid electron crystal, superionic compounds, power factor enhancements

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ZT, such as skutterudites,16,17 clathrates,18,19 and chalcopyrite.20,21 Recently, the superionic compounds such as, Cu2X,22,23 Ag2X,24−26 CuAgX27 (where X = S, Se, Te) and CuCrSe228 have been in the main research focus due to new concept phonon liquid electron crystal (PLEC). These superionic materials have a rigid anion sub lattice to maintain crystalline solid, while the cations have a “liquid-like” movements to scatter the heat carrying acoustic phonons, thus satisfying the PLEC concept. The intrinsically ultralow κl of the superionic compounds are associated with weakly bonded elements, low sound velocity, and low energy optical modes.29,30 Among the different superionic semiconductors, argyrodites n+ 2− exist with a common formula Am+ (12−n)/mB X6 , (here, A = Li, Ag, Cu; B = Si, Ge, Sn; and X = S, Se, Te with m and n as valence states of A and B, respectively) have been extensively studied as promising TE material.31 Argyrodite is a Greek term used to represent minerals rich in silver and have been widely studied for solid state batteries.32 Depending on the constituent elements, most of the argyrodite compounds are monoclinic, orthorhombic, hexagonal, or cubic in crystal structure at low temperatures. At high temperature, argyrodite compounds usually possess high symmetry cubic or hexagonal crystal structure, where cations (Ag, Cu) are disorderly

INTRODUCTION In support of a global sustainable energy solution, thermoelectric (TE) materials can be an alternate option for energy harvesting techniques. TE materials can directly convert heat energy into electricity and vice versa under a thermal gradient without harmful gas emission, leading to an ecofriendly technologies. The performance of heat to electricity conversion for any TE materials depends on dimensionless quantity called figure of merit, ZT = (S2/ρκ)T, where S is thermoelectric power, ρ is electrical resistivity, κ is total thermal conductivity, which includes electronic (κe) and lattice (κl) part, and T is an average absolute temperature, respectively.1,2 It is obvious that for high ZT, material requires high power factor (S2/ρ) and poor κ at operating temperature. However, because of interrelationship of S, ρ, and κe with carrier concentration, n, the enhancement of ZT is very challenging for any TE material.3 Thus, decoupling the interdependency is very much essential, which can be achieved by isolating the flow of heat and charge in the materials. For several decades, interestingly, the phonon glass electron crystal (PGEC)2 approach have been employed to enhance the ZT, where S2/ρ enhances through electronic band modification via resonant impurities4,5 and band convergence,6−8 while κ can be reduced by nanostructuring,9,10 nanocomposites,11,12 and atomic rattlers,13 also by soft phonon modes14 and complex structures.15 The idea is to develop a material which is glassy for phonons and crystalline for charge carriers. In the context of PGEC, various research have been reported with new TE materials with high © XXXX American Chemical Society

Received: September 28, 2018 Accepted: December 4, 2018 Published: December 4, 2018 A

DOI: 10.1021/acsaem.8b01660 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Reitveld refinement of room temperature powder X-ray diffraction patterns of (a) Ag8GeSe6 and (b) Ag8SnSe6. Insets show the unit cell crystal structure where gray, green, blue, and yellow spheres are representing Ag, Ge, Sn, and Se atoms, respectively.

distributed around the fixed sublattice formed by B cations and the chalcogenides anions.31 With the possibility of variety substitution and high ionic conductivities, argyrodite compounds are interesting for PLEC-TE research. In the present report, we have reported performance of ntype Ag8GeSe6 and Ag8SnSe6 argyrodites across structural phase transition. At low temperature, Ag8GeSe6 and Ag8SnSe6 have orthorhombic crystal structure. However, both Ag8GeSe6 and Ag8SnSe6 undergo structural phase transition from orthorhombic (space group Pmn21) to cubic (space group F4̅3m) at ∼321 and ∼355 K, respectively.30,33 We emphasis that the cubic phase for Sn-based compound is more conductive than Ge based compound. The high S value at room temperature for both the compounds has been realized by low n and large electronic band gaps, leading to an improved S2/ρ. Further, the low energy Einstein optic modes and liquid-like cations leads to the ultralow κl, hence, improving the TE performance of Ag8SnSe6 at elevated temperature.

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powder sample were collected at room temperature using a Cary 5000 UV−vis−NIR spectrometer equipped with an integrated sphere attachment and BaSO4 as a reference. The data were collected in the range of 2400 to 600 nm at a scan rate of 600 nm min−1. The gap was estimated by Kubelka−Munk relationship, α = (1 − R)2/2R, where α and R are the extinction and reflectance coefficients of the material, respectively. Raman spectroscopy measurements were performed using Horiba HR-Evolution spectrometer with 532 nm excitation laser. Linkam stage was used for high temperature Raman measurements in the temperature range of 300−453 K. Low-temperature (2−300 K) thermoelectric transport measurements were performed on samples with typical size ∼2 × 2 × 10 mm3, using thermal transport option (TTO) of quantum design make physical property measurement system (PPMS). The gold coated copper leads with Ag epoxy were used to make contacts. The measurement was carried out in a high vacuum of 10−5 Torr and continuous mode. Low-temperature heat capacity and Hall measurements were also performed using the PPMS. High-temperature ρ and S were measured using a ULVAC-RIKO ZEM-3 instrument under helium atmosphere from room temperature to 623 K. The typical dimensions of the sample for measurement are ∼2 × 2 × 10 mm3. The thermal conductivity was calculated using κ = DCpdm, where the thermal diffusivity (D) was directly measured for a sample with ∼10 mm diameter and ∼2 mm thickness in a Netzsch LFA-457 instrument in the temperature range 300−623 K by using the laser flash diffusivity method under nitrogen atmosphere. To avoid errors from thermal emissivity of the materials, samples were coated with a thin layer of graphite. The high temperature specific heat, Cp was measured by differential scanning calorimetric (DSC) using NETZSCH STA 449 F1, and the mass density dm was measured by using the Archimedes method. The measured dm of SPS-processed Ag8GeSe6 and Ag8SnSe6 were 6.77 and 6.87 g/cm3, respectively.

EXPERIMENTAL DETAILS

Bulk samples of polycrystalline Ag8MSe6 (M = Ge, Sn) were prepared by solid state melting followed by annealing technique. We have used high purity elements, Ag (shots, ∼99.99%), Ge (shots, ∼99.99%), Sn (powder, ∼99.99%), and Se (shots, ∼99.99%) in the stoichiometric compositions and placed in quartz tubes. The quartz tubes were flame-sealed under ∼10−5 mbar vacuum and heated in a furnace up to 1323 K in 12 h and held at this temperature for 24 h, then cooled down to 873 K in 24 h, followed by annealing at 873 K for 3 days. The annealing was employed to prevent the formation of the byproducts. Finally, the obtained ingot samples were grounded into powders with an agate mortar and loaded into graphite dies with an inner diameter of 10 mm and compacted before placing the die assembly into a spark plasma sintering system (SPS211-LX, Dr. Sinter Lab). The sintering system was first evacuated to 10−3 mbar. The temperature was increased to 693 K for 5 min, with a constant pressure of 45 MPa. Further, the SPS-processed high density samples with 10 mm diameter and 3 mm thickness were polished and cut into rectangular bar shape for thermal transport and electrical measurements. Crystal structure and phase purity were examined by powder X-ray diffraction (XRD) using rotating anode Rigaku Smartlab diffractometer in Bragg−Brentano geometry. XRD spectra were recorded using Cu Kα radiation (λ = 1.5406 Å), at 45 kV operating voltage and 100 mA with the scan speed of 2 deg/min and the step size of 0.02°. Field emission scanning electron microscopy (FESEM) imaging and energy dispersive spectra (EDS) of the polished surface of samples were performed using JFEI, USA, make Nova Nano SEM-450. To measure the optical band gap, optical diffuse reflectance spectra of

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RESULTS AND DISCUSSION Figure 1 shows the room temperature XRD patterns of Ag8GeSe6 and Ag8SnSe6 with orthorhombic structure (space group Pmn21) and all the reflections have been identified with ICDD Card Nos. 04-008-7238 and 04-010-6257, respectively. The absence of secondary peaks are confirmed by Reitveld refinement and the calculated lattice parameters (Section S1 and Table S1) are in good agreement with previous literature indicating high purity of prepared samples.33,34 As the ionic radius of Sn4+ (∼69 pm) is bigger than Ge4+ (∼53 pm), the unit cell of Ag8SnSe6 has larger lattice parameters than Ag8GeSe6. Because of the large electronegativity of Ge (∼2.01) as compared to Sn (∼1.96), the bond strength of Ge−Se is higher than Sn−Se. The unit cell structures have been shown in the insets. B


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Figure 2. SEM images of top surface of polished pellets of (a) Ag8GeSe6 and (f) Ag8SnSe6. The EDS elemental mapping of (b) distribution of all elements, (c) Ag, (d) Ge, and (e) Se for Ag8GeSe6 and (g) distribution all elements, (h) Ag, (i) Sn, and (j) Se for Ag8SnSe6.

Figure 3. (a) (αhν)2 as a function of photon energy hν. The extrapolation of the spectrum (blue straight line) is used to estimate the optical band gap value for Ag8GeSe6 and Ag8SnSe6. (b) DSC curve of Ag8GeSe6 and Ag8SnSe6 samples showing structural phase transition.

Microstructure and chemical composition have been studied from SEM images and the distribution of elements by EDX analysis which is shown in Figure 2. The top surface of SEM image exhibits highly dense sample with clean and uniform microstructure, supporting the obtained XRD results. The EDX elemental mapping confirms the phase purity and homogeneity throughout the samples and the estimated chemical compositions are mentioned in the Supporting Information (Section S1, Table S2). We have performed the diffuse reflectance spectrum, which is shown in Figure 3a. The optical band gap, Eg is estimated from the extrapolation of the (αhν)2 as a function of photon energy hν, based on the equation of αhν = B(hν − Eg)p, where α is the extinction coefficient, hν is the incident photon energy, B is the absorption coefficient, and p depends upon nature of transition.35,36 The obtained band gaps are 0.85 and 0.8 eV for Ag8GeSe6 and Ag8SnSe6, respectively, which are consistent with earlier report on family of these compounds.37 Clearly, the estimated Eg is higher for Ge based compound. Most of the argyrodites have structural phase transition at high temperatures. We have also measured the differential scanning calorimetry (DSC) to probe the associated structural phase transition in both the samples. Figure 3b depicts the exothermic peaks across the phase transition from orthorhombic (space group Pmn21) to cubic (space group F4̅3m) structural units for Ag8SnSe6 and Ag8GeSe6. The minor peak at 353 K in Ag8GeSe6 is associated with ε to γ phase transition while peak around 415 K for both the samples is related to crystallization of Se into γ phase.38

To understand further, we have performed Raman spectroscopy across the structural phase transition in the temperature range of 300−453 K (Figure 4). At room

Figure 4. Temperature-dependent Raman spectroscopy studies of (a) Ag8GeSe6 and (b) Ag8SnSe6, across structural phase transition.

temperatures, we have observed three peaks at ∼193, 219 and 250 cm−1 for Ag8GeSe6, while two peaks at ∼205 and 213 cm−1 for Ag8SnSe6. We have analyzed the origin of the peaks and understood that the peak at ∼193 cm−1 (for Ag8GeSe6) and ∼205 cm−1 (for Ag8SnSe6) represents symmetric stretching of Ge−Se (or Sn−Se) bond in GeSe4 (or SnSe4) C


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Figure 5. Temperature dependence of (a) resistivity, ρ (in log scale), (b) thermoelectric power, S, (c) carrier concentration, n, and (d) mobility, μ, for Ag8GeSe6 and Ag8SnSe6.

Table 1. Carrier Concentration, n, Hall Mobility, μ, Debye Temperature, θD, and Average Speed of Sound, νa, of Ag8GeSe6 and Ag8SnSe6 at 300 and 370 K sample name orthorhombic phase (300 K)

cubic phase (370 K)

n (1016 cm−3) μ (cm2 V−1 s−1) θD (K) νa (ms−1) n (1016 cm−3) μ (cm2 V−1 s−1)

Ag8GeSe6

Ag8SnSe6

ratio of parameters (Ag8SnSe6/Ag8GeSe6)

0.15 663.99 276 2067 0.61 501.48

1.25 833.33 270 2041 4.68 1112.9

8.33 1.25

units with A1 symmetry.39−41 The peak at ∼250 cm−1 observed in Ag8GeSe6 is associated with one of the triply degenerate T2 mode with asymmetric stretching (ν3) of Se atom in GeSe4 unit.47 Raman modes at ∼219 cm−1 (for Ag8GeSe6) and at ∼213 cm−1 (for Ag8SnSe6) represents vibration of Se−Se atoms coordinated with Ag atom.42 Interestingly, the Raman intensity of the modes associated with Se−Se atoms coordinated with Ag are poor in both the samples, but with increase in temperature the peak intensity enhances up to the phase transition. With increase in temperature, Ag atoms also starts vibrating, which contributes to the Raman intensity of Se−Se bond. Near phase transition ∼333 K (for Ag8GeSe6) and ∼363 K (for Ag8SnSe6), the full width at half-maximum (fwhm) for Se−Se vibrational mode decreases, while the Raman intensity enhances because of significant increase in vibration amplitude of Ag atom.43 For Ag8GeSe6, Se−Se atoms vibrational mode emerges as most intense peak at 333 K, which suggest a metastable state at transition. The rise in vibrational amplitude near phase transition enables the hopping of Ag atoms. At higher temperature and in the cubic phase, Ag atoms undergoes kinetic disorder resulting in disruption of phonon vibration of Se−Se atoms coordinated with Ag atoms and hence Raman intensity gradually decreases and merges with A1 modes. Thus, phonons responds differently across phase transition in both the samples which can be understood from increase in intensity of peaks at transition temperature.

7.67 2.22

The temperature dependent transport measurements for Ag8GeSe6 and Ag8SnSe6 are shown in Figure 5. The ρ(T) decreases with temperature for both the samples suggesting a semiconducting behavior in line with the optical absorption measurements. The ρ values at 300 K are higher for Ag8GeSe6 (∼0.064 Ω-m) than Ag8SnSe6 (∼0.006 Ω-m) and remained high by an order even at ∼623 K. The behavior is expected with larger band gap for Ag8GeSe6. While cooling, the onset of sharp rise in ρ(T) at ∼360 K (for Ag8GeSe6) and ∼331 K (for Ag8GeSe6) is associated with the structural phase transition shown in DSC and Raman data. The observed deviation can be understood by generation of cation disorder because of structural phase transition in both the samples.34 Due to the richness of Ag ions, most of the superionic conductors argyrodites are n-type materials.44 The negative S(T) values throughout the measured temperature range confirm that electrons are the majority charge carriers in both the samples (shown in Figure 5b). The S value for Ag8GeSe6 increases from -219 μV/K (at 303 K) to −248 μV/K (at 623 K), while for Ag8SnSe6, S decreases from −332 μV/K (at 303 K) to −259 μV/K (at 623 K), with an intermediate maximum value −388 μV/K at temperature ∼360 K. In case of Ag8SnSe6, the charge carriers have relatively higher thermal effective mass (shown at electronic contribution in heat capacity data, Table S4). Since, S gets directly influenced by effective mass, thus, Ag8SnSe6 with high thermal effective mass of electrons has higher S compared to Ag8GeSe6. Noticeably, the S decreases signifiD


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Figure 6. Temperature dependence of (a) thermal conductivity, κ, (b) figure of merit, ZT, and power factor, S2/ρ (inset).

cantly for Ag8SnSe6, due to increasing n at high temperatures (Figure 5c). Such higher S values are mainly attributed to the large Eg and lower n value which is shown in Figure 5c. Clearly, n for Ag8SnSe6 (∼4.7 × 1016 cm−3) is one order higher compared to Ag8GeSe6 (∼0.6 × 1016 cm−3) at 370 K (Table 1). The value of κ(T) is extremely low (0.12−0.45 W m−1 K−1) throughout the temperature range (shown in Figure 6a), for both the samples. The κe contribution to the total κ has been estimated using the Wiedemann−Franz law (κe = LT/ρ, where L is the Lorenz number, calculated from measured S value45) and found to be very negligible in comparison to κl, because of very small n. Thus, we chose to show total thermal conductivity in Figure 6a, which is still extremely low in comparison to superionic conductors and many state of the art TE materials.23,24,26,46 However, the κ slightly increases near to phase transition and then remains almost temperature independent because of mobile Ag ions which restrict the expected 1/T dependence of phonon−phonon scattering.47,48 The inherently poor κ can be understood based on the larger unit cells and anharmonicity in the crystals. The larger unit cell is expected to affect the heat capacity and in turn the lattice part κl = 1/3Cvval, where Cv is the heat capacity at constant volume, va is the average sound velocity, and l is the phonon mean free path. We have measured the low-temperature heat capacity (Section S2), and we found the electronic term, φ, in C/T versus T2 curve, is negative and thus simple Debye model is not sufficient to explain the heat capacity data. Considering the fact that argyrodites have weakly coupled binding between Ag and Se atoms, some of the low energy modes are expected to appear. To understand further, we have tested the heat capacity data by fitting first with one Debye and one Einstein modes (Figure S2 and Table S3) and second with one Debye and two Einstein modes (Figure S3 and Table S4). More details of the fitting and heat capacity data are presented in Supporting Information S2. The Einstein modes signifies the low energy optical phonon modes due to existence of weak chemical bonds, which is also observed in skutterudites and many other liquid like materials.29,49 Here, the presence of such low energy optical modes reduce the heat transfer via strong scattering with heat carriers acoustical modes. The Einstein temperatures are almost comparable in both the samples, thus the poor κ is justified with low lying optical modes in heat capacity. We also have estimated the elastic constants and Gruneisen parameter, (Table S5), and the observed large Gruneisen parameter values represents the lattice anharmonicity resulting to poor κl values in both

samples. The large unit cell should have a number of optical phonon modes with low velocity having less contribution to heat transport. Furthermore, these optical modes interacts with acoustic phonons and blocks heat transfer, which ultimately support in decoupling the charge and heat transport. Now, we will discuss on the overall TE performance. As stated earlier, that controlling the composition and with a suitable element the S2/ρ can be enhanced for these inherently poor thermal conductors. On the basis of the obtained ρ(T) and S(T), the S2/ρ is estimated and plotted in Figure 6b inset. The maximum S2/ρ values obtained at 623 K are ∼51 μW m−1 K−2 (for Ag8GeSe6) and ∼392 μW m−1 K−2 (for Ag8SnSe6). The relatively high S2/ρ for Ag8SnSe6 is mainly ascribed to the poor ρ and high μ value (shown in Figure 5c and d). Combining the obtained S2/ρ and κ, the final ZT of Ag8GeSe6 and Ag8SnSe6 are calculated and plotted in Figure 6b. Therefore, because of poor ρ and higher μ values of Ag8SnSe6 than Ag8GeSe6, the higher S2/ρ values in turn reflected in enhanced ZT ∼ 0.62 at 623 K for Ag8SnSe6. In conclusion, we have demonstrated that Ag8SnSe6 has a better TE performance over Ag8GeSe6 with 8 time higher S2/ρ at 623 K. We emphasize that the improved electrical transport properties of Ag8SnSe6 has been understood by higher value of n, large electronegativity difference between Ge and Sn and high μ values in high temperature cubic phase. Further, the appearance of low energy Einstein optical modes and weakly bonded disordered Ag cation to fixed anion sub lattice justifies the poor κ. Consequently, the TE performance of cubic Ag8SnSe6 is reported with a maximum ZT ∼ 0.62 at 623 K, which is 10 times higher than Ag8GeSe6.

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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/acsaem.8b01660.

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Structural and compositional analysis, heat capacity analysis, elastic parameters, heating−cooling cycle data for thermal measurement, and thermal diffusivity data of Ag8GeSe6 and Ag8SnSe6 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ajay Soni: 0000-0002-8926-0225 E


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(19) Shi, X.; Yang, J.; Bai, S.; Yang, J.; Wang, H.; Chi, M.; Salvador, J. R.; Zhang, W.; Chen, L.; Wong-Ng, W. On the Design of HighEfficiency Thermoelectric Clathrates through a Systematic CrossSubstitution of Framework Elements. Adv. Funct. Mater. 2010, 20, 755−763. (20) Ahmed, F.; Tsujii, N.; Mori, T. Thermoelectric Properties of CuGa1‑xMnxTe2: Power Factor Enhancement by Incorporation of Magnetic Ions. J. Mater. Chem. A 2017, 5, 7545−7554. (21) Plirdpring, T.; Kurosaki, K.; Kosuga, A.; Day, T.; Firdosy, S.; Ravi, V.; Snyder, G. J.; Harnwunggmoung, A.; Sugahara, T.; Ohishi, Y.; Muta, H.; Yamanaka, S. Chalcopyrite CuGaTe2: A High-Efficiency Bulk Thermoelectric Material. Adv. Mater. 2012, 24, 3622−3626. (22) He, Y.; Lu, P.; Shi, X.; Xu, F.; Zhang, T.; Snyder, G. J.; Uher, C.; Chen, L. Ultrahigh Thermoelectric Performance in Mosaic Crystals. Adv. Mater. 2015, 27, 3639−3644. (23) Liu, H.; Shi, X.; Xu, F.; Zhang, L.; Zhang, W.; Chen, L.; Li, Q.; Uher, C.; Day, T.; Snyder, G. J. Copper Ion Liquid-Like Thermoelectrics. Nat. Mater. 2012, 11, 422−425. (24) Day, T.; Drymiotis, F.; Zhang, T.; Rhodes, D.; Shi, X.; Chen, L.; Snyder, G. J. Evaluating the Potential for High Thermoelectric Efficiency of Silver Selenide. J. Mater. Chem. C 2013, 1, 7568−7573. (25) Ferhat, M.; Nagao, J. Thermoelectric and Transport Properties of β-Ag2Se Compounds. J. Appl. Phys. 2000, 88, 813−816. (26) Pei, Y.; Heinz, N. A.; Snyder, G. J. Alloying to Increase the Band Gap for Improving Thermoelectric Properties of Ag2Te. J. Mater. Chem. 2011, 21, 18256−18260. (27) Qiu, P. F.; Wang, X. B.; Zhang, T. S.; Shi, X.; Chen, L. D. Thermoelectric Properties of Te-Doped Ternary Cuagse Compounds. J. Mater. Chem. A 2015, 3, 22454−22461. (28) Bhattacharya, S.; Basu, R.; Bhatt, R.; Pitale, S.; Singh, A.; Aswal, D. K.; Gupta, S. K.; Navaneethan, M.; Hayakawa, Y. CuCrSe2: A High Performance Phonon Glass and Electron Crystal Thermoelectric Material. J. Mater. Chem. A 2013, 1, 11289−11294. (29) Liu, H.; Yang, J.; Shi, X.; Danilkin, S. A.; Yu, D.; Wang, C.; Zhang, W.; Chen, L. Reduction of Thermal Conductivity by Low Energy Multi-Einstein Optic Modes. Journal of Materiomics 2016, 2, 187−195. (30) Li, W.; Lin, S.; Ge, B.; Yang, J.; Zhang, W.; Pei, Y. Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6. Advanced Science 2016, 3, 1600196. (31) Kuhs, W. F.; Nitsche, R.; Scheunemann, K. The Argyrodites  a New Family of Tetrahedrally Close-Packed Structures. Mater. Res. Bull. 1979, 14, 241−248. (32) Chen, M.; Yin, X.; Reddy, M. V.; Adams, S. All-Solid-State MoS2/Li6PS5Br/In-Li Batteries as a Novel Type of Li/S Battery. J. Mater. Chem. A 2015, 3, 10698−10702. (33) Alieva, Z. M.; Bagkheri, S. M.; Alverdiev, I. J.; Yusibov, Y. A.; Babanly, M. B. Phase Equilibria in the Pseudoternary System Ag2SeAg8GeSe6-Ag8SnSe6. Inorg. Mater. 2014, 50, 981−986. (34) Li, L.; Liu, Y.; Dai, J.; Hong, A.; Zeng, M.; Yan, Z.; Xu, J.; Zhang, D.; Shan, D.; Liu, S.; Ren, Z.; Liu, J.-M. High Thermoelectric Performance of Superionic Argyrodite Compound Ag8SnSe6. J. Mater. Chem. C 2016, 4, 5806−5813. (35) Davis, E. A.; Mott, N. F. Conduction in Non-Crystalline Systems V. Conductivity, Optical Absorption and Photoconductivity in Amorphous Semiconductors. Philos. Mag. 1970, 22, 0903. (36) López, R.; Gómez, R. Band-Gap Energy Estimation from Diffuse Reflectance Measurements on Sol−Gel and Commercial TiO2: A Comparative Study. J. Sol-Gel Sci. Technol. 2012, 61, 1−7. (37) Bendorius, R.; Iržikevičius, A.; Kindurys, A.; Tsvetkova, E. V. The Absorption Spectra of Ag8MIVSe6 and Ag8Gex 6VI Compounds. Physica Status Solidi (a) 1975, 28, K125−K127. (38) Chekaylo, M. V.; Ukrainets, V. O.; Il’chuk, G. A.; Pavlovsky, Y. P.; Ukrainets, N. A. Differential Thermal Analysis of Ag−Ge−Se, Ge− Se Charge Materials in the Process of Their Heating and Ag8GeSe6, GeSe2 Compound Synthesis. J. Non-Cryst. Solids 2012, 358, 321−327. (39) Edwards, T. G.; Sen, S. Structure and Relaxation in Germanium Selenide Glasses and Supercooled Liquids: A Raman Spectroscopic Study. J. Phys. Chem. B 2011, 115, 4307−4314.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S. would like to acknowledge Board of Research in Nuclear Sciences, India, for young scientist research award (37(3)/14/ 02/2015/BRNS), Department of Science and Technology Science and Engineering Board (DST-SERB), India (YSS/ 2014/001038) and Indian Institute of Technology Mandi for research facility.

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REFERENCES

(1) Slack, G. A.; Rowe, D. M. CRC Handbook of Thermoelectrics; CRC Press, 1995. (2) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105−114. (3) Rowe, D. M. Thermoelectrics Handbook: Macro to Nano; CRC/ Taylor & Francis, 2005. (4) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States. Science 2008, 321, 554−557. (5) Zhang, Q.; Liao, B.; Lan, Y.; Lukas, K.; Liu, W.; Esfarjani, K.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z. High Thermoelectric Performance by Resonant Dopant Indium in Nanostructured Snte. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 13261−13266. (6) Acharya, S.; Anwar, S.; Mori, T.; Soni, A. Coupling of Charge Carriers with Magnetic Entropy for Power Factor Enhancement in Mn Doped Sn1.03Te for Thermoelectric Applications. J. Mater. Chem. C 2018, 6, 6489−6493. (7) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of Electronic Bands for High Performance Bulk Thermoelectrics. Nature 2011, 473, 66−69. (8) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125−6135. (9) Minnich, A. J.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Bulk Nanostructured Thermoelectric Materials: Current Research and Future Prospects. Energy Environ. Sci. 2009, 2, 466−479. (10) Soni, A.; Yanyuan, Z.; Ligen, Y.; Aik, M. K. K.; Dresselhaus, M. S.; Xiong, Q. Enhanced Thermoelectric Properties of Solution Grown Bi2Te3−xSex Nanoplatelet Composites. Nano Lett. 2012, 12, 1203− 1209. (11) Liu, W.; Yan, X.; Chen, G.; Ren, Z. Recent Advances in Thermoelectric Nanocomposites. Nano Energy 2012, 1, 42−56. (12) Wang, H.; Hwang, J.; Snedaker, M. L.; Kim, I.-h.; Kang, C.; Kim, J.; Stucky, G. D.; Bowers, J.; Kim, W. High Thermoelectric Performance of a Heterogeneous PbTe Nanocomposite. Chem. Mater. 2015, 27, 944−949. (13) Beekman, M.; Morelli, D. T.; Nolas, G. S. Better Thermoelectrics through Glass-Like Crystals. Nat. Mater. 2015, 14, 1182− 1185. (14) Acharya, S.; Pandey, J.; Soni, A. Soft Phonon Modes Driven Reduced Thermal Conductivity in Self-Compensated Sn1.03Te with Mn Doping. Appl. Phys. Lett. 2016, 109, 133904. (15) May, A. F.; Toberer, E. S.; Saramat, A.; Snyder, G. J. Characterization and Analysis of Thermoelectric Transport in N-Type Ba8Ga16‑xGe30+x. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 125205. (16) Khovaylo, V. V.; Korolkov, T. A.; Voronin, A. I.; Gorshenkov, M. V.; Burkov, A. T. Rapid Preparation of InxCo4Sb12 with a RecordBreaking ZT = 1.5: The Role of the in Overfilling Fraction Limit and Sb Overstoichiometry. J. Mater. Chem. A 2017, 5, 3541−3546. (17) Rull-Bravo, M.; Moure, A.; Fernandez, J. F.; Martin-Gonzalez, M. Skutterudites as Thermoelectric Materials: Revisited. RSC Adv. 2015, 5, 41653−41667. (18) Nolas, G. S.; Cohn, J. L.; Slack, G. A.; Schujman, S. B. Semiconducting Ge Clathrates: Promising Candidates for Thermoelectric Applications. Appl. Phys. Lett. 1998, 73, 178−180. F


Article

ACS Applied Energy Materials (40) Taube, A.; Łapińska, A.; Judek, J.; Wochtman, N.; Zdrojek, M. Temperature Induced Phonon Behaviour in Germanium Selenide Thin Films Probed by Raman Spectroscopy. J. Phys. D: Appl. Phys. 2016, 49, 315301. (41) Smith, A. J.; Meek, P. E.; Liang, W. Y. Raman Scattering Studies of SnS2 and SnSe2. J. Phys. C: Solid State Phys. 1977, 10, 1321−1333. (42) Pandiaraman, M.; Soundararajan, N. Micro-Raman Studies on Thermally Evaporated Ag2Se Thin Films. Journal of Theoretical and Applied Physics 2012, 6, 7. (43) Studenyak, I.; Rushchanskii, K.; Buchuk, R. Y.; Stephanovich, V. Phonon Spectra of Cu6PS5Br Superionic Ferroelastic: Experimental and Theoretical Studies. Condens. Matter Phys. 2007, 10, 11−16. (44) Jiang, B.; Qiu, P.; Chen, H.; Zhang, Q.; Zhao, K.; Ren, D.; Shi, X.; Chen, L. An Argyrodite-Type Ag9GaSe6 Liquid-Like Material with Ultralow Thermal Conductivity and High Thermoelectric Performance. Chem. Commun. 2017, 53, 11658−11661. (45) Kim, H.-S.; Gibbs, Z. M.; Tang, Y.; Wang, H.; Snyder, G. J. Characterization of Lorenz Number with Seebeck Coefficient Measurement. APL Mater. 2015, 3, 041506. (46) 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, 634−638. (47) Heep, B. K.; Weldert, K. S.; Krysiak, Y.; Day, T. W.; Zeier, W. G.; Kolb, U.; Snyder, G. J.; Tremel, W. High Electron Mobility and Disorder Induced by Silver Ion Migration Lead to Good Thermoelectric Performance in the Argyrodite Ag8SiSe6. Chem. Mater. 2017, 29, 4833−4839. (48) Weldert, K. S.; Zeier, W. G.; Day, T. W.; Panthöfer, M.; Snyder, G. J.; Tremel, W. Thermoelectric Transport in Cu7PsSe6 with High Copper Ionic Mobility. J. Am. Chem. Soc. 2014, 136, 12035−12040. (49) Dimitrov, I. K.; Manley, M. E.; Shapiro, S. M.; Yang, J.; Zhang, W.; Chen, L. D.; Jie, Q.; Ehlers, G.; Podlesnyak, A.; Camacho, J.; Li, Q. Einstein Modes in the Phonon Density of States of the SingleFilled Skutterudite Yb0.2Co4Sb12. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 174301.

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