High Electron Mobility and Disorder Induced by Silver Ion Migration

Article pubs.acs.org/cm

High Electron Mobility and Disorder Induced by Silver Ion Migration Lead to Good Thermoelectric Performance in the Argyrodite Ag8SiSe6 Barbara K. Heep,‡,¶ Kai S. Weldert,‡,¶,† Yasar Krysiak,‡ Tristan W. Day,§ Wolfgang G. Zeier,# Ute Kolb,‡ G. Jeffrey Snyder,*,§ and Wolfgang Tremel*,‡ ‡

Institut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg 10-14, D-55128 Mainz, Germany † Graduate School Materials Science in Mainz, Johannes Gutenberg-Universität, Staudingerweg 9, 55128 Mainz, Germany § Department of Material Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States # Physikalisch-Chemisches Institut, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring-17, 35392 Giessen, Germany ABSTRACT: Superionic chalcopyrites have recently attracted interest in their use as potential thermoelectric materials because of extraordinary low thermal conductivities. To overcome longterm stability issues in thermoelectric generators using superionic materials at evaluated temperatures, materials need to be found that show good thermoelectric performance at moderate temperatures. Here, we present the structural and thermoelectric properties of the argyrodite Ag8SiSe6, which exhibits promising thermoelectric performance close to room temperature.

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INTRODUCTION Thermoelectric materials open up possibilities for waste heat recovery and cooling applications depending on the materials temperature regimes. Above 800 °C Si−Ge-alloys and Zintl phases exhibit very good thermoelectric efficiencies.1−4 Between 500 and 800 °C PbTe and PbSe, which have been studied for more than 50 years, still show the best thermoelectric performances.5−8 However, the high toxicity and the mechanical properties of lead chalcogenides preclude wide applications. Therefore, current research is directed for example on skutterudites, half-Heusler compounds and TAGS materials,9−15 as alternatives with high thermoelectric efficiencies mostly in the mid temperature range of 100−500 °C. However, thermoelectric materials are needed for moderate temperature regimes between room temperature and 150 °C, where, so far, bismuth telluride-based materials show the most superior thermoelectric efficiencies. Since the first Peltier cooling devices were developed over 60 years ago using bismuth telluride the zT of this compound has been steadily improved. The peak zT of typical p-type bulk bismuth-telluride was raised from about 0.5 to 1 with bismuth antimony telluride alloys.16−18 Nanostructured alloys with dense dislocations even show zT values up to 1.86 at 300 K.19 However, the very high materials cost as well as the toxicity make bismuth telluride based thermoelectric generators too expensive for commercial use, which is further limited by low operation temperatures. Finding alternative materials with a similar efficiency to bismuth telluride is very challenging, as most materials exhibit high zT values only at elevated temperatures. © 2017 American Chemical Society

During the past decade, superionic conductors have emerged as promising candidates for thermoelectric energy conversion. Binary copper and silver chalcogenides like Cu2S, Cu2Se and Ag2Se show good thermoelectric performances at high temperatures due to very low thermal conductivities.20−25 Because of long-term stability issues at evaluated temperatures,26 research has focused on improving the thermoelectric properties of materials with ionic mobilities at low temperatures.27 Therefore, more complex materials such as Cu and Ag Argyrodites have been investigated that exhibit good thermoelectric performances because of their very low thermal conductivities.28 The materials inherent low lattice thermal conductivity has recently been attributed to the low sound velocities.29,30 In this work, we present the structural and thermoelectric properties of Ag8SiSe6, which shows a promising figure of merit at room temperature because of its low lattice thermal conductivity (0.5 W/mK) and its high electronic mobility (1500 cm2/(V s)). In addition, Ag8SiSe6 does not crystallize in the fast-ionic conducting cubic phase below 370 K and could therefore be expected to be a thermoelectric material without the detrimental effects of ion migration as seen in other superionic conductors. The combination of migration stability, high electronic mobility and low lattice thermal conductivity in Ag8SiSe6 show the class of argyrodites to be a highly promising Received: February 22, 2017 Revised: May 11, 2017 Published: May 12, 2017 4833

DOI: 10.1021/acs.chemmater.7b00767 Chem. Mater. 2017, 29, 4833−4839

Article

Chemistry of Materials

beam of 50−100 nm in diameter on the sample (NED setting). Crystal position tracking was performed in microprobe STEM mode and NED patterns were acquired sequentially in steps of 1° in a tilt range of 81°. The ADT3D software package was used for three-dimensional electron diffraction data processing.36

candidate for thermoelectric applications at moderate temperatures.

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EXPERIMENTAL SECTION

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Synthesis. Bulk samples of polycrystalline Ag8SiSe6 were prepared by melting and annealing techniques using elemental powders of Ag (Alfa Aesar, 99.999%) and Se (Alfa Aesar, 99.999%), as well as Si pieces (Alfa Aesar, 99.9999%), which were ground into a fine powder before use. The phase purity of the starting materials was verified by Xray diffraction, and all synthetic procedures were carried out in a N2 drybox. To ensure dry conditions, the synthesis was performed in evacuated quartz ampules, which were dried for several hours at 1073 K under dynamic vacuum before usage. For the synthesis of Ag8SiSe6, the starting elements were thoroughly ground, sealed in quartz ampules and heated to 1173 K for 12 h. The obtained chunks were crushed, ground to a fine powder, sealed in quartz ampules again, and reannealed at 773 K for 72 h. The second annealing step was necessary to prevent the formation of the binary byproduct. All procedures were carried out in horizontal tube furnaces with heating and cooling rates of 5 K/min. The obtained powders were manually ground and consolidated into 1−1.5 mm thick, 10 mm diameter pellets at 473 K for 24 h under a force of 50−75 kN by hot pressing in steel dies. The resulting discs had more than 95% theoretical density, determined from the geometric density. Characterization. High-resolution synchrotron powder diffraction data were collected using beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory, using an average wavelength of 0.414621 Å. Discrete detectors covering an angular range from −6 to 16° 2θ are scanned over a 34° 2θ range, with data points collected every 0.001° 2θ and scan speed of 0.01°/s. Pawley fits were performed with TOPAS Academic V5.0 applying the fundamental parameter approach.31 As a result of the high absorption coefficient μR of Ag8SiSe6 the sample had to be diluted. Ground borosilcate capillaries were used for dilution. The thermal diffusivity α was measured with a Netzsch laser flash diffusivity instrument (LFA 457) under continuous argon flow. In order to maximize the emissivity, samples were spray-coated with a thin layer of graphite before the measurement. Thermal conductivity was determined via κ = αCpd with the heat capacity Cp and the geometric density d. Cp was estimated using the Dulong−Petit approximation (Cp = 3kB per atom), and the theoretical densities were calculated from the molar mass and the refined lattice parameters. Electrical transport was characterized via measurements of the Seebeck coefficient, Hall coefficient and electrical resistivity. Electrical resistivity and Hall coefficients were measured simultaneously using the van der Pauw technique with pressureassisted contacts under dynamic vacuum.32 The Seebeck coefficient was calculated from the slope of the voltage versus temperature gradient measurements from chromel-Nb thermocouples under dynamic vacuum with a maximum ΔT of 7.5 K for all temperatures.33 Simultaneous thermogravimetry and differential thermal analysis (TGDTA) have been performed with a Netzsch STA 449 F3 Jupiter device between room temperature and 875 at 10 K/min, under argon flow, to ensure phase stability at high temperatures For transmission electron microscopy (TEM) and automated diffraction tomography (ADT) investigations, powdered samples were dispersed in hexane using an ultrasonic bath and sprayed on carboncoated copper grid using a sonifier.34 TEM and ADT measurements were carried out with a FEI TECNAI F30 S-TWIN transmission electron microscope at 300 kV equipped with a field emission gun. TEM images and nano electron diffraction (NED) patterns were acquired with a CCD camera (16-bit 4096 × 4096 pixel GATAN ULTRASCAN4000) using the Gatan Digital Micrograph software DM3. Scanning transmission electron microscopy (STEM) images were collected by a FISCHIONE high-angular annular dark field (HAADF) detector and acquired by Emispec ES Vision software. Three-dimensional electron diffraction data were collected with a FISCHIONE tomography holder using an automated acquisition module developed for FEI microscopes.35 A condenser aperture of 10 μm and mild illumination settings were used to produce a semiparallel

RESULTS AND DISCUSSION Chemical Characterization. The crystal structure of Ag 8 SiSe 6 has been investigated previously. 37−40 As a representative example of the argyrodite structure type the Ag8SiSe6 structure is based on a face centered cubic close packing of Se2− anions, with additional Se2− anions occupying half of the tetrahedral voids and [SiSe4]4− units at the octahedral sites as illustrated in Figure 1. This anionic

Figure 1. Crystal structure of fully disordered, face-center-cubic (F4̅3m) Ag8SiSe6 (Se orange, Si blue spheres). The left cell shows the [SiSe6]8− anion sublattice. Se2−-ions form a cubic close packed framework with Se2− anions and [SiSe4]4− units in the tetrahedral voids. Ag+ cations (gray spheres) in the right cell are delocalized. For simplicity, all possible interstitial sites are shown with a 100% occupancy.

framework is stuffed with disordered Ag+ cations, with mobility along preferential diffusion paths within the [SiSe6] framework at higher temperatures. Ag8SiSe6 exhibits at least one structural phase transition.41,42 Above 370 K, Ag8SiSe6 crystallizes in a face-centered-cubic structure (F4̅3m) with a fully disordered cation sublattice. A partial localization of the Ag+ ions occurs below 370 K, which leads to a simple-cubic structure (P213) where partial Ag+ ordering occurs at the highest probability density sites of the high-temperature phase diffusion paths. The synchrotron powder diffraction data of the synthesized samples collected at room temperature is shown in Figure 2.

Figure 2. Synchrotron powder diffraction data for Ag8SiSe6 at 295 K; reflections corresponding to a simple cubic unit cell are indexed (experimental data, red circles), Pawley fit (black line), and corresponding difference plot (red line). 4834


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As expected, at high temperatures the reflections of the simple cubic argyrodite phase are observed, indicating that a reduced symmetry of the room temperature phase in Ag8SiSe6 is responsible for the additional reflections in the room temperature X-ray powder diffraction data. In other words, at lower temperatures, the Ag cations become localized leading to a superstructure due to the no longer mobile Ag+ cations.45 The structural distortions at lower temperatures is a common theme in the argyrodite phases. To gain further information about the structural features of the room temperature phase, electron diffraction experiments were carried out. The acquisition of an ADT data set from a single nanocrystal as shown in Figure 4b allows reconstruction

Positions and intensities of the main reflections can be assigned to the face-centered-cubic argyrodite phase. However, between 2Θ = 8° and 12° many additional reflections were observed, indicating that the symmetry of Ag8SiSe6 is lower than the symmetry of the room temperature phase of Cu7PSe6.43,44 Because of the large number of additional superstructure reflections it is not possible to solve the structure using synchrotron powder diffraction data. The major reflections can be indexed, as indicated in Figure 2, using the simple cubic room temperature structure of Cu7PSe6. Pawley fits were used in to obtain the unit cell size of a hypothetical cubic structure. The Pawley fit is shown in Figure 2 and leads to a unit cell size of 10.90(1) Å, which is reasonable as the unit cell size is slightly larger than the unit cell size of Ag7PSe6 with 10.77 Å,45 which contains one silver cation less per unit formula. In order to solve the room temperature structure of Ag8SiSe6, single crystal X-ray diffraction data is necessary. Unfortunately, the size and quality of the synthesized crystals was not sufficient enough to carry out a single crystal X-ray diffraction experiment. As it is not possible to determine the room temperature structure of Ag8SiSe6 based on that of the known α-, β-, and γAg8SiSe6 phases39 and corrobotate phase purity, high-temperature (380 K) synchrotron X-ray diffraction experiments were performed to show that these additional reflections are indeed due to the distorted structure of the room temperature polymorph with low symmetry. Figure 3 shows the collected

Figure 4. (a) Projection of the three-dimensional reciprocal lattice of the room-temperature phase. The cubic subcell is drawn in red, the supercell is implied in yellow. (b) Selected crystal for ADT measurements.

of the three-dimensional reciprocal space preventing reflection overlap. As indicated in red lines in Figure 4a a primitive cubic lattice could be found in the reciprocal volume, with lattice parameter a = 11.20(1) Å by taking into account only the strongest reflections. Strong additional reflections indicate a superlattice with two equal d-values of approximately 40.5 Å (see yellow line Figure 4a), presumably because of a partial ordering of statistically distributed silver ions distributed over a number of sites with distorted tetrahedral, trigonal, and linear coordination. These structural data show that Ag8SiSe6 exhibits the cubic lattice above 370 K with mobile Ag cations and superionic conduction. At room temperature, the Ag+ cations are localized, resulting in supercell reflections. The localized cations below 370 K suggest excellent stability against migration of cations (Soret effect) under an applied temperature gradient. Electronic Transport Properties. The temperaturedependent electrical resistivity of Ag8SiSe6 is shown in Figure 5a, and the temperature dependence of the Seebeck coefficients for two consecutive heating and cooling cycles are shown in Figure 5b. In the temperature region between 300 and 380 K a decrease of the electrical resistivity is observed, as expected for a semiconductor. Between 380 and 390 K a phase transition from the low symmetry room temperature phase to the simplecubic high temperature phase is observed with a rate-dependent hysteresis-like behavior for the heating and cooling data at the phase transition. The cation disordering during the heating process and the cation ordering during the cooling process appears gradually over a temperature range of 20 to 30 K. Above 410 K the hysteresis disappears. However, the temperature dependence of the electrical resistivity above the phase transition cannot be investigated as the sample starts to decompose above 440 to 450 K. At these temperatures, selenium evaporates, which reduces and decomposes the

Figure 3. High-temperature synchrotron powder diffraction data of Ag8SiSe6. The red dots represent the synchrotron data at 380 K, Rietveld refinement (black line), and corresponding difference plot (red line). Reflections could be indexed to the face-centered cubic cell with a = 10.9413(1) Å, Rwp = 7.3%, and GOF = 1.1. Additional reflections are marked with an asterix.

diffraction data at 380 K, above the phase transition. Most of the reflections can be indexed to the face-centered cubic phase (space group F4̅3m). With the unit cell parameter of a = 10.9413(1) Å, Rwp = 7.3%, and GOF = 1.1, the calculated data are in very good agreement with the reported parameters.39 Small additional reflections cannot be assigned to the starting elements, resulting binary compounds, related oxides or hydrates (see Figure 3). We assume that those reflections are caused by borosilicate glass that was used to dilute the sample because of the large absorption coefficient of Ag8SiSe6. As only the high intensity beam of the synchrotron can observe these reflections, these can be regarded as minor impurities that are not present in the samples used for transport measurements. 4835


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Figure 5. Electrical transport properties of Ag8SiSe6 including electrical resistivity (a), Seebeck coefficient (b), Hall mobility (c), and Hall carrier concentration (d). Circles represent heating data; squares represent cooling data. At 380 K a phase transition between the room temperature phase and the cubic high temperature phase is observed. Ag8SiSe6 shows extrinsic electron transport as a result of intrinsic defects.

material. The thermopower S increases from about −120 μV/K at room temperature to −90 μV/K at 430 K. The negative values indicate that electrons are the major charge carriers, resulting from intrinsic defects. The decreasing absolute values indicate that intrinsic conduction in this lightly doped semiconductor caused by the thermal excitation of charge carriers plays a role even at moderate temperatures as a result of a small band gap. An Arrhenius plot (not shown) estimated from the electrical resistivity data below the phase transition is compatible with a small band gap of approximately 0.1 eV, but the accuracy of this might be limited because of the small investigated temperature interval. It is not possible to extract the Goldsmid−Sharp band gap because of the lack of a Seebeck maximum.47 In contrast to Cu7PSe6 the overall values of the electrical resistivity are much smaller.28 Temperature-dependent Hall measurements were performed to obtain Hall carrier mobilities μH and Hall carrier concentrations nH. In contrast to Cu7PSe6, the Hall carrier mobilities are extremely large and Hall coefficient is negative indicating n-type conduction (Figure 5c). The carrier mobilities at room temperature are around 1500 cm2 (V s)−1 and decrease with increasing temperature. These values are, to the best of our knowledge, the highest mobilities reported for this class of thermoelectric materials and only surpassed by CuAgSe.48 At the phase transition the Hall mobilities drop to 500 cm2 (V s)−1, which may be attributed to the increase of disorder in the cation sublattice leading to additional charge carrier scattering. The Hall carrier concentration shows a slightly

increasing trend from 300 to 350 K (Figure 5d), indicative of the lightly doped nature of the material. At higher temperatures, the Hall carrier concentration is around 1019 cm−3. At first glance it seems suspicious that the phase transition affects the Hall data and not the Seebeck coefficient. However, for superionic phase transitions the change in the Seebeck coefficient at the transition is expected to occur in a very small temperature range of ∼5 K only, and is hence not reflected in the steps measured here.49 Thermal Transport Properties. The total thermal conductivity of Ag8SiSe6 is shown in Figure 6. The absolute values are around 0.9 W (K m)−1 before the phase transition and drop to 0.75 W (K m)−1 after the phase transition. However, the electrical resistivity in Ag8SiSe6 is much lower compared to the electric resistivity in Cu7PSe6; therefore, the electronic contribution to the total thermal conductivity is a significant over the whole temperature range investigated. The electronic contribution to the thermal conductivity was estimated using the Wiedemann−Franz law (κel = LTρ−1), with L = 1.81 × 10−8 W Ω K−2 calculated from the Seebeck coefficient.50 The obtained values of the lattice contribution to the thermal conductivities are around 0.4 W (K m)−1 at room temperature and decrease slightly to around 0.3 W (K m)−1 at higher temperatures. These values are similar to the lattice thermal conductivities in Cu7PSe6, and the drop in the total thermal conductivity can be attributed to the electronic contribution to the lattice thermal conductivity. In addition, the lattice thermal conductivity slightly increases at the phase 4836


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Figure 6. Thermal conductivity κ in Ag8SiSe6. Circles represent heating data, squares represent cooling data.

Figure 7. Temerature dependence of the dimensionsless figure of merit zT.

transition and then remains temperature independent. This flat dependence is a clear sign of mobile Ag ions, which disturb the usual 1/T dependence from phonon−phonon scattering. This change in the temperature dependence of the lattice thermal conductivity can be seen as indirect proof that Ag ions are not mobile below the phase-transition temperature of 370 K. In the case of Ag8SiSe6 the combination of a low symmetry crystal structure and high cation disorder leads to these observed low lattice thermal conductivities. The values of the lattice contribution to the thermal conductivities are significantly lower than those for most of the state of the art thermoelectric materials, which usually are ≥1 W (K m)−1.2,5,7−10,13,20 Most crystalline solid materials have a temperature-dependent thermal conductivity proportional to T−1 at high temperatures, indicating Umklapp-scattering to be the major scattering mechanism.51 Highly disordered materials like glasses often show less temperature dependence of the thermal conductivity.52 For the argyrodite Cu7PSe628 and Ag8SnSe630 the superionic character leads to a very high positional disorder resulting in very low, temperature independent lattice thermal conductivities. The liquid-like behavior of the mobile Ag+ ions is responsible for a very short phonon mean free path, which results in κlat values in range of the glass limit. In addition, the low thermal conductivity can be attributed to the very large unit cell of Ag8SiSe6, composed of 60 × 4 = 240 atoms. This leads to a larger number of optical modes with low group velocities, and thus small contributions to the thermal conductivity.53 Figure of Merit. Figure 7 shows the figure of merit zT of Ag8SiSe6 as a function of the temperature. The maximum zT of 0.6 to 0.8 at 300 to 350 K is very promising as exclusively an intrinsic n-type doped bulk material was studied. This value is comparable to the zT value for nonoptimized bulk Bi2Te3 at room temperature, which is around 0.8.16 To analyze the transport and predict the optimum carrier densities for a maximum figure of merit, a single parabolic band model has been applied to the thermoelectric transport data of Ag8SiSe6.46 The resulting prediction of zT versus Hall carrier concentration is shown in Figure 8. Assuming only acoustic phonon scattering to limit the carrier mobility, Ag8SiSe6 is predicted to reach a maximum zT of 1.1 at 300 K. This value is comparable to the state-of-the-art zT value for bulk Bi2−xSbxTe3 at room temperature,54 Ag8SiSe6 intrinsically exhibits a Hall carrier concentration of 5 × 1018 carriers per cm3, much greater than the optimum value of 8 × 1017

Figure 8. Single parabolic band model of the transport data for Ag8SiSe6 leading to estimates of the effective mass m* and the mobility parameter μ0 that allow to predict the thermoelectric figure of merit zT for different carrier concentrations. The model assumes acoustic phonon scattering limits the carrier mobility. Reducing the electroncarrier concentration in Ag8SiSe6 by 1 order of magnitude is predicted to result in a maximum zT of 1.1 at 300 K.

cm−3. Doping this material with an electron acceptor may reduce the amount of electrons in Ag8SiSe6 and therefore lower the carrier concentrations into a range of much higher figures of merit. A similar strategy has recently been shown successful in Nb doped argyrodites.29 However, as recently shown for Cu2−xSe,27 lowering the carrier concentration may affect the 4837


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(6) Bell, L. E. Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 2008, 321, 1457−1461. (7) LaLonde, A. D.; Pei, Y.; Snyder, G. J. Reevaluation of PbTe1−xIx as high performance n-type thermoelectric material. Energy Environ. Sci. 2011, 4, 2090−2096. (8) Zhang, Q.; Cao, F.; Liu, W.; Lukas, K.; Yu, B.; Chen, S.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z. Heavy doping and band engineering by potassium to improve the thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe1‑ySey. J. Am. Chem. Soc. 2012, 134, 10031− 10038. (9) Visnow, E.; Heinrich, C. P.; Schmitz, A.; de Boor, J.; Leidich, P.; Klobes, B.; Hermann, R. P.; Muller, W. E.; Tremel, W. On the True Indium Content of In-Filled Skutterudites. Inorg. Chem. 2015, 54, 7818−7827. (10) Sales, B. C.; Mandrus, D.; Williams, R. K. Filled Skutterudite Antimonides: A New Class of Thermoelectric Materials. Science 1996, 272, 1325−1328. (11) Zeier, W. G.; Schmitt, J.; Hautier, G.; Aydemir, U.; Gibbs, Z. M.; Felser, C.; Snyder, G. J. Engineering half-Heusler thermoelectric materials using Zintl chemistry. Nat. Rev. Mater. 2016, 1, 16032− 16041. (12) Tang, Y.; Gibbs, Z. M.; Agapito, L. A.; Li, G.; Kim, H.-S.; Nardelli, M. B.; Curtarolo, S.; Snyder, G. J. Convergence of multivalley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites. Nat. Mater. 2015, 14, 1223− 1228. (13) Shi, X.; Yang, J.; Salvador, J. R.; Chi, M.; Cho, J. Y.; Wang, H.; Bai, S.; Yang, J.; Zhang, W.; Chen, L. Multiple-filled skutterudites: high thermoelectric figure of merit through separately optimizing electrical and thermal transports. J. Am. Chem. Soc. 2011, 133, 7837−7846. (14) Skrabek, E. A.; Trimmer, D. S. In CRC Handbook of Thermoelectrics: Properties of the General TAGS System; Rowe, D. M., Ed.; CRC Press: Boca Raton, FL, 1995; Chapter 22. (15) Thompson, A. J.; Sharp, J. W.; Rawn, C. J. Microstructure and Crystal Structure in TAGS Compositions. J. Electron. Mater. 2009, 38, 1407−1411. (16) Smith, G. E.; Wolfe, R. Thermoelectric Properties of BismuthAntimony Alloys. J. Appl. Phys. 1962, 33, 841−846. (17) Navrátil, J.; Starý, Z.; Plechácĕ k, T. Thermoelectric properties of p-type antimony bismuth telluride alloys prepared by cold pressing. Mater. Res. Bull. 1996, 31, 1559−1566. (18) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; et al. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008, 320, 634−638. (19) 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.; et al. Thermoelectrics. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics. Science 2015, 348, 109−114. (20) 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. (21) Ferhat, M.; Nagao, J. Thermoelectric and transport properties of β-Ag2Se compounds. J. Appl. Phys. 2000, 88, 813−816. (22) El Akkad, F.; Mansour, B.; Hendeya, T. Electrical and thermoelectric properties of Cu2Se and Cu2S. Mater. Res. Bull. 1981, 16, 535−539. (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) He, Y.; Day, T.; Zhang, T.; Liu, H.; Shi, X.; Chen, L.; Snyder, G. J. High thermoelectric performance in non-toxic earth-abundant copper sulfide. Adv. Mater. 2014, 26, 3974−3978. (25) Day, T. W.; Zeier, W. G.; Brown, D. R.; Melot, B. C.; Snyder, G. J. Determining conductivity and mobility values of individual components in multiphase composite Cu1.97Ag0.03Se. Appl. Phys. Lett. 2014, 105, 172103−172107.

scattering mechanism depending on the dopant, and could prevent the desired properties. Nevertheless, these data show the promising properties of Ag8SiSe6 and that successful doping strategies of reducing carrier densities need to be developed for a possible improvement of the thermoelectric transport.

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CONCLUSION We have synthesized the fast silver ion conducting argyrodite Ag8SiSe6 and determined its thermoelectric transport properties. While Ag8SiSe6 crystallizes in the argyrodite structure type above 370 K with a superionic Ag disorder, at room temperature the ions are localized because of a structural modulation. Ag8SiSe6 shows promising potential as a thermoelectric material because of low thermal conductivity arising from Ag disorder and this superstructure modulation. In addition to the low thermal conductivity, Ag8SiSe6 exhibits electron mobilities as high as 1800 cm2/(V s) leading to promising figures of merit around room temperature. The localized nature of the Ag+ cations should ultimately prevent thermodiffusion and long-term deterioration of this material due to the Soret effect. The combination of low thermal conductivity, high electron mobility and immobile cations at room temperature make Ag8SiSe6 and the class of argyrodites a candidate for thermoelectric cooling at room temperature.

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AUTHOR INFORMATION

Corresponding Authors

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

Wolfgang G. Zeier: 0000-0001-7749-5089 Wolfgang Tremel: 0000-0002-4536-994X Author Contributions ¶

B.K.H. and K.S.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support through the Excellence Initiative (DFG/GSC 266) is acknowledged by K. W., and support for Y. K. and U. K. came from the Stiftung für Innovation Rheinland-Pfalz. T. W. Day and G. J. Snyder acknowledge support from the U.S. Air Force Office of Scientific Research. We gratefully thank David Brown (CalTech) for fruitful discussions. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

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