Extremely Low Thermal Conductivity in Thermoelectric Ge0.55Pb0


In this contribution, we report that an extremely low lattice thermal conductivity can be achieved in a Ge0.55Pb0.45Te-based thermoelectric system thr...
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Extremely low thermal conductivity in thermoelectric Ge0.55Pb0.45Te solid solutions via Se substitution Junqin Li, Haijun Wu, Di Wu, Caiyan Wang, Zhiping Zhang, Yu Li, Fusheng Liu, Wei-Qin Ao, and Jiaqing He Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02772 • Publication Date (Web): 14 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016

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Extremely

low

thermal

conductivity

in

thermoelectric

Ge0.55Pb0.45Te solid solutions via Se substitution Junqin Li,†,* Haijun Wu,‡, § Di Wu,‡ Caiyan Wang,† Zhiping Zhang,† Yu Li,† Fusheng Liu,† Wei-qin Ao,† and Jiaqing He ‡,* †

College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, China



Department of Physics, South University of Science and Technology of China, Shenzhen 518055, China

§

Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117575, Singapore

The authors declare no conflict of interest. Corresponding Authors: [email protected] ; [email protected]

Abstract In this contribution, we report that an extremely low lattice thermal conductivity can be achieved in Ge0.55Pb0.45Te based thermoelectric system through gradually replacing Te by Se. It was revealed that substitution of Se promotes the solid solubility of Pb in GeTe, hence leading to a reduction of PbTe precipitation. The PbTe precipitation was found completely disappeared when 60% Te was replaced by Se. In the optimized-composition alloy (x=0.5), the substantial amount of solute Pb and Se atoms in GeTe-based phase, together with the twinning microstructures in GeTe-based matrix phase, enhance phonon scattering sufficiently, leading to the extremely low thermal conductivities (0.67 Wm-1K-1 at 300 K and 0.45 Wm-1K1

at 723 K), which are lowest ever reported. The maximum ZT of 1.55 at 723 K was obtained in the alloy

Ge0.55Pb0.45Te0.5Se0.5, 8 times higher than that of its Se-free counterpart Ge0.55Pb0.45Te.

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Introduction Thermoelectricity, which enables a direct inter-conversion between electrical energy and thermal energy, is capable of scavenging electric power from sources of waste heat and believed one of the most promising choices for relieving global energy problems. Meanwhile, its inverse conversion (from electricity to heat) can behave as an effective solid-state refrigerator for chilling up electronic devices from overheat, a common issue restricting the lifetime and reliability of electronic devices. The performance of a thermoelectric (TE) material is evaluated by a dimensionless figure of merit ZT=α2σT/(κe+κL), where α, σ, T, κe and κL are representing respectively the Seebeck coefficient, the electrical conductivity, the absolute temperature, the electronic and lattice thermal conductivity. However, an inevitable problem for now is that the commercial applications of thermoelectric materials are limited by the low ZT value.1 Improved ZT can be achieved either by enhancing the power factor or via reducing the thermal conductivity. Attempts of enhancing the power factor include carrier concentration optimization and/or band structure engineering (e.g. resonant states and band convergence),2-7 while thermal conductivity can be reduced through solid-solution alloying, the development of materials with intrinsically low thermal conductivity, or the recently realized nanostructuring.8 Continuously significant enhancements on ZT have been achieved in various TE systems via the above approaches.2-13 Considering the strong and intricate interdependence among α, σ and κe, microstructure optimization to extensively and intensively lower the solely independent parameter κL is more feasible for most TE systems. Thus how to finely tune the phase and microstructure to improve the performance in bulk TE materials is still an open issue, leading the tide of researches on thermoelectrics. Semiconductors based on germanium telluride GeTe is deemed as an appropriate candidate for TE applications in the intermediate temperature range from 500-900 K, just as lead telluride PbTe. Nevertheless, unlike PbTe, GeTe experiences a second-order phase transition from the low-temperature rhombohedral (R3m) to the high-temperature cubic (Fm-3m) structure at ~700 K.14 It was also noted that GeTe can still possess a good mechanical stability during this phase transition.15 GeTe exhibits relatively lower thermoelectric performance than PbTe due to its intrinsically high carrier concentration (1021 cm−3). 16 The general strategy to enhance the performance of GeTe is to alloy with a rock-salt component, e.g., AgSbTe2, 17 SnTe, and PbTe.18-19 The most famous example of GeTe-based TE material is AgSbTe2-GeTe (TAGS), which exhibits a peak ZT of 1.36 at 700 K mainly coming from its extremely low thermal conductivity.17 Gelbstein18 investigated the microstructural evolution upon the phase separation of SnTe-GeTe at 673 K and found that fine alternating laminates and twins are formed in the alloy during the phase separation. The pseudo-binary PbTe-GeTe phase diagram shows an unlimited mutual solubility of its end-members in both liquid state and high-temperature solid state, but phase separation driven by spinodal decomposition

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(miscibility gap) which appears below 587 oC (860 K)20,21 infers that reduction of lattice thermal conductivity can be achieved in this region by complicating the microstructures. Gorsse et al.19 reported that the microstructures be finely controlled through solution, quenching and aging treatment based on the spinodal decomposition in the PbTe-GeTe system. A high ZT was achieved in Ge0.87Pb0.13Te attributed to the nucleation of sub-micron phase-separated domains, twins with comparable size, and dislocation networks.22 Recently, upon the further introduction of 3 mol % Bi2Te3, a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te.[23,24] Bismuth promotes the solubility of Pb in the GeTe matrix, reducing the carrier concentration and enhancing the solid-solution phonon scattering, and thus leading to concurrent optimization of electrical and thermal transport properties.24 In the present work, we found that the substitution of Se for Te in PbTe-GeTe alloys can promotes the solid solubility of Pb in GeTe more sufficiently, leading to a substantial solution of Pb and Se atoms in the GeTe based lattice. These intensive Ge-Pb and Te-Se alloying, together with the twinning structures in GeTe-based phase, and the small amount of secondary PbTe and Ge nano-precipitates, enhanced phonon scattering sufficiently, leading to the lowest thermal conductivity ever reported in this system.

Materials synthesis and characterizations Samples with nominal compositions of Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8 were synthesized by the following procedures. Starting materials of Pb, Ge, Te and Se powders or chunks of purities > 99.99% were mixed in appropriate molar ratio in quartz tubes and sealed under vacuum of 3.0 × 10-3 Pa. The mixtures were heated at 1273 K for 20 h for complete reaction and homogeneity, and then slowly cooled to 873 K and held for 4 h followed by quenching in liquid nitrogen. The quenching process was used to the high-temperature (Pb, Ge)(Te,Se) unlimited solid solution state of the alloy to room temperature in this work based on the PbTe-GeTe pseudo-binary phase diagram. The obtained ingots were then pulverized by ball milling in a planetary ball miller at 200 rpm for 6 h. The weight ratio of balls to powders was kept at about 20:1, and the mill vial was evacuated and then filled with H2 to avoid possible oxidation during the milling process. The obtained powders were then consolidated into bulk by spark plasma sintering (SPS) at 773 K for 5 min under an axial pressure of 50 MPa. The relative densities of the consolidated samples were more than 95%. Bar specimens with dimensions of 12.0 mm × 5.0 mm × 5.0 mm were cut out for the electrical properties measurement, and disk-shape specimens with 10.0 mm × 2.0 mm (diameter × thickness) for the thermal conductivity measurement. Powder X-ray diffraction experiments were performed using Bruker D8 Advance SS/18kW diffractometer with Cu Kα radiation operating at 40 kV and 250 mA. The crystallographic phases of these samples were analyzed with JADE 5.0. The Rietveld refinements of the XRD patterns were performed using the Topas 3.1 software. The room temperature carrier concentration np and mobility µ were measured by Hall effect measurement along the length direction of the sample (ET9005-CCS1, East Chenjing, China). The

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Seebeck coefficient (S) and electrical conductivity (σ) measurements were carried out in a commercial ZEM-2 (Ulvac-Riko, Japan) along the length direction of the sample in a helium atmosphere. The thermal diffusivity and heat capacity were measured by a laser flash apparatus (ULVAC-RIKO TC-9000 ) along the thickness direction of the sample under vacuum of 1.0×10-3 Pa, with the uncertainty of 5 % for thermal diffusivity and heat capacity measurements. The bulk density of the sample was calculated from the sample’s geometry and mass. The thermal conductivity (κ) was calculated using the equation κ = λCpd, where λ is the thermal diffusivity, Cp the heat capacity, and d the bulk density of the sample. (Scanning) transmission electron microscopy ((S/TEM) and energy dispersive spectroscopy (EDS) were carried out using a Tecnai F30 microscope operated at 200 kV. TEM specimens were prepared with caution by conventional methods, including cutting, grinding, polishing, dimpling, Ar-ion milling with a liquid nitrogen cooling.

Results and Discussions The powder X-ray diffraction (XRD) patterns for the SPSed samples Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8 were shown in Fig. 1. It is seen that the Se-free sample is a combination of GeTebased rhombohedral and PbTe-based rock-salt phases. The lattice parameters and the amount of the phases in the alloys were refined by Rietveld refinement of their XRD patterns using software Topas 3.1. It refines the parameters by minimizing a quantity through the Newton-Raphson alorithm:25 n

x2 = ∑ i =1

1 {yi − y c,i (α )}2 yi

where yi is the observed intensity at a certain 2θ; yc,i is the calculated intensity at the same angle and α = (α1, α2, ……, αp) are the parameters to be refined. The calculated intensity yc,i can be calculated by

y c ,i = ∑ Sφ ∑ I φ ,h Ω(Ti − Tφ ,h ) φ

h

where Sφ is the scale factor of the phase φ; Iφ,h is the calculated intensity of the peak h of the phase φ based on the structure model and Ω(Ti-Tφ,h) is the reflection profile fuction. The percentage of the phase φ is proportional to its scale factor Sφ. The structure models and refinement results for the representative samples are shown in Fig.2 and table 1. The Se atoms were considered in Te atom sites randomly in both PbTe-based and GeTe-based phases. The atomic percentage of Ge atom in Pb atom site is 20 at. % for PbTe-based phase while that of the Pb atoms in Ge atom site is varied in GeTe-based phase based on the solid solubilities of both phases, which can be tuned via Se alloying. The low agreement factors Rwp and Rp indicate that the Rietveld refinements are satisfactory. The amount of the PbTe-based phase is 60.0 wt.% for the sample with x = 0.2, 4.0 wt.% for the sample with x = 0.5, shown in Fig. 2a and 2b, while it is further reduced to 0 for the sample with x = 0.6. The enlarge patterns for the samples with x = 0.5, 0.7 and 0.8 between 2θ = 25 to 50o are shown in the insert of Fig. 1 for identifying the phases more clearly

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with marks. The amount of the PbTe-based phase and GeTe-based phase in the samples with Se content x is shown in the insert figure of Fig. 2(b). The GeSe-based solid solution appeared in the sample with x = 0.7, while became a main phase in the sample with x = 0.8, shown in Fig. 1f and Fig. 2c. The PbSe-based phase also appears in the sample with x = 0.8. For x = 0.8 sample, the peaks cannot well match with the pure GeSe due to the heavy solution of Pb and Te in GeSe matrix. Therefore, the match XRD pattern of the GeSe-based solution in Fig. 1 was calculated with Diamond 3.1 software based on GeSe structure and the parameters obtained by Rietveld refinement. A small amount of Ge appears in some samples as can be seen from their XRD patterns for the reason that GeTe-based alloys are characterized by a deviation from stoichiometry toward tellurium.26 The main nonstoichiometric defects are doubly ionized metal vacancies. As a result of this deviation from stoichiometry, GeTe always exhibits p-type conduction, and excess Ge appears in the stoichiometry of GeTe-based alloys.14 From PbTe-GeTe phase diagram, the room temperature solubility of Pb in GeTe is around 5%,20,21 the substitution of Se for Te in Ge0.55Pb0.45Te alloys promotes the solid solubility of Pb in GeTe-based phase. The reason may be that the lattice contraction due to the substitution of the Te atoms (a.r.: 0.142 nm) by smaller Se atoms (a.r.: 0.122 nm) in GeTe may compensate the expansion when larger Pb atoms replace (a.r.: 0.181 nm) smaller Ge atoms (a.r.: 0.152 nm). As the Se content x reaches 0.6 in the sample Ge0.55Pb0.45Te1-xSex, all Pb atoms are dissolved in the GeTe-based solid solution. The substitution of Se for Te in Ge0.55Pb0.45Te1-xSex causes significant change of phases in the samples and thus influences their microstructures and thermoelectric properties. As seen in a later section, intense phonon scattering due to Te-Se and Pb-Ge alloying leads to a significant reduction of lattice thermal conductivity, both experimentally and simulated within the Callaway model. The positive Hall coefficients at room temperature for the samples Ge0.55Pb0.45Te1-xSex with x = 0.2, 0.5, 0.6 and 0.7 indicate that all the samples are of p-type conduction. Fig. 3 shows the carrier concentration np and mobility µ as a function of x. The Hall measurement wasn’t performed on the sample with x = 0.8 due to its poor thermoelectric properties with major GeSe-based phase. It is seen that the carrier concentration increases from 7.8 × 1018 cm-3 for x = 0.2 to 1.2 × 1019 cm-3 for x = 0.7, Fig. 3(a), which may be contributed to the increased cation vacancies due to the substitution of Se atoms for Te atoms in the alloy. The carrier mobility decreases from 400 cm2V-1S-1 for x = 0.2 to 118 cm2V-1S-1 for x = 0.7 due to the enhanced carriers scattering by dissolved Pb and Se impurities in GeTe. The further substitution of Se into Te site of PbTe-GeTe alloy can significantly enhance the solid solubility of Pb in Ge sites and thus reduce the carrier concentration of GeTe due to the reduction of the super-high vacancy concentration of GeTe by alloying with Pb; 24 such enhancement is obviously more intense than that of the introduction Bi into the same Ge sites as Pb. To illustrate it, Fig. 3b compares the carrier concentration np and mobility µ of Sesubstituted and Bi-substituted PbTe-GeTe alloys.24 the former ones exhibit much lower carrier concentra-

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tion (20 times less) than the latter ones, reflecting higher Pb solid solubility. The reduction of carrier concentration would benefit for the Seebeck coefficient enhancement, although deteriorate the electrical conductivity to some degree, and the overall electrical transport properties would still be improved, see later. The temperature dependence of electrical resistivity, Seebeck coefficient, power factor, and thermal conductivity for the alloys Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8 is shown in Fig. 4. The measurement data are repeatable. The representative repeated resistivities measured twice for the sample with x = 0.5, shown in Fig. 4(a) with label x = 0.5 and x = 0.5r, are in very good agreement. The electrical resistivity ρ and Seebeck coefficient S of Ge0.55Pb0.45Te1-xSex, shown in Fig. 4a and 4b, increase first and then decrease with elevating temperature, showing an abnormal temperature dependence of the electronic transport properties. The same phenomenon is also observed in the (GeTe)x(AgSbTe2)1-x and 3% Bi2Te3doped Ge0.87Pb0.13Te alloys, resulting from the phase transition from the low-temperature rhombohedral to the high-temperature cubic phases of GeTe-based phase confirmed by DSC.23 It is noted that the transition temperature decreases from 573 K to 473 K with Se content x increases from x = 0 to 0.7. The electrical resistivity depends on the competition between the increased carrier concentration p and decreased mobility µ due to the Se substitution, based on ρ = 1/neµ. The electrical resistivity values for Ge0.55Pb0.45Te1-xSex, showing an increasing trend with the Se content x, are 13, 26, 89, 120, 200 and 350 µΩ m at room temperature, 21, 42, 137, 140, 280 and 500 µΩ m at 473 K, and 25, 33, 83, 100, 150 and 150 µΩ m at 723 K, for the samples with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8, respectively. The Seebeck coefficients of the studied samples, shown in Fig. 4b, are found to be positive over the entire temperature range, indicating that the hole-type carriers dominate the thermoelectric transport. Similar to the behavior as temperature-dependent electrical resistivities, the Seebeck coefficients of Ge0.55Pb0.45Te1-xSex increase first and then decrease with elevating temperature, showing slope change curves due to the phase transition of GeTe-based phase as discussed above. The Seebeck coefficients for the sample Ge0.55Pb0.45Te1-xSex increase with the Se content x, from 50 µVK-1 for x = 0 to 257 µV K-1 for x = 0.7 at room temperature, from 85 µVK-1 for x = 0 to 337 µVK-1 for x = 0.7 at 473 K, and from 130 µVK-1 for x = 0 to 290 µVK-1 for x = 0.7 at 723 K. This increase may be mainly the result of the enhanced effective mass of carriers since the carrier mobility µ reduces but the carrier concentration increases with the Se substitution based on the equations: 27 S=

8π 2 k 2 * π 2/3 m T( ) 3eh 3n p

and

µ = eτ/ m*

where k is the Boltzmann constant; e is the electron charge; h is the Planck’s constant; m* is the effective mass of carrier; np is the carrier concentration, τ is the relaxation time and T is the absolute temperature.

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The power factors calculated from formula P = S2/ρ for the Ge0.55Pb0.45Te1-xSex samples are shown in Fig. 4c, which increases monotonically with elevating temperature except for the sample with x = 0.8. The power factor increases from 186 µWK-2 at room temperature to 659 µW m-1 K-2 at 723 K for x = 0, and from 300 µW m-1 K-2 at room temperature to 974 µW m-1 K-2 at 723 K for x = 0.5, as examples. With increasing Se content x, the power factor increases first, reaches the maximum as x = 0.5 and then starts to decrease, specifically, the power factors of Ge0.55Pb0.45Te1-xSex are 186, 261, 542, 459, 429 and 29 µW m-1 K-2 at room temperature, and 659, 614, 974, 851, 694 and 36 µW m-1 K-2 at 723 K for x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8, respectively. The thermal conductivity of the Se-free sample decreases with increasing temperature, from 2.8 Wm1

K-1 at room temperature to 1.7 Wm-1K-1 at 723 K, while that of Se substituted samples do not change no-

tably with increasing temperature, e.g., from 0.67 Wm-1K-1 at room temperature to 0.45 Wm-1K-1 at 723 K for the sample with Se content x = 0.5, shown in Fig. 4d. The substitution of Se for Te in Ge0.55Pb0.45Te affects its thermal conductivity significantly. With increasing Se content x, the thermal conductivity for Ge0.55Pb0.45Te1-xSex decreases first and reaches the minimum values as x = 0.5, and increases afterwards. Besides the electronic contribution to the thermal conductivity, the microstructure and solute atoms have strong effects on the lattice thermal conductivity. Coupling all these effects, the thermal conductivity turn to the lowest when x = 0.5, which contains the nano PbTe-based precipitates, twins microstructure and substantial solute atoms. The increases of the thermal conductivities in the samples with x > 0.5 may be due to the reduction of the phonon scattering from the PbTe nano-precipitates which is disappeared, or present of GeSe phase. The thermal conductivities of Ge0.55Pb0.45Te1-xSex are 2.51, 1.03, 0.67, 0.92, 0.80 and 0.80 Wm-1K-1 at room temperature, and 1.71, 1.18, 0.45, 0.67, 0.69 and 0.87 Wm-1K-1 at 723 K for x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8, respectively. Se substituted Ge0.55Pb0.45Te alloys exhibit much lower thermal conductivity than that of pure GeTe (nearly 90% reduction at 300 K) and even fairly lower than the most optimized alloy (Ge0.87Pb0.13Te3%Bi) (nearly 70% reduction at 300 K) reported in literatures, Fig.5a.22,24 The total thermal conductivity κtot is the sum of the electronic (κele) and lattice thermal conductivity (κlat). κele is proportional to the electrical conductivity σ through the Wiedemann-Franz relation, κele=LσT, where L is the Lorenz number. The contribution to total thermal conductivity from electronic part is quite small when x is larger than 0.5 (below 1%), because the corresponding electrical conductivity values are too low. The lattice thermal conductivity of Ge0.55Pb0.45Te-50%Se is compared with pure GeTe, optimized PbTe-GeTe alloy (Ge0.97Pb0.13Te) and Ge0.97Pb0.13Te-3%Bi with the highest properties ever reported. Ge0.55Pb0.45Te-50%Se exhibits obvious lower lattice thermal conductivities at all temperature range, with nearly 60% further reduction. Such astonishing reduction of thermal conductivities in Se-substituted PbTe-GeTe alloys strongly suggest the heavily solid solution of both Pb and Se in GeTe lattice (atomic-scale defects) can

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play a significant role on phonon scattering, which is always underrated compared with the nanoscale precipitates in nanostructured thermoelectric materials. To explore the underlying mechanism of extremely low thermal conductivity of Ge0.55Pb0.45Te1-xSex, various analytical (S)TEM techniques were employed to study the microstructural features of Ge0.55Pb0.45Te0.5Se0.5, shown in Fig. 6 and 7. As discussed above, the substitution of Se for Te could significantly enhance the sold solubility of PbTe in the GeTe, from below 5%24, 28 to above 40%. STEM observation, together with energy dispersive X-ray (EDX), could provide more convincing evidence on the solubility than XRD, because of its higher resolution. Previous works reported that large-scale PbTe precipitates (several hundred nanometers in size) would spread inside the matrix, when the fraction of Pb is much higher than the solubility limit,24 which can be expected in Ge0.55Pb0.45Te sample. After 50% substitution of Se for Te, it is hard to find large-scale PbTe precipitates (formed after spinodal decomposition) in the sample, but the original GeTe rhombohedral twin structure can be maintained. TEM bright-field image (Fig. 6(a)) clearly shows the typical (110)-type twin. The electron diffraction pattern obtained from the twin boundary exhibits peak splitting direction parallel with the unsplitted row [1-10], Fig. 6(b), which is corresponding to the twin plane, Fig. 6(a). The width of twin variants are ranging from tens nanometers down to hundreds nanometers. As shown in XRD analysis results, the Ge0.55Pb0.45Te0.5Se0.5 sample also contains small amount of PbTe-based precipitates and Ge precipitates, Fig. 7. TEM bright-field image (Fig. 7(a)) and high-angle annular dark-field (HAADF) image (Fig. 7(b)) from the same area exhibit the precipitates, and the EDX point analysis in Fig. 7(c) and (d) reveals that the precipitates is PbTe-based phase (seen from higher intensity of Pb at around 2.6 keV and 10.6 keV), compared with the matrix. Specially, the composition can be identified via the semi-quantitative analysis of EDX spectra, and the Pb fraction in the matrix was measured as 40-45% (averaged from tens spots), which is a strong evidence on enhanced solid solubility of Pb in GeTe via the substitution of Se for Te. What’s more, there are some relatively large precipitates (100-200 nm in size) at the grain boundaries, as shown in Fig. 7(e, f), and its point EDX spectrum (inset in Fig. 7(e)) and line EDX spectrum (Fig. 7(f)) reflect that the precipitates are pure Ge. The microstructures of Ge0.55Pb0.45Te0.5Se0.5 can be mainly believed to be the coexistence of heavy solid solution (atomic-scale point defect or strains) and slim twins (nano-scale in width and meso-scale in length), collectively leading to its extremely low thermal conductivity. The low density of precipitates and micro-scale of grains also contribute the overall phonon scattering to some degree. Twin boundaries can act as a mediate-wavelength phonon scattering source while exhibiting a negligible effect on the electronic transport since the twin boundary is always coherent interfaces, which cannot disrupt the electronic transport with very short wavelength.29 Although the density of PbTe precipitates in Ge0.55Pb0.45Te0.5Se0.5 is much lower than those in Ge0.55Pb0.45Te1-xSex (x < 0.5), the precipitate-free (twinned) Ge0.55Pb0.45Te0.5Se0.5 sample presents much lower thermal conductivity. It is seen that point defects and

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associated strain (due to solid solution from high-solubility Pb and Se in GeTe) in GeTe-PbTe thermoelectric materials are the significant phonon scattering centers, quite similar with the case in PbTe-PbS system.30 These atomic-scale defects can provide effective scattering of short-wavelength phonons and contribute much more to reduce the thermal conductivity. To evaluate this unexpected role of solid solution, we did theoretical calculation of lattice thermal conductivity based on the modified Callaway model, which was described in detail in our previous work.24 For the present Ge0.55Pb0.45Te0.5Se0.5 with heavy solid solution and the Ge0.97Pb0.13Te-3%Bi with high density of nanoscale precipitates and light solid solution, the calculated results can well fit experimental results, Figure 5(b). It is reasonable to conclude that the low thermal conductivities of Ge0.55Pb0.45Te1-xSex should mainly originate from the enhanced phonon scattering from the solution of Pb and Se in GeTe as the main phonon scattering centers; the twinning microstructures in the GeTe-based solid solution, and the dispersed PbTe-based and Ge precipitates in the GeTe matrix also contribute for it to some extent. The figure of merit (ZT) for the studied samples can be calculated, shown in Fig.8, based on the equation ZT = S2T/ρκ. It appears that the values of ZT for all the Ge0.55Pb0.45Te1-xSex samples increase with temperature; meanwhile, the ZT value increases with increasing Se content from x = 0 to 0.5 but decreases with further increasing Se content x largely due to the increased thermal conductivity. The maximal ZT of 1.55 was eventually obtained in the sample with Se content x = 0.5 at 723K as a result of synergetic optimization of lowest thermal conductivity and modest electrical transport properties. This value is much higher than ZT = 0.2 of the Se-free alloy Ge0.55Pb0.45Te and ZT = 0.51 of pure GeTe prepared at the same temperature.

Conclusion The Se-substituted Ge0.55Pb0.45Te1-xSex alloys were prepared and the effects of Se substitution on the phases and thermoelectric properties were investigated. The substitution of Se for Te in Ge0.55Pb0.45Te was found to be able to significantly promote the solid solubility of Pb in GeTe-based phase, resulting in the intensive alloying scattering on phonon transport hence the substantial reduction in the thermal conductivity. As a result, without severely deteriorating the power factor, the figure of merit ZT can be enhanced for 50% Se substitution. The lowest thermal conductivity, i.e., 0.67 Wm-1K-1 at room temperature and 0.45 Wm-1K-1 at 723K, which was found in Ge0.55Pb0.45Te0.5Se0.5 assured a highest ZT of 1.55 at 723 K. This value is much higher than that of its alloys-free counterpart Ge0.55Pb0.45Te.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51571144 and 11504239) and Shenzhen Science and Technology Research Grant (Nos. JCYJ20140418182819176 JCYJ20150827155136104,

JCYJ20150324141711684,

ZDSYS20141118160434515

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JCYJ20140612140151884). Transmission electron microscopy work was performed in Center of Electron Microscopy in Zhejiang University, Hangzhou, China (H.J.W).

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(15) Cook, B.A.; Kramer, M.J.; Wei, X.; Harringa, J.L.; Levin, E.M. Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material. J. Appl. Phys. 2007, 101, 053715. (16) Levin, E.M.; Besser, M.F.; Hanus, R. Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials. J. Appl. Phys. 2013, 114, 083713. (17) Salvador, J.R. Yang, J. Shi, X.; Wang, H.; Wereszczak, A.A. Transport and mechanical property evaluation of (AgSbTe)1-x(GeTe)x (x = 0.80, 0.82, 0.85, 0.87, 0.90). J. Solid State Chem. 2009, 182, 2088-2095. (18) Gelbstein, Y. Phase morphology effects on the thermoelectric properties of Pb0.25Sn0.25Ge0.5Te. Acta Mater. 2013, 61, 1499-1507. (19) Gorsse, S.; Pereira, P.B.; Decourt, R.; Sellier, E. Microstructure engineering design for thermoelectric materials: an approach to minimize thermal diffusivity. Chem. Mater. 2010, 22, 988-993. (20) Parker, S.G.; Pinnell, J.E.; Swink, L.N. Determination of liquidus-solidus curves for system PbTe-GeTe. J. Mater. Sci. 1974, 9, 1829-1832. (21) Ramachandrarao, S.P. Misra, S.; Anantharaman, T.R. Thermodynamic and constitutional studies of the PbTe-GeTe system. J. Mater. Sci. 1975, 10, 1849-1855. (22) Gelbstein,Y.; Davidow, J.; Girard, S.N.; Chung, D.Y.; Kanatzidis, M. Controlling metallurgical phase separation reactions of the Ge0.87Pb0.13Te alloy for high thermoelectric performance. Adv. Energy Mater. 2013, 3, 815 – 820. (23) Davidow, J.; Gelbstein, Y. A comparison between the mechanical and thermoelectric properties of three highly efficient p-type GeTe-rich compositions: TAGS-80, TAGS-85, and 3% Bi2Te3-Doped Ge0.87Pb0.13Te. J. Electron. Mater. 2013, 42, 1542. (24) Wu, D.; Zhao, L.D.; Hao, S.Q.; Jiang, Q.K.; Zheng, F.S.; Doak, J.W.; Wu, H.J.; Chi, H.; Gelbstein, Y.; Uher, C.; Wolverton, C.; Kanatzidis, M.; He, J.Q. Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping. J. Am. Chem. Soc. 2014, 136, 11412−11419. (25) McCusker, L.B.; Von Dreele, R.B.; Cox, D.E.; Louer, D.; Scardi, P. Rietveld Refinement Guidelines. J. Appl. Cryst. 1999, 32, 36-50. (26) Korzhuev, M.A. Germanium Telluride and Its Physical Properties. Moscow: Nauka, 1986, 104 (in Russian). (27) Snyder, G..J.; Toberer, E.S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105-114. (28) Volykhov, A.A.; Yashina, L.V.; Shtanov, V.I. Phase relations in pseudobinary systems of germanium, tin, and lead chalcogenides. Inorg. Mater. 2006, 42, 596-604. (29) Pei, Y.L. ; Wu, H.J.; Sui, J.H.; Li, J. ; Berardan, D. ; Barreteau, C. ; Pan, L.; Dragoe, N.; Liu, W.S.; He, J.Q.; Zhao, L.D. High thermoelectric performance in n-type BiAgSeS due to intrinsically low thermal conductivity. Energy Environ. Sci. 2013; 6:1750–1755. (30) Wu, D.; Zhao, L.D.; Tong, X.; Li, W.; Wu, L.J.; Tan, Q.; Pei, Y.L.; Huang L.; Li, J.F.; Zhu, Y.M.; Kanatzidis, M.G.; He, J.Q. Superior thermoelectric performance in PbTe-PbS pseudo-binary: extremely low thermal conductivity and modulated carrier concentration. Energy Environ. Sci. 2015, 8, 2056-2068.

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Tables

Table 1 Structure and refined parameters for Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5 and 0.6. Samples Ge0.55Pb0.45Te1-xSex

x = 0.2

x = 0.5

x = 0.6

40.12

95.42

100

Cell parameters a (nm)

0.4165(2)

0.4248(3)

0.4227(2)

c (nm)

1.0651(4)

1.0650(1)

1.0615(6)

Volume of unit cell (nm3)

0.1600(1)

0.1664(3)

0.1642(2)

Calculated density (gcm-3)

6.602(5)

6.473(1)

7.025(9)

3a (0,0,0.4752) atoms

Ge0.9Pb0.16

Ge0.7Pb0.3

Ge0.55Pb0.45

3a (0,0,0) atoms

Te0.8Se0.2

Te0.5Se0.5

Te0.4Se0.6

59.88

3.97

0

0.6293(2)

0.6189(1)

Volume of unit cell (nm3)

0.2492(3)

0.2371(2)

Calculated density (gcm-3)

7.947(9)

7.945(5)

4a (0,0,0) atoms

Pb0.8Ge0.2

Pb0.8Ge0.2

4b (1/2,1/2,1/2) atoms

Te0.8Se0.2

Te0.5Se0.5

0

0.61

GeTe –based phase

wt.% Space group: R3m Structure type: GeTe

Occupation (wt.%) PbTe –based phase

Space group: Fm-3m Cell parameters a (nm) Structure type: NaCl

Occupation (wt.%)

0

Space group: Fd-3m Ge-phase

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Cell parameters a (nm)

0.5656(1)

Structure type: C Volume of unit cell (nm3)

0.1809(1)

Calculated density (gcm-3)

5.330(3)

Occupation

Ge

8a (0,0,0) Rp

5.34

3.15

6.37

Rwp

6.79

4.39

8.79

Reliability factors

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Figures

Fig. 1

Fig.1 Powder X-ray diffraction patterns for the samples Ge0.55Pb0.45Te1-xSex with x = (a) 0, (b) 0.2, (c) 0.5, (d) 0.6, (e) 0.7 and (f) 0.8. Inset are enlarged patterns for x=0.5, 0.7 and 0.8.

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Fig. 2

Fig. 2 Rietveld refinement for the XRD patterns for the samples Ge0.55Pb0.45Te1-xSex with x = (a) 0, (b) 0.5 and (c) 0.8; Insert figure of Fig. 2(b): the amount of the PbTe-based phase and GeTebased phase in the samples with Se content x. 14

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Fig. 3

Fig. 3 (a) and (b) compositional dependent of the carrier concentration n and mobility µ for Ge0.55Pb0.45Te1-xSex with x = 0.2, 0.5, 0.6 and 0.7 at room temperature, compared with GeTe, Ge0.87Pb0.13Te and Ge0.87Pb0.13Te-3%Bi in (b).

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Fig. 4

Fig. 4 Temperature dependence of the (a) electrical resistivity ρ, (b) Seebeck coefficient S, (c) Power factor P and (d) total thermal conductivity κ for the samples Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8.

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Fig. 5

Fig. 5 Temperature dependence of the (a) total thermal conductivity κtot and (b) lattice thermal conductivity κlat of GeTe[23], Ge0.87Pb0.13Te[23], Ge0.87Pb0.13Te-3%Bi[23] and Ge0.55Pb0.45Te-50%Se. The dashed lines are for the calculated κlat of Ge0.87Pb0.13Te-3%Bi and Ge0.55Pb0.45Te-50%Se.

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Fig. 6

Fig. 6 (a) TEM image obtained from Ge0.55Pb0.45Te-50%Se showing twin structure, the twin plane is along (1-10); (b) electron diffraction pattern along [110] zone axis, obtained from twin boundary, the peak splitting direction is parallel with the unsplitted row [1-10].

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Fig. 7

Fig. 7 (a) TEM image and (b) HAADF image showing precipitates; (c) and (d) EDX spot spectrum of the precipitate and the matrix reflecting the precipitate is Pb-rich; (e) HAADF image showing large scale precipiate at the grain boundary, the inset is EDX spot spectrum of the precipitate showing Ge; (f) EDX line scan across the precipitate showing Ge enrichment at the precipitate. 19

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Fig. 8

Fig. 8 Temperature dependence of the figure of merit ZT for the samples Ge0.55Pb0.45Te1-xSex with x = 0, 0.2, 0.5, 0.6, 0.7 and 0.8.

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