Stacking Fault-Induced Minimized Lattice Thermal Conductivity in the

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Stacking Fault Induced Minimized Lattice Thermal Conductivity in the HighPerformance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying Junqin Li, Yucheng Xie, Chunxiao Zhang, Kuan Ma, Fusheng Liu, Weiqin Ao, Yu Li, and Chaohua Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04984 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 18, 2019

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ACS Applied Materials & Interfaces

Stacking Fault Induced Minimized Lattice Thermal Conductivity in the High-Performance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying Junqin Li, Yucheng Xie, Chunxiao Zhang, Kuan Ma, Fusheng Liu, Weiqin Ao, Yu Li and Chaohua Zhang*

College of Materials Science and Engineering, Shenzhen Key Laboratory of Special Functional Materials, Shenzhen University, Shenzhen, 518060, China, E-mail: [email protected]

Abstract Materials with low lattice thermal conductivity (κlat) are crucial for the applications of thermal insulation and thermoelectric energy conversion. Stacking faults induced phonon scattering within interfaces has been put forward theoretically by Klemens in 1950s. However, unlike other traditional defects like point defects, grain boundaries and dislocations, the role of stacking faults for reducing κlat remains poorly understood and has yet to be revealed experimentally. The layered Bi2Te3 with a van der Wass gap shows different stacking structures than the non-layered GeTe, which is used to introduce stacking faults into the GeTe-based alloys in this work. Based on the experimental and theoretical modeling results, this paper reveals the significant contribution of stacking-fault phonon scattering for minimizing the κlat. Besides the achieved extreme low κlat (~0.39 Wm-1K-1 at 573 K), optimized carrier density and band convergence are also realized in the GeTe-based alloys upon Bi2Te3 alloying, leading to

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a significant high thermoelectric figure of merit ZT>2 at 773 K and an averaged ZT>1.4 within 300-773 K. This stacking-fault engineering strategy provide a different avenue to reduce the κlat for enhancing the performance of thermal insulation and thermoelectric materials.

Keywords: stacking fault, GeTe, thermoelectric, phonon scattering, theoretical modeling

1. Introduction Reducing the thermal conductivity (κ) is important for thermal insulation and especially thermoelectric (TE) energy conversion. TE materials enable direct energy conversion between heat and electricity, showing great promise in the solid-state cooling and waste-heat recovery.1-2 Widespread use of TE materials and products requires improvements of the TE performance, which is usually characterized by a dimensionless figure of merit ZT. Defined as ZT = (S2σ)(1/κ)T, ZT with high value at specific temperature (T) requires low κ and high power factor (PF, =S2σ, where S and σ is the Seebeck coefficient and electrical conductivity respectively). The κ is usually composed of lattice thermal conductivity κlat and electrical thermal conductivity κele, where κele is calculated by LσT based on the Wiedemann-Franz law (L is the Lorenz number).1 To enhance the ZT values, strategies like band engineering3-4 for enhancing PF and all-scale-hierarchical-architecture engineering5-6 for reducing κlat have been widely developed. Because the thermal parameter κlat is less coupling with other electronic parameters (S, σ, κlat),7 appropriate reduction of κlat without compromising much of the PF has been proved to be an effective way to enhance ZT. The heat transport through the lattice vibration can be considered as the contribution of


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phonons from all spectrum of frequencies (ω),8-9 and κlat can be expressed as a sum of the spectral lattice thermal conductivity κs(ω) from different frequencies: 𝜅𝑙𝑎𝑡 = ∫𝜅𝑠(𝜔)𝑑𝜔. According to the Debye-Callaway model,9-11 the κlat can be reduced by different scattering mechanisms. Usually, phonon-phonon scattering in the Umklapp process determines the intrinsic κlat of a material.8 On the other hand, boundary scattering targeting low-frequency phonons and point-defect scattering targeting high-frequency phonons can further reduce the κlat.8,

12

Besides the three traditional scattering mechanisms, other specific scattering

mechanisms from different microstructures such as dislocations (dislocation strains and dislocation cores),13-14 resonant dopants15 and nanoprecipitates16 have also been proposed to push the κlat to the amorphous limit and lower. Designing additional scattering process to minimize the κlat and simultaneously keeping good electrical performance is important for enhancing the ZT. Because of the increasing ZT values in this material system recently, GeTe-based TE materials have attracted great attention like other group-IV telluride (PbTe and SnTe based materials) for power generation.17-25 Generally, the ZT of GeTe can only reach to ~1 around 700 K due to the high hole carrier density (~1021 cm-3),17 which is ascribed to many Ge vacancies formed in the crystal structure. Moreover, the GeTe also undergoes a phase transition from rhombohedral phase at low temperature to cubic phase at high temperature,23, 26-27 which can also influence the TE performance. Thus, to enhance the TE performance of the GeTebased materials, carrier-density-modulation and phase-structure-tuning strategies are widely performed by doping and alloying with other elements such as Pb,19, 28 Pb-Se,20, 29-30 Pb-Bi,27 Pb-Sb,31-32 Sb,23 Sb-Se,24, 33 Sb-Bi,26 Se-S,34 Bi,25 Bi-Cu,35 Bi-Cd,36-37 Bi-Mn,38 In,39 In-Sb,40 as



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well as with other compounds like AgSbTe2,41 Bi2Te3,21, 42 Sb2Te322, 43-44 and MnTe.45 Band engineering mechanisms like band convergence21, 26, 36, 45 and resonant levels39-40 have been put forward to understand the high PF in the GeTe-based alloys, and the point-defect scattering is the most proposed mechanism to explain the reduction of κlat in the GeTe-based alloys.31, 38 Usually, the Ge vacancies in the point-defect forms are firstly discussed for understanding the optimization of carrier density and reduction of κlat.31,

38

Finding additional phonon

scattering process is especially important for further reducing the κlat in this material systems. Recently, the planar defects due to Ge vacancies (also named as planar vacancies, defect layers and Ge vacancy gaps) have also been observed in the GeTe-based alloys such as Sb2Te3(GeTe)n,43-44 GenSb2(Te1-xSex)n+333and Ge1-x-yCdxBiyTe.36 These planar defects can be considered as one kind of stacking faults,13, 36 whose contribution to the reduction of κlat has not been fully revealed and uncoupled from other traditional phonon scattering processes. Although Y. Gelbstein et. al have observed an abnormal reduction of κlat in the Bi2Te3-doped GexPb1-xTe based alloys that cannot be explained from the traditional phonon scattering process, they ascribed this κlat reduction to the phase separation features without further discussions.28 Di Wu et. al have especially discussed the band degenerate and the corresponding electrical properties of the Ge0.87Pb0.13Te alloying with 3 mol % Bi2Te3.21 However, the underlying mechanism of phonon scattering by stacking faults is less discussed before and needs further clarification theoretically and experimentally. In this work, the stacking-fault phonon scattering mechanism is proposed to understand the low κlat in the Bi2Te3-doped GeTe-based alloys. The layered Bi2Te3 has different structure stacking orders compared with GeTe, making it a good medium for introducing stacking faults


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into the GeTe-based materials. The reduction of κlat by stacking-fault engineering is revealed by both experiments and theoretical modeling. Moreover, the Bi2Te3 is also favorable for reducing the very high hole carrier density and modulating the band structures of the GeTebased alloys. Combining these effects for PF enhancement and κlat minimization, highperformance GeTe-based thermoelectric materials can be obtained. This stacking-fault engineering strategy can also be great potential for applying in thermal barrier materials and other TE materials as well, such as PbTe and SnTe.

2. Experimental Section 2.1 Synthesis process. The bulk Pb (99.99%), Ge (99.99%), Te (99.99%) and powder Bi2Te3 (99.999%) were used as starting materials. The staring materials for preparation of GeTe and the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples with the stoichiometric ratio x = 0, 0.01, 0.03, 0.05 and 0.07 were sealed in the evacuated quartz tube (6×10-3 Pa). Then the samples were slowly heated to 1050 oC, held for 20 h for complete mixing and homogeneity of the liquid phase, and slowly cooled to 600 oC. They were held at 600 oC for 10 h and subsequently quenched in liquid nitrogen to obtain the solid solution. The quenched alloys were powdered in agate mortar and then consolidated by spark plasma sintering (SPS) under axial pressure of 60 MPa at 600 oC for 30 minutes to obtain the high-density bulk samples. 2.2 Characterization methods: Transmission electron microscopy (TEM) characterization was carried out using the instruments JEM-2100F and FEI Titan G2-300. Scanning electron microscopy (SEM, Hitachi SU-70) equipped with energy dispersive spectrometer (EDS) was performed for morphology and elemental analysis. Powder X-ray diffraction (XRD) patterns



were obtained using Bruker D8 Advance SS/18kW diffractometer with Cu Kα radiation operating at 40 kV×200 mA. The electrical resistivity and the Seebeck coefficient were measured by the instrument (ZEM-2, Ulvac-Riko, Japan, ±5% uncertainty) in a helium atmosphere. The thermal conductivity (κ) of the samples was calculated using the equation κ = λCpd, where the thermal diffusivity λ was measured by a laser flash method (LFA 467, NETZSCH, ±3% uncertainty), as shown in Figure S1. The anisotropy of the samples is nearly negligible, as confirmed by the measurement of λ in two different directions, as shown in Figure S2. Thus, to simplify the test process, we used the measured λ parallel to the SPS pressing direction to represent the λ perpendicular to the SPS pressing direction, which is also the testing direction of electrical properties. The density d of the samples was measured by an Archimedes method, as displayed in Table S1. Different from previous report using Dulong-Petit value for the Cp of the GeTe-based alloys, we use a modified fitting value for determining the heat capacity: 𝐶𝑝 (𝑘𝐵 per atom) = 3.15 + 4.7 × 10 ―4 × (𝑇/K ―300). Similar fitting equations have also been applied in other TE materials such as SnTe46, PbTe47 and PbSe48. As shown in the Figure S3, this equation fitted well with the experimental Cp data measured by the differential scanning calorimeter (DSC 404F3, NETZSCH) except the phase-change region, with about ±5% uncertainty between both the Dulong-Petit value and DSC measured data.

3. Results and Discussions 3.1 Stacking-fault engineering for low lattice thermal conductivity As shown in Figure 1a, the GeTe (or PbTe) and Bi2Te3 can be seen as the stacking arrangement of the repeating unit of -Te-Ge- (or -Te-Pb-) and -Te-Bi-Te-Te-Bi- respectively. The van der


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Wass gap exists between adjacent Te atomic layers in the Bi2Te3. When the (Ge, Pb)Te is alloyed with layered Bi2Te3 to form pseudo-binary alloyed compounds (Ge, Pb)Te-Bi2Te3, the layer-like van der Wass gap (or viewed as planar vacancies, defect layers and Ge vacancy gaps) can be brought into the alloyed compound. These pseudo-layered crystal structures have also been well accepted to understand the crystal structure of the well-known phase-change-material GenSb2Ten+3 (GST),33, 44, 49 which can be seen as the alloying of GeTe with layered Sb2Te3. In the GeTe-rich (Ge, Pb)Te-Bi2Te3 samples, the planar Ge vacancies as planar defects separates two halves of the crystal, which can be viewed as one kind of stacking faults as introduced by Klemens.13, 50 As shown in Figure 1a, the amount of Ge vacancies in the (Ge0.87Pb0.13Te)1x(Bi2Te3)x

alloys can be increased with the adding amount of Bi2Te3, which thus leads to the

increase of the stacking faults concentration. This kind of planar defect lies within grains, which is different from the twin grain boundary that separates two neighboring crystals with very specific relative crystallographic orientations.23 This planar defect is also different from the normal grain boundary that separates two crystals randomly rotated relative to each other, and is also different from dislocations that regard as linear defects.13, 50



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i

(Ge,Pb)Te

Bi 2 Te 3

Stacking faults

(Ge,Pb) Te Bi

(Ge,Pb)Te

j

Figure 1. (a) Side-view schematic of the formation of stacking faults by alloying (Ge,Pb)Te compound with layered Bi2Te3. (b) Spectral lattice thermal conductivity κs(ω) vs angular frequency at 300 K and (c) temperature-dependent lattice thermal conductivity κlat of the (Ge0.87Pb0.13Te)0.97(Bi2Te3)0.03 sample when progressively considering the phonon scatterings from Umklapp processes (U), normal phonon scattering (N), grain boundaries (B), point defects (PD) and stacking faults (SF). (d) Theoretical modelled (e) experimental temperaturedependent κlat of GeTe, Ge0.87Pb0.13Te and (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples. The theoretical data without considering stacking faults is also included in dot lines as shown in (d).


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Based on the Debye-Callaway model,9-11 the of κlat is calculated by Equation (1): 1 𝜔

𝜅𝑙𝑎𝑡 = ∫𝜅𝑠 (𝜔)𝑑𝜔 = 3∫0 𝑎𝐶𝑠 𝑣𝑔 2𝜏𝑡𝑜𝑡𝑑𝜔

(1)

where the ωa, Cs, νg and τtot is the cut-off frequency, spectral heat capacity, the phonon group velocity and total relaxation time respectively. The τtot is the reciprocal sum of the relaxation times from different scattering mechanisms according to the Matthiessen’s rule,51 as shown in Equation (2). 𝜏𝑡𝑜𝑡 ―1 = 𝜏𝑈 ―1 + 𝜏𝑁 ―1 + 𝜏𝑃𝐷 ―1 +𝜏𝐵 ―1 +𝜏𝑆𝐹 ―1 +⋯

(2)

where 𝜏𝑈 ―1, 𝜏𝑁 ―1, 𝜏𝑃𝐷 ―1, 𝜏𝐵 ―1 and 𝜏𝑆𝐹 ―1 are the contributions from the Umklapp phonon-phonon scattering, normal phonon-phonon scattering, point-defect scattering, boundary scattering and stacking fault scattering respectively. The details for describing each phonon scattering can be found in the Supporting Information. To understand the role of phonon scattering by stacking faults, we have used the relaxation rate 𝜏𝑆𝐹 ―1 introduced by Klemens,13,

50

considering the specular reflection of phonons at

stacking faults, as shown in Equation (3): 𝑎𝑙𝑎𝑡2𝛾2𝑁𝑠

𝜏𝑆𝐹 ―1 = 0.7

𝑣𝑠

𝜔2

(3)

where alat, γ, νs and Ns is the average lattice parameter, Grüneisen parameter, sound speed and number of stacking faults per length respectively. The phonon scattering by stacking faults is previously ignored by most researchers probably because the appearance of stacking faults is also accompanied with the linear defects of dislocations at the end of this planar defect,50 which have different frequency dependent relaxation rate (ω3 dependence for dislocation cores, ω dependence for dislocation strain).48 However, as introduced by Kumar et al.,52 the phonon



scattering by dislocations can also be represented by a term of ω2 dependence relaxation rate when considering the overlapping of phonon scattering between point defects and dislocations. Thus, to simplify the theoretical modelling process, we use ω2 dependence relaxation rate in Equation (3) to also include the phonon scattering contribution from those additional dislocations derived from the stacking faults. To reveal the phonon scattering by stacking faults, the experimental and theoretical modelled κlat of GeTe, Ge0.87Pb0.13Te and (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples are compared with one another. As shown in Figure 1b, the intrinsic κs(ω) considering only the phonon-phonon scattering (U+N, Umklapp process and normal process) is nearly constant in all frequency. Adding boundary (B), point defect (PD) and stacking faults (SF) can increase the scattering of low-frequency, high-frequency and mid-frequency phonons respectively (Figure 1b). As shown in Figure 1c, the phonon-phonon scattering process (U+N) leads to the T-1 dependent κlat above 300 K, and further reduction of κlat at each temperature can be realized by additional phonon scatterings especially from point defects and stacking faults (Figure 1c). Figure 1d and 1e are the theoretical modelled and experimental κlat respectively, and those theoretical fitting parameters are listed in Table S2-S4. The basic fitting parameters for phonon-phonon scattering, boundary scattering and point-defect scattering are determined from the experimental κlat of GeTe and Ge0.87Pb0.13Te (Figure 1e), which are also used for the following theoretical modelling of the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples. The change of velocity from the mass difference is also considered in the theoretical modelling (Table S3). However, if the stackingfault scattering process is not considered (Ns=0, dot lines in Figure 1e), the theoretical modelled κlat of the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples can show large deviation from the experimental


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results (Figure 1d-e and Table S4). To theoretically model the phonon scattering process by stacking faults, the 𝜏𝑆𝐹 ―1 shown in equation (3) is introduced to fit the κlat of the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x samples by setting the Ns in proportion to the composition x : Ns=xN0, where N0 is fitted as 1.7 × 106 m-1. As shown in Figure 1d and 1e, the experimental data and theoretical results are in good accordance with each other, proving the existence of stackingfault phonon scattering mechanism. As the bipolar thermal conductivity and the phase-change phenomena is not considered in this theoretical model, the deviation between the theoretical and experimental data still exists, such as the increase of the κlat at high temperature (>500 K) and sharp change of the κlat at specific temperature. As shown in Figure 1d and 1e, by increasing the density of stacking faults that is realized by increasing the Bi2Te3 composition in the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys (from x=0 to x=0.07), the κlat is gradually reduced close to the amorphous limit of GeTe (~0.39 W m-1K-1, calculated by the Cahill-Watson-Pohl Model,53 see details in Supporting Information). The greatly reduced κlat tends to be temperature independent for samples x=0.05 and 0.07, (Figure1e), which is mainly due to the strong phonon scattering from stacking faults as demonstrated in Figure 1d as well as the slightly increased bipolar thermal conductivity in the low-hole-concentration samples. As shown in Figure 1e, a few data of κlat is below the amorphous limit, which may indicate that other mechanism is also involved such as the change of phonon dispersion taking into account of periodic boundary condition.54

3.2 Morphology and structure characterization



Figure 2. Transmission electron microscopy (TEM) characterizations. (a)(b) High-resolution TEM (HRTEM) images and (c) the corresponding selected area electron diffraction (SAED) pattern in the examined region. (d) HRTEM image near a grain boundary. (e) (f) High-angle annular dark field (HAADF) scanning TEM (STEM) images. Randomly distributed streaks/defect layers as highlighted with white dot lines in (a) are observed in these TEM images, which is due to the missing cation atoms (mostly Ge) as revealed in the magnified HAADF STEM image in (f). These TEM images are taken on a standard (Ge0.87Pb0.13Te)0.95(Bi2Te3)0.05 sample.

To reveal the evidence of the defect layers as proposed in Figure 1a, transmission electron microscopy (TEM) studies are performed on a standard (Ge0.87Pb0.13Te)0.95(Bi2Te3)0.05 sample, as shown in Figure 2. By rotating the observed axis to along the [001] direction as referenced


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by the selected area electron diffraction pattern shown in Figure 2c, many randomly distributed streaks can be clearly seen within grains (Figure 2a-b) and near grain boundaries (Figure 2d). The evidence of defect layers observed by high-resolution transmission electron microscopy (HRTEM) have also been found in the GST materials33, 43-44and Ge1-x-yCdxBiyTe,36 which is ascribed to the missing Ge atoms in the gaps. High-angle annular dark field (HAADF) scanning TEM (STEM) images (Figure 2e-f) were also taken to present the detailed atomic arrangement structures of these stacking faults. In the HAADF STEM mode, the intensity of heavier elements (Te) is higher than that of lighter elements (Ge).36 The slight change of the contrast in Ge atomic columns within two adjacent bright Te columns indicates the formation of planar Ge vacancies (Figure 2f).

Figure 3.

(a) X-ray diffraction (XRD) patterns of GeTe and (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys

with different composition x. (b) Lattice parameters and interaxial angles based on the Rietveld refinement of the XRD data in (a).

The phase structure evolution by alloying with Bi2Te3 is also characterized by roomtemperature X-ray diffraction (XRD) patterns (Figure 3a). After spark plasma sintering (SPS) process, a small amount of PbTe based phase shows up in the Ge0.87Pb0.13Te compound due to



the spinodal decomposition (Figure 3a and Figure S4). As the PbTe, GeTe and Bi2Te3 have quite similar lattice constant55-56 , they have good solid solubility in their formed alloys. By alloying Bi2Te3 with the GeTe-PbTe based alloys, the increase of the solubility of PbTe in the GeTe based matrix is mainly due to the increase of entropy in these complex compounds, leading to single-phase compound from x=0.03 and above (Figure 3a and Figure S5-S7). The appearance of (024) and (220) peaks indicates the rhombohedral phase (PDF#47-1079) of GeTe at room temperature. With increasing the amount of Bi2Te3, the (024) and (220) peaks gradually approach and coalesce into one broad peak that close to the (220) peak of cubic GeTe phase (PDF#52-0849), which can be ascribed to the slight change of lattice parameters (0.6-0.607 nm) and shift of interaxial angles from 88.3o to 89.6o (Figure 3b), as carried out by the Rietveld refinement of the XRD data in Figure 3a. The increased cubic nature of GeTe based alloys by increasing the Bi2Te3 content is mainly due to the decreased rhombohedral-to-cubic phasetransition temperature, as revealed in the quasi-binary phase diagram of Bi2Te3-GeTe.42


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Figure 4. Temperature-dependent (a) total thermal conductivity, (b) electrical conductivity, (c) Seebeck coefficient and (d) power factor of the GeTe and (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys. (a)-(d) share the same annotations as shown in (a). The inset in (b) corresponds to the enlarged image of the samples with x=0.05 and 0.07.

3.3 Thermoelectric transport properties Besides its effect on introducing stacking faults for the reduction of κlat, the effect of Bi2Te3 alloying on other TE transport properties is also presented as shown in Figure 4. The Lorenz number for calculating the κlat is determined from the measured Seebeck coefficient (Figure S8a). The total thermal conductivity is gradually reduced as the amount of Bi2Te3 increase (Figure 4a), which is mainly ascribed to the reduction of electrical thermal conductivity κele



(Figure S8b) from the reduced electrical conductivity σ (Figure 4b) and to the reduction of κlat as discussed in Figure 1e. As shown in Figure 4b and its inset, the trend of electrical conductivity is interrupted near the phase-change temperatures. By alloying with Bi2Te3, the Seebeck coefficient S (Figure 4c) and the power factor PF (Figure 4d) calculated by S2σ from Figure 4b-c can be greatly enhanced. As revealed by the carrier-density-dependent roomtemperature Seebeck coefficient (Figure 5a), the enhanced Seebeck coefficient can be ascribed to the reduced carrier density and the increased effective mass (ms*, set as the ratio to the electron mass me). The solid lines in Figure 5a correspond to the theoretical plots from the degenerate single Kane band model (see details in Supporting Information) with different ms*. The reported literature results31, 34, 36, 43 and this work mostly fall in the region from ms*=1.2 to 2.8. The increased ms* by increasing the amount of Bi2Te3 indicates the band convergence for enhanced S as proposed by previous literatures.21,

26, 36, 45

Although the carrier mobility μ

decreases (Figure 5b) as the amount of Bi2Te3 increase in the (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys, the great reduction of κlat due to stacking faults (Figure 1e) can leads to an enhanced ratio of μ/κlat (optimized at x=0.03), which is usually considered as a quality factor for judging whether one strategy is beneficial for high ZT or not.7 The measurement repeatability and SPS-process dependence of the TE properties have also been performed (Figure S9-10), which shows that our samples have good measurement repeatability and are less effected by the SPS pressure and sintering time.


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500

(Ge0.87Pb0.13Te)1-x(Bi2Te3)x - This work:

m*s=2.8

GeTe 0.03

400

S (VK-1)

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


300

0 0.05

0.01 0.07

Ge1-x-yCdxBiyTe - Ref.36 Sb2Te3(GeTe)n - Ref.43

m*s=1.7

GeTe1-2xSexSx - Ref.34 Ge1-x-yPbxSbyTe - Ref.31

200 m*s=1.2

100 0

1

nH (1020 cm-3)

10

Figure 5. (a) Hall-carrier-density-dependent room-temperature Seebeck coefficient for this work and other literature results.31,

34, 36, 43

(b) Room-temperature composition-dependent

carrier mobility (μ) and the ratio of carrier mobility vs lattice thermal conductivity (μ/κlat).

3.4 Enhanced figure of merit



Figure 6. (a) Temperature-dependent ZT of the GeTe and (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys. The dot lines correspond to the data using the Dulong-Petit specific heat capacity. (b) Carrier density-dependent ZT at 298 K and 573 K, and the dot lines are the theoretical results from different modelling conditions. Comparations of (c) the temperature-dependent ZT and (d) average ZT values within 300-773 K. 22, 25, 26, 29, 38, 45

Compared with that of the GeTe and Ge0.87Pb0.13Te, the ZT of (Ge0.87Pb0.13Te)1-x(Bi2Te3)x alloys are greatly enhanced by alloying with Bi2Te3 (Figure 6a), which is mainly ascribed to the optimized carrier density (Figure 5a) and enhanced quality factor μ/κlat (Figure 5b). The dot lines in Figure 6a correspond to the ZT values by replacing the specific heat capacity with the corresponding Dulong-Petit value, which is usually used in the reported GeTe-based alloys.21-


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22, 25-26, 31, 36, 45 Figure

6b shows the carrier-density-dependent ZT for our samples at 298 K and

573 K along with the corresponding theoretical calculated results with different effective mass, using the single band model and setting the κlat set as 0.43 W m-1K-1 (other details given in the Supporting Information). From these plots in Figure 6b, we can see the benefits of optimized carrier density and band convergence for enhanced ZT values. Compared with other GeTebased alloys,22, 25-26, 29, 38, 45 our samples show much higher ZT in the low temperature region (2 at 773 K (Figure 4c) and an averaged ZT as high as 1.41 within 300-773 K (Figure 6d) can be achieved in our samples, which is higher and comparable with other reported high-ZT GeTebased alloys (Figure 4d).22, 25-26, 29, 38, 45, 57-59

4. Conclusion As revealed by theoretical model prediction and experimental results, our work proves the significant contribution of stacking faults for reducing the lattice thermal conductivity in the GeTe-based alloys by introducing pseudo-layered structures. Stacking faults targeting on midfrequency phonons can be further developed as a common strategy to reduce the lattice thermal conductivity like other well-known grain boundary and point defects targeting on low- and high-frequency phonons respectively. Using this stacking-fault strategy, the lattice thermal conductivity can be reduced close to the amorphous limit of GeTe of ~0.39 Wm-1K-1. Along with other effects like carrier density optimization and band convergence by Bi2Te3 alloying, a high ZT peak >2 and average ZT~1.41 within 300-773 K can be obtained in the GeTe-based alloys. Therefore, this stacking-fault strategy offers great potential for further enhancing the



performance of other thermoelectric materials and thermal barrier materials as well.

Supporting Information The Supporting Information is available on line. Details of theoretical modelling methods and parameters, SEM and EDX data, temperature-dependent thermoelectric properties, Lorenz number, electrical thermal conductivity, thermal diffusivity, density, DSC measured specific heat capacity. Acknowledgements This work is supported by the National Natural Science Foundation of China (grant No. 21805196, 51571144), Natural Science Foundation of Guangdong Province, China (grant No.2018A030310416), Natural Science Foundation of Shenzhen University (grand No.2017003), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. 2017KQNCX178), Shenzhen Science and Technology Innovation Commission (grant No. JCYJ20150827155136104). We also thank the Electron Microscope Center of Shenzhen University for the TEM measurements. Conflict of Interest The authors declare no conflict of interests.

References (1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105114. (2) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with


Page 20 of 28

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


Thermoelectric Systems. Science 2008, 321, 1457-1461. (3) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125-61335. (4) Heremans, J. P.; Wiendlocha, B.; Chamoire, A. M. Resonant Levels in Bulk Thermoelectric Semiconductors. Energy Environ. Sci. 2012, 5, 5510-5530. (5) Zhao, L.-D.; Dravid, V. P.; Kanatzidis, M. G. The Panoscopic Approach to High Performance Thermoelectrics. Energy Environ. Sci. 2014, 7, 251-268. (6) Biswas, K.; He, J.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-Performance Bulk Thermoelectrics with All-Scale Hierarchical Architectures. Nature 2012, 489, 414-418. (7) Zhu, T.; Liu, Y.; Fu, C.; Heremans, J. P.; Snyder, J. G.; Zhao, X. Compromise and Synergy in High-Efficiency Thermoelectric Materials. Adv. Mater. 2017, 29, 1605884. (8) Toberer, E. S.; Zevalkink, A.; Snyder, G. J. Phonon Engineering through Crystal Chemistry. J. Mater. Chem. C 2011, 21, 15843-15852. (9) Callaway, J. Model for Lattice Thermal Conductivity at Low Temperatures. Phys. Rev. 1959, 113, 1046-1051. (10)Callaway, J. Low-Temperature Lattice Thermal Conductivity. Phys. Rev. 1961, 122, 787790. (11)Callaway, J.; von Baeyer, H. C. Effect of Point Imperfections on Lattice Thermal Conductivity. Phys. Rev. 1960, 120, 1149-1154. (12)Kim, W. Strategies for Engineering Phonon Transport in Thermoelectrics. J. Mater. Chem. C 2015, 3, 10336-10348.



(13)Singh, B. K.; Menon, V. J.; Sood, K. C. Phonon Conductivity of Plastically Deformed Crystals: Role of Stacking Faults and Dislocations. Phys. Rev. B 2006, 74, 184302. (14)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.; Snyder, G. J.; Kim, S. W. Dense Dislocation Arrays Embedded in Grain Boundaries for High-Performance Bulk Thermoelectrics. Science 2015, 348, 109-114. (15)Pohl, R. O. Thermal Conductivity and Phonon Resonance Scattering. Phys. Rev. Lett. 1962, 8, 481-483. (16)He, J.; Girard, S. N.; Kanatzidis, M. G.; Dravid, V. P. Microstructure-Lattice Thermal Conductivity Correlation in Nanostructured PbTe0.7S0.3 Thermoelectric Materials. Adv. Funct. Mater. 2010, 20, 764-772. (17)Perumal, S.; Roychowdhury, S.; Biswas, K. High Performance Thermoelectric Materials and Devices Based on GeTe. J. Mater. Chem. C 2016, 4, 7520-7536. (18)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. (19)Gelbstein, Y.; Dado, B.; Ben-Yehuda, O.; Sadia, Y.; Dashevsky, Z.; Dariel, M. P. Highly Efficient Ge-Rich GexPb1−xTe Thermoelectric Alloys. J. Electron. Mater. 2009, 39, 2049-2052. (20)Li, J. Q.; Lu, Z. W.; Wu, H. J.; Li, H. T.; Liu, F. S.; Ao, W. Q.; Luo, J.; He, J. Q. High Thermoelectric Performance of Ge1−xPbxSe0.5Te0.5 Due to (Pb, Se) Co-Doping. Acta Mater. 2014, 74, 215-223. (21)Wu, D.; Zhao, L. D.; Hao, S.; Jiang, Q.; Zheng, F.; Doak, J. W.; Wu, H.; Chi, H.; Gelbstein,


Page 22 of 28

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


Y.; Uher, C.; Wolverton, C.; Kanatzidis, M.; He, J. Origin of the High Performance in GeTeBased Thermoelectric Materials Upon Bi2Te3 Doping. J. Am. Chem. Soc. 2014, 136, 1141211419. (22)Fahrnbauer, F.; Souchay, D.; Wagner, G.; Oeckler, O. High Thermoelectric Figure of Merit Values of Germanium Antimony Tellurides with Kinetically Stable Cobalt Germanide Precipitates. J. Am. Chem. Soc. 2015, 137, 12633-12638. (23)Perumal, S.; Roychowdhury, S.; Negi, D. S.; Datta, R.; Biswas, K. High Thermoelectric Performance and Enhanced Mechanical Stability of P-Type Ge1–xSbxTe. Chem. Mater. 2015, 27, 7171-7178. (24)Li, J.; Zhang, X.; Lin, S.; Chen, Z.; Pei, Y. Realizing the High Thermoelectric Performance of GeTe by Sb-Doping and Se-Alloying. Chem. Mater. 2016, 29, 605-611. (25)Li, J.; Chen, Z.; Zhang, X.; Sun, Y.; Yang, J.; Pei, Y. Electronic Origin of the High Thermoelectric Performance of GeTe among the P-Type Group IV Monotellurides. NPG Asia Mater. 2017, 9, e353. (26)Perumal, S.; Bellare, P.; Shenoy, U. S.; Waghmare, U. V.; Biswas, K. Low Thermal Conductivity and High Thermoelectric Performance in Sb and Bi Codoped GeTe: Complementary Effect of Band Convergence and Nanostructuring. Chem. Mater. 2017, 29, 10426-10435. (27)Li, J.; Zhang, X.; Chen, Z.; Lin, S.; Li, W.; Shen, J.; Witting, I. T.; Faghaninia, A.; Chen, Y.; Jain, A.; Chen, L.; Snyder, G. J.; Pei, Y. Low-Symmetry Rhombohedral GeTe Thermoelectrics. Joule 2018, 2, 976-987. (28)Gelbstein, Y.; Davidow, J. Highly Efficient Functional GexPb1-xTe Based Thermoelectric



Alloys. Phys. Chem. Chem. Phys. 2014, 16, 20120-20126. (29)Li, J.; Wu, H.; Wu, D.; Wang, C.; Zhang, Z.; Li, Y.; Liu, F.; Ao, W.-q.; He, J. Extremely Low Thermal Conductivity in Thermoelectric Ge0.55Pb0.45Te Solid Solutions Via Se Substitution. Chem. Mater. 2016, 28, 6367-6373. (30)Li, J.; Chen, Z.; Zhang, X.; Yu, H.; Wu, Z.; Xie, H.; Chen, Y.; Pei, Y. Simultaneous Optimization of Carrier Concentration and Alloy Scattering for Ultrahigh Performance GeTe Thermoelectrics. Adv. Sci. 2017, 4, 1700341. (31)Li, J.; Zhang, X.; Wang, X.; Bu, Z.; Zheng, L.; Zhou, B.; Xiong, F.; Chen, Y.; Pei, Y. HighPerformance GeTe Thermoelectrics in Both Rhombohedral and Cubic Phases. J. Am. Chem. Soc. 2018, 140, 16190-16197. (32)Zhang, X.; Li, J.; Wang, X.; Chen, Z.; Mao, J.; Chen, Y.; Pei, Y. Vacancy Manipulation for Thermoelectric Enhancements in GeTe Alloys. J. Am. Chem. Soc. 2018, 140, 15883-15888. (33)Rosenthal, T.; Urban, P.; Nimmrich, K.; Schenk, L.; de Boor, J.; Stiewe, C.; Oeckler, O. Enhancing the Thermoelectric Properties of Germanium Antimony Tellurides by Substitution with Selenium in Compounds GenSb2(Te1–xSex)n+3 (0 ≤ x ≤ 0.5; n (0 ≤ xChem. Mater. 2014, 26, 2567-2578. (34)Samanta, M.; Biswas, K. Low Thermal Conductivity and High Thermoelectric Performance in (GeTe)1-2x(GeSe)x(GeS)x: Competition between Solid Solution and Phase Separation. J. Am. Chem. Soc. 2017, 139, 9382-9391. (35)Shimano, S.; Tokura, Y.; Taguchi, Y. Carrier Density Control and Enhanced Thermoelectric Performance of Bi and Cu Co-Doped GeTe. APL Mater. 2017, 5, 056103. (36)Hong, M.; Wang, Y.; Liu, W.; Matsumura, S.; Wang, H.; Zou, J.; Chen, Z.-G. Arrays of


Page 24 of 28

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


Planar Vacancies in Superior Thermoelectric Ge1−x−yCdxBiyTe with Band Convergence. Adv. Energy Mater. 2018, 8, 1801837. (37)Li, J.; Li, W.; Bu, Z.; Wang, X.; Gao, B.; Xiong, F.; Chen, Y.; Pei, Y. Thermoelectric Transport Properties of CdxBiyGe1-x-yTe Alloys. ACS Appl. Mater. Interfaces 2018, 10, 3990439911. (38)Liu, Z.; Sun, J.; Mao, J.; Zhu, H.; Ren, W.; Zhou, J.; Wang, Z.; Singh, D. J.; Sui, J.; Chu, C.-W.; Ren, Z. Phase-Transition Temperature Suppression to Achieve Cubic GeTe and High Thermoelectric Performance by Bi and Mn Codoping. Proc. Natl. Acad. Sci. USA 2018, 115, 5332-5337. (39)Wu, L.; Li, X.; Wang, S.; Zhang, T.; Yang, J.; Zhang, W.; Chen, L.; Yang, J. Resonant Level-Induced High Thermoelectric Response in Indium-Doped GeTe. NPG Asia Mater. 2017, 9, e343. (40)Hong, M.; Chen, Z. G.; Yang, L.; Zou, Y. C.; Dargusch, M. S.; Wang, H.; Zou, J. Realizing Zt of 2.3 in Ge1-x-ySbxInyTe Via Reducing the Phase-Transition Temperature and Introducing Resonant Energy Doping. Adv. Mater. 2018, 30, 1705942. (41)Kim, H.-S.; Dharmaiah, P.; Madavali, B.; Ott, R.; Lee, K.-H.; Hong, S.-J. Large-Scale Production of (GeTe)x(AgSbTe2)100−x (x=75, 80, 85, 90) with Enhanced Thermoelectric Properties Via Gas-Atomization and Spark Plasma Sintering. Acta Mater. 2017, 128, 43-53. (42)Madar, N.; Givon, T.; Mogilyansky, D.; Gelbstein, Y. High Thermoelectric Potential of Bi2Te3 Alloyed GeTe-Rich Phases. J. Appl. Phys. 2016, 120, 035102. (43)Xu, X.; Xie, L.; Lou, Q.; Wu, D.; He, J. Boosting the Thermoelectric Performance of Pseudo-Layered Sb2Te3(GeTe)n Via Vacancy Engineering. Adv. Sci. 2018, 5, 1801514.



(44)Rosenthal, T.; Schneider, M. N.; Stiewe, C.; Döblinger, M.; Oeckler, O. Real Structure and Thermoelectric Properties of GeTe-Rich Germanium Antimony Tellurides. Chem. Mater. 2011, 23, 4349-4356. (45)Zheng, Z.; Su, X.; Deng, R.; Stoumpos, C.; Xie, H.; Liu, W.; Yan, Y.; Hao, S.; Uher, C.; Wolverton, C.; Kanatzidis, M. G.; Tang, X. Rhombohedral to Cubic Conversion of GeTe Via Mnte Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance. J. Am. Chem. Soc. 2018, 140, 2673-2686. (46)Zheng, L.; Li, W.; Lin, S.; Li, J.; Chen, Z.; Pei, Y. Interstitial Defects Improving Thermoelectric SnTe in Addition to Band Convergence. ACS Energy Lett. 2017, 2, 563-568. (47)Tan, G. J.; Shi, F. Y.; Hao, S. Q.; Zhao, L. D.; Chi, H.; Zhang, X. M.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Non-Equilibrium Processing Leads to Record High Thermoelectric Figure of Merit in PbTe-SrTe. Nat. Commun. 2016, 7, 12167. (48)Chen, Z.; Ge, B.; Li, W.; Lin, S.; Shen, J.; Chang, Y.; Hanus, R.; Snyder, G. J.; Pei, Y. Vacancy-Induced Dislocations within Grains for High-Performance PbSe Thermoelectrics. Nat. Commun. 2017, 8, 13828. (49)Ibarra-Hernández, W.; Raty, J.-Y. Ab Initio Density Functional Theory Study of the Electronic, Dynamic, and Thermoelectric Properties of the Crystalline Pseudobinary Chalcogenide (GeTe)x/(Sb2Te3) (x=1,2,3). Phys. Rev. B 2018, 97, 245205. (50)Klemens, P. G., Thermal Conductivity and Lattice Vibrational Modes. In Solid State Physics, Seitz, F.; Turnbull, D., Eds. Academic Press: 1958; Vol. 7, pp 1-98. (51)Agrawal, B. K.; Verma, G. S. Lattice Thermal Conductivity at Low Temperatures. Phys. Rev. 1962, 126, 24-29.


Page 26 of 28

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


(52)Kumar, A.; Srivastava, B. K.; Ansari, M. A. Lattice Thermal Conductivity of Deformed Lithium Fluoride Crystals at Low Temperatures. Phys. Stat. Sol. B 1986, 138, 423-432. (53)Cahill, D. G.; Watson, S. K.; Pohl, R. O. Lower Limit to the Thermal Conductivity of Disordered Crystals. Phys. Rev. B 1992, 46, 6131-6140. (54)Chen, Z.; Zhang, X.; Lin, S.; Chen, L.; Pei, Y. Rationalizing Phonon Dispersion for Lattice Thermal Conductivity of Solids. Natl. Sci. Rev. 2018, 5, 888-894. (55)Kim, J. N.; Kaviany, M.; Shim, J.-H. Optimized ZT of Bi2Te3−GeTe compounds from First Principles Guided by Homogeneous Data. Physical Review B 2016, 93. (56)Bauer Pereira, P.; Sergueev, I.; Gorsse, S.; Dadda, J.; Müller, E.; Hermann, R. P. Lattice Dynamics and Structure of GeTe, SnTe and PbTe. Phys. Status solidi (b) 2013, 250, 1300-1307. (57)Perumal, S.; Roychowdhury, S.; Biswas, K. Reduction of Thermal Conductivity through Nanostructuring Enhances the Thermoelectric Figure of Merit in Ge1−xBixTe. Inorg. Chem. Front. 2016, 3, 125-132. (58)Samanta, M.; Roychowdhury, S.; Ghatak, J.; Perumal, S.; Biswas, K. Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x(AgSbSe2)100-x. Chem. Eur. J. 2017, 23, 7438-7443. (59)Roychowdhury, S.; Samanta, M.; Perumal, S.; Biswas, K. Germanium Chalcogenide Thermoelectrics: Electronic Structure Modulation and Low Lattice Thermal Conductivity. Chem. Mater. 2018, 30, 5799-5813.



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Abstract Graphic (Ge,Pb)Te

Bi 2 Te 3

Stacking faults

(Ge,Pb) Te Bi

(Ge,Pb)Te


j

Stacking Fault-Induced Minimized Lattice Thermal Conductivity in the

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