Enhancement of Thermoelectric Properties in Pd-In Co-doped SnTe

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Enhancement of Thermoelectric Properties in PdIn Co-doped SnTe and Its Phase Transition Behavior Zheng Ma, Jingdan Lei, De Zhang, Chao Wang, Jianli Wang, Zhenxiang Cheng, and Yuanxu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08564 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Enhancement of Thermoelectric Properties in Pd-In Co-doped SnTe and Its Phase Transition Behavior Zheng Ma,† Jingdan Lei,† De Zhang,† Chao Wang,∗,† Jianli Wang,† Zhenxiang Cheng,∗,†,‡ and Yuanxu Wang∗,† †Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng, 475004, China ‡Institute for Superconducting and Electronic Materials, University of Wollongong, Squires Way, North Wollongong, 2522, Australia E-mail: [email protected]; [email protected]; [email protected] Abstract SnTe have attracted more and more attention due to the similar band and crystal structure with high performance thermoelectric materials PbTe. Here, we introduced Pd into SnTe and the valence band convergence was confirmed by first principles calculation. In the experimental process, we found that Pd-doped SnTe exhibit a reduced thermal conductivity because of softening chemical bonds and grain refining effects. To further improve the thermoelectric performance, Pd-In co-doped SnTe samples were prepared and the abnormal change of thermal conductivity was observed. The results of synchrotron powder diffraction suggest that the local phase transition (local structural distortions) near 400 K results in the first turn on thermal conductivity. Similarly, the second local phase transition in near 600 K observed by neutron powder diffraction lead to a decrease thermal conductivity of the sample. Finally, a peak

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thermoelectric figure of merit (ZT) ∼ 1.51 has been obtained in Sn0.98 Pd0.025 In0.025 Te at 800 K.

Keywords SnTe, thermoelectric performance, local phase transition, NPD, SPD

Introduction At present, the growing shortage of non-renewable energy, such as oil, coal and natural gas, has severely restricted the development of society. Therefore, actively exploring the new energy sources has become a consensus among countries in the world. Thermoelectric materials have gained more and more attention because they can directly convert waste heat into electricity. The performance of a thermoelectric material is described by the dimensionless figure of merit ZT =S 2 σT/(κe +κL ), where σ is the electrical conductivity, S the Seebeck coefficient, κe the electronic thermal conductivity, κL the lattice thermal conductivity and T the absolute temperature, respectively. Optimizing thermoelectric performance, in other words, to increase ZT value, has always been a major task for researchers in this field. However, as we all know, the constituent thermoelectric parameters are interdependent. Therefore, it is necessary to break or weaken the dependencies between various parameters in order to achieve the high performance thermoelectric materials. Until now, we already know many methods could improve thermoelectric performance, for example, lattice anharmonicity, 1,2 liquid-like lattice, 3,4 rattling impurities, 5 nanostructures, 6,7 dislocations, 8 low sound velocity, 9 and point defects including substitutions, 10 band engineering (band convergence and resonant level). So far, lead chalcogenides, which have the rock-salt structure, are some of the most studied thermoelectric materials and have a record figure of merit (ZT) between 1.4 and 2.5. 6,11–13 It has been found that lead telluride (PbTe) and its alloys have the best thermo2


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electric performance in the middle temperature range. However, due to the toxicity of Pb, the application of PbTe-based alloy would be greatly limited in our daily life. As a lead-free compound, tin telluride (SnTe), discovered in the last century, has attracted interest in recent years because it has the similar crystalline and band structure with PbTe. However, contrary to the expectation of most researchers, this material exhibits poor thermoelectric properties. The main reason is that this material possesses a very high carrier concentration (1020 cm−3 ∼1021 cm−3 ) due to the intrinsic Sn vacancies, which led to low Seebeck coefficient and high electronic thermal conductivity. Small band gap (0.18 eV at 300 K) and large energy separation (0.35 eV at 300 K) between the light and heavy valence bands in SnTe material 14–16 (0.30 and 0.17 eV for PbTe at 300 K, respectively) are also unfavourable to the modulation of thermoelectric properties. Additionally, compared with PbTe, SnTe has not only high electron thermal conductivity but also high lattice thermal conductivity. For example, the lattice thermal conductivity of SnTe is 3 Wm−1 K−1 whereas the lattice thermal conductivity of PbTe is only 1.5 Wm−1 K−1 at 300 K. 17 Because of the similarities of the structure and band between SnTe and PbTe, the research on PbTe could give us more direct help during exploring the thermoelectric properties of SnTe. PbTe is the most successful example of band engineering. Valence band convergence in PbTe was achieved by various dopants, such as Mg, Ca, Ba, Sr, Mn and Cd. 13,18–21 Similar with PbTe, valence band convergence in SnTe can increase the band gap to reduce the energy separation between upper light-hole band at L point and lower heavy-hole band at Σ point, 22–24 thus significantly increase the Seebeck coefficient of SnTe at high temperature region. For example, for Mn-doped SnTe, band convergence occurs and the Seebeck coefficient could reach ∼270 µVK−1 at 920 K. 25 The other doping elements, such as Cd, Mg, Hg and Cu, also exhibit band convergence in SnTe. 26 The introduction of resonant impurity states near the Fermi level is another effective method to improve the Seebeck coefficient of SnTe. This phenomenon gives people inspiration to combine band convergence with resonant level to improve the thermoelectric properties of SnTe. In-Cd co-doped SnTe makes ZT up

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to 1.1 at 850 K 27 and the ZT value in In-Mn co-doped SnTe could reach 1.5 at 840 K. 28 Just as we described above, there are many elements that can introduce valence band convergence in SnTe, but no one has done theoretical and experimental research on Pd element. According to the results reported by Tan et al., strong band convergence would happen in SnTe when the energy of s orbital of doping element is greater than the energy of Te p orbital. 29 The energy of Te p orbital is -9.5 eV and the energy of Pd s orbital is -6.91 eV, 30 which means Pd element meet the criteria and band convergence could be expected in Pd doped SnTe. Interestingly, in our work, doped Pd can also reduce the lattice thermal conductivity by introducing chemical bond softening and grain refinement effects. To further enhance the electrical properties of SnTe, Pd-In co-doped SnTe samples were prepared and characterized in our work. Additionally, SnTe with rocksalt structure at room temperature has lattice instability originating from resonant bonding and undergoes a temperature dependent structure transition below 100 K. 31,32 Banik et. al modulated the crystal structure of SnTe by doping Ge and the local rhombohedral distortions could be found in global cubic SnTe near room temperature, resulting in an ultralow lattice thermal conductivity. 33 In order to reveal the subtle features of the crystal structure of SnTe, neutron and synchrotron powder diffraction measurements were applied to Pd-In co-doped SnTe sample and its temperature-dependent structural transformation was presented in this work. Our results proved that Pd-In co-doped SnTe also could occur phase transition in high temperature area and lower lattice thermal conductivity could be achieved. Finally, a high value of the figure of merit ZT ∼ 1.51 achieved for the Sn0.98 Pd0.025 In0.025 Te.

Experimental Section The first-principle calculation on the electronic structure of SnTe were performed using density functional theory (DFT) within projector augmented wave pseudopotentials as implemented in the VASP program. 34,35 The electronic-electronic exchange interaction was treated

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using generalize gradient approximation (GGA) given by Perdew-Burke-Ernzerhof (PBE). The structures used in the calculation were 3×3×3 supercell constructed by a rhombohedral primitive cell, which contains 27 Sn and 27 Te atoms. In order to make the theoretical calculation closer to the actual experiment, we introduce Sn vacancies by deleting one Sn atom in the supercell, which corresponds to a hole concentration consistent with the experimental fact. 36 Furthermore, one Sn site in Sn0.96 Te was substituted by Pd atom during the study on Pd doped SnTe. A 5 × 5 × 5 k-point grid generated according to the Monkhorst-Pack scheme was used in the sampling of the Brillouin Zone. The plane-wave energy cutoff was chosen to be 500 eV. The tolerance in the self-consistent field (SCF) calculation was 1 × 10−7 eV/atom.The spin-orbit coupling (SOC) was considered in our calculation to include the relativistic effects on the electronic structure of SnTe. To avoid producing the large number of Sn vacancies due to the volatilization, the ratio of Sn to Te was set to 1.03:1 during the preparation process of Sn1.03 Te samples. To synthesize Sn1.03 Te, starting materials with suitable proportion, Sn (powder, Aladdin, 99.9%) and Te (powder, Aladdin, 99.9%), were fully mixed and placed into a quartz tube, then the quartz tube was evacuated to high vacuum (∼2.5×10−2 Pa) and sealed by a flame. The sealed quartz tube was slowly heated to 1173 K in a tube furnace and held at this temperature for 15 hours. After the quartz tube cooled to room temperature, the burned material was put into a ball mill to crush into powder. Finally, the crushed material was sintered in a 12.7 mm diameter mold into a wafer by a spark plasma sintering system (SPS-211LX, Fuji Electronic Industrial Co. Ltd.). The similar preparation process was used to fabricate Pd doped SnTe and Pd-In co-doped SnTe except Pd (powder, Aladdin, 99.5%) and In (powder, Aladdin, 99.5%) with suitable proportion were added into starting materials. Crystal structure of the obtained samples were characterized by X-ray diffraction using Bruker D8 Advance with Cu Kα radiation. The neutron powder diffraction (NPD with a wavelength of 2.41788 Å) patterns were collected on the high-resolution powder diffractometer Wombat, OPAL (Australian Nuclear Science and Technology Organization) with the

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temperature ranging from 330 to 750 K. The Synchrotron powder diffraction (SPD) patterns were collected on the Powder Diffraction beamline at the Australian Synchrotron with a wavelength of 0.729806 Å. During the SPD measurement, the sample was first heated from 300 to 460 K and then cooled down to 310 K. The microstructure examined on a freshly broken surface of the samples was observed by a field emission scanning electronic microscope (FESEM, JSM-7001F, JEOL Co., Ltd.). The obtained pellets after spark plasma sintering processed were cut into bars with dimensions of 12 mm × 2 mm × 3 mm that were used to measure electrical conductivity and Seebeck coefficient simultaneously using an static DC thermoelectric property measurement system (ZEM-3, ULVAC-RIKO, Inc.) under a low-pressure He atmosphere from room temperature to 800 K. Carrier concentration and mobility were analyzed at room temperature using DC Hall measurement system (ET9005, East Changing Technologies, Inc.). The thermal conductivity was determined from the thermal diffusivity obtained by the laser flash method (DLF-1/EM1200, TA Inc.) in a Ar atmosphere. The Raman measurements were performed using a Confocal laser Raman spectrometer (RM-1000) spectrometer with a 633 nm excitation laser in the frequency range of 250-50 cm−1 with 1 cm−1 resolution.

Results and discussion To explore the effects of Sn vacancies and Pd impurity atoms on the band structure of SnTe, first we have performed the density functional theory (DFT) calculations for SnTe, Sn0.96 Te and Sn0.93 Pd0.04 Te. Fig. 1a) show the band structure of SnTe and it could be clearly seen from Fig. 1d) that the direct band gap (Eg) is 0.108 eV at L point, which is consistent with the previous reports (0.110 eV) and smaller than the experimental values (0.18 eV). 16,37 The energy separation between the light and heavy valence band, ∆EL−Σ , is about 0.254 eV and slightly smaller than the experimental values (0.30 eV). 16 It is well known that GGA would result in an underestimation of the band gap. However, the shape and trend

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b)

a)

Sn0.96Te

SnTe

c)

d) Sn0.93Pd0.04Te

Figure 1: Energy band structure with SOC for a) SnTe, b) Sn0.96 Te and c) Sn0.93 Pd0.04 Te. d) The band gap Eg and the energy separation of valence bands ∆EL−Σ for SnTe, Sn0.96 Te and Sn0.93 Pd0.04 Te. The inset shows the isoenergy surface at E=-0.38 eV in the first Brillouin zone of SnTe. of band predicted from the GGA calculations are expected to be correct. Considering the balance between the accuracy and time spend, we only used GGA during all the DFT calculations. It could be observed from Fig. 1b) and d) that Sn vacancy would lead to the narrow band gap, which is coincident with previous report. 37 The Sn vacancy in SnTe also give rise to the sharper band shape near valence band maximum (VBM) and conduction band minimum (CBM). Additionally, the energy separation ∆EL−Σ of Sn0.96 Te kept nearly unchanged comparing with that of SnTe. However, once Pd doped into Sn0.96 Te, as shown in Fig. 1d), the energy separation ∆EL−Σ would dramatically decrease from 0.25 to 0.12 eV, which is even smaller than the value of Mg or Mn doping Sn0.96 Te (0.13 eV). 37 Comparing with SnTe and Sn0.96 Te, Sn0.93 Pd0.04 Te show a much flatter band structure at VBM, which meant that the hole effective mass should be increased and the electronic states around the Fermi level would be more localized. In other words, the Seebeck coefficient would be enhanced by Pd-doping. Therefore, the enhancement of Seebeck coefficient caused by the band convergence and the increasing density of states near Fermi level could occur at high

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temperature when Pd is doped into SnTe. In order to more intuitively observe the band convergence in SnTe, we also present the isoenergetic surface in the first Brillouin zone of SnTe in Fig. 1d). As we all know, the valence band extremum L lies at the center of the (111) Brillouin zone faces and the holes are confined to highly anisotropic and nonparabolic pockets. 38 Once we shifts energy downward by 0.38 eV from Fermi level, hole pockets would interconnect with the Σ band and merge to an open network of tubes, which meant both L and Σ band contributed to carrier transport.

a)

b) Te

Sn0.99Pd0.04Te Sn1.00Pd0.03Te Sn1.01Pd0.02Te Sn1.02Pd0.01Te Sn1.03Te Te PDF#44-0925 SnTe PDF#46-1210

Figure 2: a) XRD patterns for Sn1.03−x Pdx Te samples; b) the lattice parameter of Sn1.03−x Pdx Te. Based on the theoretical prediction, we next studied the thermoelectric performance of Pd doped Sn1.03 Te through experimental methods. The XRD patterns of Sn1.03−x Pdx Te (0.01 ≤ x ≤ 0.04) shown in Fig. 2a) exhibit a single phase that can be indexed to the rock-salt SnTe (Fm¯3m) structure. In addition, based on the refined XRD data, we made the diagram of the relationship between lattice parameters and Pd doping concentration shown in Fig. 2b). The lattice parameter initially decreases linearly from a = 6.332 to a = 6.318 with the increasing Pd concentration in SnTe following the Vegard’s law, which indicates the solid solution nature of Sn1.03−x Pdx Te (up to x = 0.03) samples. However, the lattice parameters of Sn1.03−x Pdx Te compositions with x > 3 do not follow the Vegard’s law and deviate from linearity, as shown in Fig. 2b). This indicates that Pd can be completely dissolved into the SnTe substrate when the doping concentration of Pd is less than or equal 8


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a)

b)

c)

d)

Figure 3: a) Electrical conductivities, b) Seebeck coefficients, c) total thermal conductivities and d) ZT values as the function of temperature for Sn1.03−x Pdx Te. to 3%. With the increase of Pd doping concentration, Te diffraction peak appears. The formation of this Te phase is because of the fact that the doping of Pd breaks the inversion symmetry center of SnTe. 22 The destruction of inversion symmetry results in deterioration of the chemical bonds around Te atoms, which eventually leads to the aggregation of Te in preparation. From Fig. 3a), we can clearly see that the electrical conductivity gradually decreases with increasing temperature, which exhibits typical metallic transport behavior of heavily doped semiconductors. Additionally, we could find that there was only a slight change of the electrical conductivity after the incorporation of Pd element. However, most of other elements, for example, Mn or Mg, which could lead to valence band convergence in SnTe, would make the electrical conductivity dramatically decrease. 39,40 Fig. 4a) shows the carrier concentration and mobility of Pd doped Sn1.03 Te. With doping Pd concentration increasing, the carrier concentration exhibits the increasing trend, which also could be inferred from Fig. 1c) because Pd element acts as the acceptor in SnTe. In addition, the increasing hole effective mass resulting from doping Pd leads to the decreasing mobility of Sn1.03 Te. This two factors together result in the nearly unchanged electrical conductivity. The Seebeck coefficient of Pd

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doped Sn1.03 Te at the high temperature was enhanced because of valence band convergence, as shown in Fig. 3b). This result is exactly what the theoretical calculation predicted. The room-temperature Seebeck coefficient as a function of the hole concentration, the so-called Pisarenko curve, is presented in Fig. 4c), which are calculated according to two-valence-band model. Clearly, when doping Pd concentration is low (≤ 2%), data points for Sn1.03−x Pdx Te basically lie at the Pisarenko curve. At these doping concentration, Σ band takes part in the electrical transport, however, the energy separation ∆EL−Σ is consistent with the value of 0.35 eV using in the calculation for Pisarenko curve. Once doping Pd concentration increases beyond 2%, Seebeck coefficient lie above the plot, which means that ∆EL−Σ are reduced. In other words, the band convergence would occur in Sn1.03−x Pdx Te when Pd doping concentration is beyond 3%, which confirms our theoretical predictions.

b)

a)

c)

I

II

Figure 4: a) The carrier concentration and mobility of Sn1.03−x Pdx Te at room temperature; b) Raman spectroscopy of Sn1.03−x Pdx Te; c) Room temperature Seebeck coefficient versus carrier concentration (the solid line is Pisarenko curve based on two-valence-band model). In the calculation, Eg =0.18 eV; ∆E=0.35 eV; mlight =0.168; mheavy =1.92. Additional data include: Bi doped SnTe; 41,42 Cu doped SnTe. 14 As we all know, one of the main reasons why SnTe is considered as a poor thermoelectric material is the high thermal conductivity. The thermal conductivity of Sn1.03 Te at room temperature observed in Fig. 3c) is as high as 6.06 Wm−1 K−1 . However, it is interesting 10


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that the incorporation of Pd lead to a decrease of thermal conductivity. To further investigate the reason for the lower thermal conductivity of Sn1.03−x Pdx Te, we calculated the electronic thermal conductivity κe and lattice thermal conductivity κL . κe is calculated base on the formula κe = LσT, where L is the Lorenz number. The κL is obtained by the subtraction of the electrical part κe from the total thermal conductivity κtotal . Here, the Lorenz number is obtained based on the two band model and the calculated details is given in Supporting Information. 43–49 Comparing with pristine SnTe, most Pd doped samples show even higher electronic thermal conductivity in Fig. 5b). Therefore, the decrease of κtotal mostly arise from a decrease of κL . It is known that the lattice thermal conductivity κL of SnTe could be reduced by the weakening of chemical bonds. 50 Thus, the Raman measurements were performed and the spectra are shown in Fig. 4b). The Raman spectra present two typical peaks in the frequency range of 50 to 250 cm−1 . 51,52 A1 is optical phonon and the ET O is transverse optical phonon. 50 With the increase of the doping concentration, the peaks of A1 and ET O shift toward low wavenumber or frequency, which means that Pd could break the center of the inversion symmetry of SnTe and the force constant of Te-Sn/Pd bonds is changed. 22 The relation between the frequency of vibration of a phonon f and the force constant of a bond K could be described by the equation based on classical vibration model, 53 f ∼(K/µ)1/2 , where µ is the reduced mass. The mass of Pd atom is smaller than that of the Sn atom, so µ will decrease when Pd atom replaces the Sn atom in the crystal lattice. According to the above equation, Raman peaks should shift to high frequency, while the experimental results are inverse. Therefore, the force constant of the bond around Te site should be significantly reduced or the chemical bond significantly softened. The weakening of the chemical bond will reduce the phonon group velocities and lattice thermal conductivity. Fig. 5a) shows the lattice thermal conductivity κL of Sn1.03−x Pdx Te over the entire temperature range and the clear decline at low temperature (≤ 550 K) could be observed after doping Pd. Certainly, there is another important factor, crystal size, which has effect on κL at low temperature and should be considered. It can be seen in Fig. 2a)

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a)

b)

Figure 5: a) Lattice thermal conductivity as the function of temperature for Sn1.03−x Pdx Te; b) Electronic thermal conductivity as functions of temperature for Sn1.03−x Pdx Te samples. that 1 mol% Pd doped sample show relatively low intensity of peaks comparing with pristine Sn1.03 Te, while other samples exhibit the higher intensity of peaks. The enhanced intensity of peaks, to some extent, suggests the better crystallinity in samples. Fig. 6 and Fig. S1 show that low doping Pd concentration (≤ 2%) lead to significant grain refinement and this effect would disappear with increasing doping Pd concentration. It is well known that grain refinement enhances the grain boundary scattering of low and mid-frequency phonons. For the low doping concentration, both grain refinement and softening chemical bonds would take effect to reduce the lattice thermal conductivity at low temperature. However, once Pd doping concentration increases beyond 3 %, large crystal grain size could be seen in Fig. S1d) and e), which means that the decrease of lattice thermal conductivity maybe mainly result from softening chemical bonds. In order to explore the distribution and dissolution of Pd elements in SnTe matrix, EDS element analysis was conducted on SnPd0.03 Te and Sn0.99 Pd0.04 Te samples, as shown in Fig. 6e)−f). Fig. 6e3 ) demonstrates that Pd is uniformly distributed throughout the entire matrix. This means that when the doping concentration of Pd is 3%, it can completely dissolve into the matrix. However, when the doping concentration of Pd reaches 4%, the aggregation of Pd elements in the matrix occurs, as shown in the area marked by the red dotted line in Fig. 6f3 ). This phenomenon indicates that the dissolution limit of Pd in SnTe is around 3%mol. The results are also consistent with those obtained 12


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Figure 6: SEM images of a) Sn1.03 Te, b) Sn1.02 Pd0.01 Te, c) and e) SnPd0.03 Te, d) Sn0.98 Pd0.025 In0.025 Te, f) Sn0.99 Pd0.04 Te; e1 )−e3 ) the elemental energy dispersive spectroscopy (EDS) mapping of the SnPd0.03 Te sample, f1 )−f3 ) the elemental energy dispersive spectroscopy (EDS) mapping of the Sn0.99 Pd0.04 Te sample. from the refined XRD data. In summary, Pd doping can optimize the electrical properties of SnTe through valence band convergence, at the same time optimize the thermal properties by chemical bond softening. Finally, an optimal doping Pd concentration of 3% can make ZT increase to 0.6 at 800 K, as shown in Fig. 3d). For SnTe, the incorporation of In can lead to a increase of Seebeck coefficient through resonant level effect, while the electrical conductivity would dramatically decrease, which could be seen in Fig. S2a) and b). This strong coupling effect between electrical conductivity and Seebeck coefficient has plagued researchers. 54 However, we could find from Fig. 7a) that In and Pd co-doping can relatively increase the electrical conductivity of SnTe comparing with only doping In. The increase of electrical conductivity after Pd incorporation maybe 13



a)

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b)

c)

d)

1

2

Figure 7: a) Electrical conductivities, c) Seebeck coefficients and d) total thermal conductivities as the function of temperature for Sn1.03−2x Pdx Inx Te; b) The carrier concentration and mobility of Sn1.03−2x Pdx Inx Te at room temperature. result from a increase of the carrier concentration. In can effectively compensate the Sn vacancies and decrease the hole population in SnTe, 28 while Pd could act as an acceptor thus leading to a increase of carrier concentration. It can be seen in Fig. 7b) that the hole concentration gradually increases with the increasing co-doping concentration. with Pd and In co-doping, the enhanced Seebeck coefficient can be observed over the entire temperature range in Fig. 7c), which could be attributed to the combination of resonant level and valence band convergence. In Fig. 4c), we can also see the Seebeck coefficients of Pd and In codoped SnTe are much higher than the value predicted by the two-valence-band model at the same carrier concentration. Clearly, an incline of Seebeck coefficient could be found with increasing co-doping concentration, especially for 3 and 3.5 % co-doping concentration. In other words, both Seebeck coefficient and hole concentration could be enhanced by Pd and

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In co-doping. However, a decrease of mobility with increasing co-doping concentration could be seen in Fig. 7b) (exclude for pristine SnTe). Therefore, enhanced power factors were achieved in Pd and In co-doped SnTe, as shown in Fig. S3a).

a)

b)

Sn1.03-2xPdxInxTe, x=0.025

Cu

（220）（200）

1

（420）（400）

2

c)

Temperature (K)

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


d) 740.5 638.3 535.2 433.1 330.0 37.50

41.60

45.68

49.78

53.87

2q Figure 8: a) Lattice thermal conductivity as a function of temperature for Sn1.03−2x Pdx Inx Te; b) Neutron powder diffraction (NPD) patterns of Sn0.98 Pd0.025 In0.025 Te with the temperature ranging from 330 to 750 K (Cu diffraction peaks come from the sample holder); c) Rietveld refined lattice parameter of Sn0.98 Pd0.025 In0.025 Te as a function of temperature. Inset is the derivative of the lattice parameter with respect to temperature. d) NPD peak at 44◦ shifts with increasing temperature. In Fig. 7d), after co-doping Pd and In, the great reduction in thermal conductivity at the low temperature range could be observed. It is worth paying special attention to two significant changes at the temperature region 1 and 2 in the thermal conductivity trends. In fact, for co-doped samples, the electronic thermal conductivity remains almost unchanged as the temperature increases, which could be seen in Fig. S3b). Therefore, the two turn points mainly originates from the change of lattice thermal conductivity. Actually, we could find these abnormal changes in the lattice thermal conductivity in Fig. 8a). Generally, the abnormal changes as shown in Fig. 8a) involve phase transition or crystal structure transition. 15



In order to explore the reason of abnormal changes in thermal conductivity with increasing temperature, we first studied the neutron powder diffraction of Sn0.98 Pd0.025 In0.025 Te sample. The neutron powder diffraction (NPD) patterns were collected from 300 to 700 K. In Fig. 8b), it can be seen that the doped sample is face-centered cubic (Fm¯3m) structure. However, with the temperature increasing, the intensity of the diffraction peak gradually decreases. The decrease in intensity of the peaks with increasing temperature is due to increased thermal vibrations. Additionally, it can be seen in Fig. 8c) that the lattice parameter grows larger as the temperature increase. The shift of the diffraction peak in Fig. 8d) also confirms that the lattice parameter increases with increasing temperature. However, the enlargement of the lattice parameter is diminished above 625 K. This crossover temperature is also confirmed by the derivative of the lattice parameter with respect to the temperature, as shown in the inset (Fig. 8c). According to the Dulong-Petit law based on the thermal expansion, the deviation of the lattice parameter could be ascribed to the intrinsic structural transformation of SnTe. 55 We suppose that the rock salt structure of the material is undistortion in the low temperature region but appears locally distortion on warming, though without a corresponding change in the average crystal structure. This phenomenon of local structure change is similar to the results reported by Banik et al. 33 The presence of In-Pd atoms in the cubic structure of SnTe breaks the symmetry and lead to the local rhombus distortion. This persistent local structural distortion and the instability strongly affect the high temperature thermal conductivity, and thus the overall thermoelectric properties. Therefore, the thermal conductivity ( or lattice thermal conductivity) of the Sn1−x−y Pdx Iny Te sample decrease significantly with the increase of temperature in the region 2. Because neutron powder diffraction resolution accuracy is not high compared to synchrotron radiation, the anomaly in region 1 cannot be well explained only by the neutron diffraction pattern. In order to explore the essential causes of the abnormal change at temperature region 1, Sn0.98 Pd0.025 In0.025 Te sample was studied using the synchrotron powder diffraction (SPD) with the temperature increasing from 300 to 460 K and then cooling down to 310 K. Diffrac-

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a)

b)

rm

up

wa

SnO2 P42/mnm P3121 Te

Figure 9: a) Synchrotron powder diffraction (SPD) patterns of Sn0.98 Pd0.025 In0.025 Te collected with the temperature increasing from 300 to 460 K and then cooling down to 310 K; b) Rietveld refined lattice parameter of Sn0.98 Pd0.025 In0.025 Te as a function of temperature. Inset is the derivative of the lattice parameter with respect to temperature. tion intensities of the Sn0.98 Pd0.025 In0.025 Te sample have been refined. Results of the Rietveld refinements performed at low-temperature structure is shown in Fig. S4. The fits are exceptionally good, which vouches for the high quality of our data and lends confidence to the claim that the cubic model accurately describes the average and local structure at this temperature. In Fig. 9a), the presence of impurity phase, identified as SnO2 with the P 42/mnm structure, could be clearly observed. Similar result for the pure phase is shown in Fig. S5. Pereira et al. also found the P 42/mnm phase of SnO2 in SnTe by high energy X-ray diffraction (λ=0.124559), 56 where SnTe was fabricated by a similar method described in our work. SnO2 maybe comes from the Sn oxidation during the preparation process. In Fig. 9b), we can clearly see that the sample has a significant local phase transition near 425 K. We should pay attention to the appearance of diffraction peaks marked with red triangle in Fig. 9a), which can be indexed to the P 3121 structure of Te. At the low measuring temperature (

Enhancement of Thermoelectric Properties in Pd-In Co-doped SnTe

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