Phase-Transition Temperature Suppression to Achieve Cubic Gete and High Thermoelectric Performance by Bi and Mn Codoping

Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping Zihang Liua,b,c, Jifeng Sund, Jun Maob,c, Hangtian Zhub,c, Wuyang Renb,c,e, Jingchao Zhoua,b,c, Zhiming Wange, David J. Singhd, Jiehe Suia,1, Ching-Wu Chub,c,1, and Zhifeng Renb,c,1 aState Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, 150001 Harbin, China; bDepartment of Physics, University of Houston, Houston, TX 77204-5005; cTexas Center for Superconductivity, University of Houston, Houston, TX 77204-5002; dDepartment of Physics and Astronomy, University of Missouri-Columbia, Columbia, MO 65211; and eInstitute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, 610054 Chengdu, China Contributed by Ching-Wu Chu, April 6, 2018 (sent for review February 5, 2018; reviewed by Austin J. Minnich and Li-Dong Zhao) Germanium telluride (GeTe)-based materials, which display in- energy harvesting and therefore much scientific interest has triguing functionalities, have been intensively studied from both shifted to Pb-free systems. fundamental and technological perspectives. As a thermoelectric GeTe, one of the analogs of PbTe, has recently received in- material, though, the phase transition in GeTe from a rhombohe- tense attention from the thermoelectric community in its aim to dral structure to a cubic structure at ∼700 K is a major obstacle replace traditional PbTe (30–36). GeTe undergoes a ferroelec- impeding applications for energy harvesting. In this work, we dis- tric phase transition from the low-temperature rhombohedral covered that the phase-transition temperature can be suppressed structure α-GeTe (space group R3m) to cubic structure β-GeTe to below 300 K by a simple Bi and Mn codoping, resulting in the (space group Fm3m) at the critical temperature (Tc) around 700 K high performance of cubic GeTe from 300 to 773 K. Bi doping on (37). Due to the presence of a high concentration of Ge vacancies the Ge site was found to reduce the hole concentration and thus to (38), undoped rhombohedral GeTe is a typical degenerate p-type enhance the thermoelectric properties. Mn alloying on the Ge site semiconductor with intrinsically high hole concentration, simultaneously increased the hole effective mass and the Seebeck which results in relatively low ZT. To overcome this short- coefficient through modification of the valence bands. With the Bi coming, In, Bi, or Sb doping as well as Pb alloying on the Ge and Mn codoping, the lattice thermal conductivity was also largely site and Se alloying on the Te site have been proven to be reduced due to the strong point-defect scattering for phonons, effective in reducing the hole concentration and further en- resulting in a peak thermoelectric figure of merit (ZT)of∼1.5 ZT – ZT ∼ hancing (30 36). However, the thermoelectric properties of at 773 K and an average of 1.1 from 300 to 773 K in cubic all compositions previously investigated show the evident fea- Ge Mn Bi Te. Our results open the door for further studies 0.81 0.15 0.04 ture of phase transition in the measured temperature range. It is of this exciting material for thermoelectric and other applications. well known that phase-transition behavior is detrimental for applications because the sudden change in the thermal expan- thermoelectric | phase transition | germanium telluride | Mn alloying | sion coefficient would induce high internal stress between the band-structure engineering materials and the contacts in the device that would lead to crack generation and consequently to deteriorating perfor- hermoelectric power generation (TEG), capable of directly mance or failure under high thermal stress. Therefore, developing Tconverting heat into electricity, has reliably provided power for spacecraft explorations (1), but the low efficiency has im- Significance peded broader application. Due to the significantly improved performance realized in the last decade (2–4), TEG has drawn wide attention for energy harvesting from waste heat and natural Phase-transition behavior in thermoelectric materials is detri- heat that would provide an alternative approach to tackle the mental for their application in thermoelectric devices. Here we challenges of energy sustainability (5). The conversion efficiency designed, and experimentally realized the high thermoelectric of TEG is mainly determined by the material’s dimensionless performance of cubic GeTe-based material by suppressing the 2 phase transition from a cubic to a rhombohedral structure to thermoelectric figure of merit (ZT), ZT = [S σ/(κlat + κele)]T, below room temperature through a simple Bi and Mn codoping where S, σ, κlat, κele, and T are the Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal on the Ge site. Bi doping reduced the hole concentration while conductivity, and absolute temperature, respectively. Conven- Mn alloying largely suppressed the phase-transition tempera- tional methods to enhance the ZT mainly include optimizing ture and also induced modification of the valence bands. Our work provides the basis for studying phase transitions in carrier concentration and strengthening point-defect phonon other thermoelectric materials to optimize these materials for scattering (6, 7), but peak ZT was limited to around unity from applications. the 1950s to the 1990s (8). Recently proposed effective concepts ‘‘ ’’ or strategies, including phonon glass electron crystal to design Author contributions: Z.L., J. Sun, J. Sui, C.-W.C., and Z.R. designed research; Z.L. and J. Sun new compounds (6), band-structure engineering to maximize the performed research; J.M., H.Z., W.R., J.Z., Z.W., and D.J.S. analyzed data; and Z.L., J. Sun, 2 power factor (PF = S σ)(9–13), microstructure engineering to J. Sui, C.-W.C., and Z.R. wrote the paper. suppress the κlat (14–17), and point-defect engineering to opti- Reviewers: A.J.M., California Institute of Technology; and L.-D.Z., Beihang University. mize performance (18–21), have led to the remarkable progress The authors declare no conflict of interest. – in the thermoelectric area (22 26). It should be noted that PbTe, Published under the PNAS license. one of the oldest and most-studied thermoelectric materials (27), 1To whom correspondence may be addressed. Email: [email protected], [email protected], plays a major role in evoking enthusiasm for current thermo- or [email protected]. electric study since most conceptual breakthroughs have come This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. from the recent study of the PbTe system (11, 15, 28, 29). 1073/pnas.1802020115/-/DCSupplemental. However, the toxicity of Pb largely hinders applications for Published online May 7, 2018. 5332–5337 | PNAS | May 22, 2018 | vol. 115 | no. 21 www.pnas.org/cgi/doi/10.1073/pnas.1802020115 Downloaded by guest on October 2, 2021 Results and Discussion A B 400 10 GeTe Ge Bi Te 0.96 0.04 The room-temperature X-ray diffraction (XRD) patterns of Ge0.92Bi0.08Te Ge1−xBixTe samples closely match that of α-GeTe (SI Appendix, ) -1 Fig. S1), confirming their room-temperature crystal structure as Ohm m) 1 200 -5 rhombohedral (37), but the phase-transition temperature from (VK S α β x = x = (10 -GeTe to -GeTe decreases from 700 K ( 0) to 585 K ( 0.08) (SI Appendix,Fig.S2). Benefiting from the reduced hole 0.1 0 concentration nH upon Bi doping (Table 1), the electrical re- 300 400 500 600 700 800 300 400 500 600 700 800 ρ Temperature (K) Temperature (K) sistivity shows an obvious increase to the desired value for 50 good thermoelectric performance over the entire temperature C D 8 range (Fig. 1A). As expected, Seebeck coefficient S increases ) ) -2 40 -1 6 upon Bi doping (Fig. 1B), in accordance with the tendency of ρ. K K -1 -1 Assuming the single parabolic band (SPB) model with acoustic 30 4 phonon scattering as the dominant mechanism for carriers (6, (W m (Wcm 20 tot 2 42), the calculated total density of states (DOS) effective mass PF m* continuously increases with Bi doping concentration (Table 10 0 S 300 400 500 600 700 800 300 400 500 600 700 800 1). Therefore, the enhancement of could be ascribed to the Temperature (K) Temperature (K) combination of reduced nH and band modification upon Bi dop- E F 2.0 ing. Compared with the pristine α-GeTe, Bi doping decreases PF, 4 especially in the high-temperature range (Fig. 1C). The total 3 -1.2 T 1.5 thermal conductivity κtot shows a significant suppression upon Bi ) 2 -1 κlat K doping due to the decreased lattice thermal conductivity ,as 1.0 -1 ZT well as the electronic thermal conductivity κele.Theκlat is obtained 1 κ κ D κ (W m 0.5 by subtracting ele from tot (Fig. 1 ), where ele is calculated using lat the Wiedemann–Franz relationship, κele = LσT, in which L is the 0.0 κlat 300 400 500 600 700 800 300 400 500 600 700 800 calculated Lorenz number. There is an obvious reduction of κ SCIENCES Temperature (K) after Bi doping, e.g., room-temperature lat decreased from Temperature (K) −1· −1 α −1· −1 α 2.4 W m K for -GeTe to 1.0 W m K for -Ge0.92Bi0.08Te APPLIED PHYSICAL Fig. 1. Temperature-dependent thermoelectric properties of α-Ge1−xBixTe (Fig. 1E). Bi doping on the Ge site introduces large mass fluctu- = ρ κ κ samples (x 0, 0.04, and 0.08). (A) ,(B) S,(C) PF,(D) tot,(E) lat,and ations and surrounding local strain-field fluctuations due to the (F) ZT. significant difference in the atomic mass and ionic radius between Bi and Ge atoms (43). In the low-temperature range from 300 to 523 K, α-GeTe shows the typical feature of Umklapp scattering − high-performance GeTe-based materials without the detri- with T 1.2 dependence (Fig.

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