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. 1E), basically consistent with the mental phase transition from α-GeTe to β-GeTe remains a −1 theoretical value T . In contrast, κlat of α-Ge Bi Te is almost significant challenge to be addressed. Based on the pseudo- 0.92 0.08 – temperature independent, which may be related to the induced high binary phase diagram of GeTe MnTe solid solution (39), it is degree of disorder and stronger anharmonicity by Bi doping (44, possible that Mn alloying on the Ge site would be an effective 45). The possibly incomplete subtraction of the electronic contri- method to reduce the phase-transition temperature. Although bution may also have some effects because of the complex band Ge1−xMnxTe systems have been reported (40, 41), the primary structure. Due to the significantly suppressed κlat, Bi doping largely focus of these studies was on the low-temperature magnetic enhances the ZT over the whole temperature range. A peak ZT of properties of the systems. ∼1.4 was achieved for α-Ge0.96Bi0.04Te, more than 50% higher than Here we successfully achieved suppression of the phase-transition that of the pristine α-GeTe (Fig. 1F). It should also be noted that temperature from around 700 K to below 300 K, resulting in the pristine α-GeTe in our work exhibits a higher PF and ZT due to the high thermoelectric performance of cubic GeTe, by a simple its relatively lower nH in comparison with the previously reported Bi and Mn codoping on the Ge site using mechanical alloying samples that were synthesized by the method of melting and and hot pressing. Bi doping reduced the hole concentration while annealing (31, 32, 36). In general, the mechanical alloying method is Mn alloying induced significant valence band modification in able to fabricate materials with the needed chemical constituents, addition to the large suppression of the phase-transition tem- resulting in the lower carrier concentration in our current work. perature. A peak ZT of ∼1.5 at 773 K and a corresponding av- This result indicates that the combination of mechanical alloying erage ZT of ∼1.1 from 300 to 773 K were achieved in cubic and hot pressing is a more appropriate method to fabricate high- Ge0.81Mn0.15Bi0.04Te. performance GeTe-based thermoelectric materials.

Table 1. Electrical transport properties of α-Ge1−xBixTe and α-Ge0.96−xMnxBi0.04Te samples 20 −3 2 −1 −1 0 2 −1 −1 Composition nH,10 cm μH,cm V ·s rH m*,m μW,cm V ·s

GeTe 4.2 95.3 1.0 1.6 197.9

Ge0.96Bi0.04Te 2.4 64.2 1.06 1.9 168.5

Ge0.92Bi0.08Te 1.0 51.3 1.10 2.1 159.7

Ge0.91Mn0.05Bi0.04Te 3.2 30.6 1.07 2.6 130.6

Ge0.86Mn0.1Bi0.04Te 4.1 16.9 1.08 3.9 123.0 Ge0.81Mn0.15Bi0.04Te 5.5 9.4 1.1 5.6 124.7

Ge0.76Mn0.2Bi0.04Te 10.0 4.4 1.11 9.9 136.7

Ge0.66Mn0.3Bi0.04Te 56.4 0.5 1.12 36.7 122.0

nH is Hall carrier concentration (or hole concentration); μH is Hall carrier mobility (or hole mobility); rH is Hall 0 factor; m* is total DOS effective mass; m is the electron rest mass; and μW is weighted mobility.

Liu et al. PNAS | May 22, 2018 | vol. 115 | no. 21 | 5333 Downloaded by guest on October 2, 2021 n A B increased the H of Ge0.96−yMnyBi0.04Te (Table 1). Lewis et al. 5.99 n – x=0.3 90.0 (38) found that the H of GeTe MnTe solid solution increases 5.98 x=0.2 89.8 with Mn concentration as a result of the increased Ge vacancies 5.97 89.6 (38). The number of Ge vacancies in the GeTe system is directly x=0.15 5.96 89.4 89.2 n x=0.1 5.95 related to the H because each Ge vacancy, acting as an acceptor 89.0 center, donates one or two carriers to the valence band (38). In x=0.05 5.94 88.8 5.93 88.6 our first-principles calculations (addressed below) we indeed find

Intensity (a.u.) x=0 5.92 88.4 that Mn is divalent in GeTe and that it adopts a high spin state. Interaxial angle (Degree) Lattice parameter (Å) 88.2 -GeTe 5.91 -GeTe 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Mn alloying intensifies the scattering of holes, leading to the 20 30 40 50 60 70 Mn concentration (x) μ ρ 2 (Degree) significantly decreased Hall mobility H (Table 1). Thus, C D gradually increases over the entire temperature range with in-

) 0.5 -1 x=0 x=0.01 creasing Mn concentration (Fig. 3A). Despite the increased nH, S K

-1 x=0.05 x=0.1 300 K 0.4 continuously increases with increasing Mn concentration (Fig. x=0.15 x=0.2 Jg Cooling B x=0.3 3 ), which will be addressed in detail below. It should be noted 0.3 473 K that the reduction of both ρ and S at high temperature for x ≥ Heating 300 K 0.1 is caused by the bipolar effect, rather than the phase transi- 0.2 tion, while both the bipolar effect and phase transition contrib-

Intensity (a.u.) -GeTe ute to those reductions for x ≤ 0.05. After Mn alloying, PF 0.1 Heat capacity ( 300 400 500 600 700 800 20 30 40 50 60 decreased somewhat due to the increased ρ (Fig. 3C). Basically, Temperature (K) 2 (Degree) 3/2 weighted mobility μW = μH(m*/m0) , where m0 is the free- PF Fig. 2. (A) XRD patterns of Ge0.96−xMnxBi0.04Te samples (x = 0, 0.05, 0.1, electron mass, determines the maximum assuming that the 0.15, 0.2, and 0.3). (B) Lattice parameter and interaxial angle dependence carrier concentration is optimal (48). The calculated room- on Mn concentration. (C) Temperature-dependent heat capacity of temperature μW displays the same variation trend as that of PF = Ge0.96−xMnxBi0.04Te samples (x 0, 0.01, 0.05, 0.1, 0.15, 0.2, and 0.3). (D)XRD (Fig. 3D), both of which indicating that Mn alloying is not a valid patterns of Ge0.66Mn0.3Bi0.04Te sample after heating up to 473 K and cooling method to enhance PF in this system. down to 300 K in air. To understand and quantify the abnormal behavior of the concurrently increased nH and S of Ge0.96−xMnxBi0.04Te with m* Although Bi doping effectively reduces nH and thus enhances increasing Mn concentration, the corresponding were cal- ZT, the obvious phase-transition phenomenon remains. Based culated based on the SPB model, shown in Table 1. Obviously, on the pseudobinary phase diagram of GeTe–MnTe solid so- Mn alloying leads to the significant enhancement of m*, which is lution (SI Appendix,Fig.S3) (39), Mn alloying on the Ge site is also demonstrated by the calculated Pisarenko plots displayed employed to possibly reduce the phase-transition temperature as dashed lines in Fig. 4A. This is consistent with the measured and obtain the cubic structure even at room temperature. XRD low μH of Mn alloyed samples, because heavy carriers generally patterns of Ge0.96−xMnxBi0.04Te samples are shown in Fig. 2A. diffuse with low velocities in a semiconductor. Experimental Samples with low Mn alloying concentration (x ≤ 0.1) continue data of previously studied compositions, including Ge1+xTe, to crystallize in rhombohedral structure while samples with high Ge1−xSbxTe, and GeTe1−xSex, fall on the solid black line (33), Mn alloying concentration (x ≥ 0.15) crystallize in cubic struc- which is calculated by the modified two-band model (33), while n ture (37, 39). In the literature, the reported critical Mn alloying at the same H,Ge0.96−xMnxBi0.04Te samples exhibit a much composition in the pseudobinary phase diagram of GeTe–MnTe higher S than the theoretical prediction. As a result of the high is about x = 0.18 (39). This discrepancy can be attributed to the S, our PFs were observably higher than those of the previous contribution of Bi doping, which also decreases the phase- reports (Fig. 4B). transition temperature to a certain extent. Fig. 2B shows the First-principles calculations, including electronic DOS and calculated lattice parameter and interaxial angle dependence on band-structure calculations, were performed to shed light on the Mn alloying concentration. It is apparent that Mn alloying leads to an almost linear decrease of lattice parameters in solid solution. Since α-GeTe is a slightly distorted rock-salt lattice along the (111) direction (37), the interaxial angle change from A 10 B 300 non-−90° to 90° after Mn alloying is consistent with XRD 250 ) measurement. Heat capacity measurements are displayed in -1 200

C Ohm m) Fig. 2 , which clearly show that Mn alloying gradually de- 1 VK -5

( 150 creases the phase-transition temperature. However, it is diffi- x=0 x=0.05 S (10 cult to detect the phase-transition temperature for x ≥ 0.1 by x=0.1 x=0.15 100 x=0.2 x=0.3 differential scanning calorimetry (DSC) measurement due to the 0.1 50 300 400 500 600 700 800 300 400 500 600 700 800 very small or perhaps zero latent heat. Additionally, the XRD Temperature (K) Temperature (K) measurements of Ge Mn Bi Te sample when heating up C D 0.66 0.3 0.04 50 24 180

to 473 K and cooling down to 300 K in air are performed, as ) ) Power factor ) -2 -2 -2 K D 40 K shown in Fig. 2 . It is obvious that all of the obtained XRD Weighted mobility 160 K -1 -1 20 patterns well match the cubic GeTe structure without the ap- -1 30 140

pearance of phase transition within the XRD detection limit. The Wcm 16 Wcm (

(Wcm 120 broad peaks in the DSC measurements may be due to the high 20 ( w PF heating rate during the DSC measurements causing an incomplete PF 12 100 10 phase transition. 300 400 500 600 700 800 0.00.10.20.3 Mn element is well known for its complex oxidation state, Temperature (K) Mn concentration (x) spanning from +2to+7, and the most common and stable oxi- ρ + Fig. 3. Temperature-dependent (A) ,(B) S,and(C) PF of Ge0.96−xMnxBi0.04Te dation state is 2 (46). It was previously reported that Mn in samples (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.3). (D) PF and weighted mobility μW GeTe–MnTe solid solution also showed the +2 that is identical dependence on Mn concentration at room temperature. The solid and dashed to that of the host atom Ge (40, 47), but Mn alloying gradually lines in D are included as guides for the eye.

5334 | www.pnas.org/cgi/doi/10.1073/pnas.1802020115 Liu et al. Downloaded by guest on October 2, 2021 A B will introduce spin scattering, which is detrimental to the mobility. 400 Ge Mn Bi Te 0.96-y y 0.04 25 Thus, it will be promising and also challenging to investigate )

Ge1+xTe -2 other alloying elements in the future to find nonmagnetic or K

) 300 20 -1

-1 GeTe Se m* = 5.5 m 1-x x weakly magnetic element ions that similarly allow carrier con- m* = 4 m Ge1-xSbxTe 15 centration optimization and stabilization of the cubic phase, 200 0 (VK ZT 0 m* = 10 m perhaps with even higher . (Wcm S 10 m* = 1.5 m m* = 2.5 m The κtot shows a significant reduction upon Mn alloying (Fig. 100 0 PF 5 A κ κ 0 7 ), as a result of both the decreased lat and ele. Heavy Mn 0 κ 110100110100alloying leads to the obvious suppression of lat due to the in- n 20 -3 H (10 cm ) n 20 -3 creased point-defect scattering. For example, room-temperature H (10 cm ) −1 −1 κlat decreases from 1.6 W m ·K for α-Ge0.96Bi0.04Te to −1 −1 −1 −1 Fig. 4. Hall carrier-concentration-dependent (A) S and (B) PF of Ge0.96−x 1.2 W m ·K for α-Ge0.86Mn0.1Bi0.04Te and to 1.1 W m ·K Mn Bi Te and previously studied compositions, including Ge + Te, Ge − x 0.04 1 x 1 x for β-Ge0.76Mn0.2Bi0.04Te (Fig. 7B). Additionally, the Debye– SbxTe, and GeTe1−xSex (33). The dashed lines in A were calculated by the Callaway model, shown as the solid line in Fig. 7B (Inset), basi- SPB model with m* = 1.5, 2.5, 4, 5.5, and 10 m , respectively, while the red 0 cally explains the decreasing trend of κlat with increasing Mn solid black line was obtained based on the modified two-valence-band model. The dashed line in B is included as a guide for the eye. concentration (43, 51), in which the longitudinal (3,400 m/s) and transverse (1,890 m/s) sound velocities of pure GeTe are obtained from ref. 36. To confirm the origin of the reduction of κlat role of Mn alloying in the significantly higher m* of rhombohe- upon Mn alloying, phonon dispersion and phonon density of α α dral and cubic GeTe. Fig. 5 compares the difference of the cal- states (PDOS) of both -GeTe and -Ge0.875Mn0.125Te were culated DOS between pure GeTe and after Mn alloying in calculated. Mn alloying in α-GeTe does not significantly alter rhombohedral and cubic GeTe, respectively. Introducing Mn the phonon dispersion (Fig. 7C), including acoustic modes made the DOS steeper in both rhombohedral and cubic GeTe, and optical modes with low frequency. In addition, the PDOS especially near the valence band edge (e.g., from −0.05 to at the low-frequency range from acoustic phonons is almost −0.2 eV for α-GeTe and from −0.25 to −0.3 eV for β-GeTe). unchanged upon Mn alloying. Computational results show This sharper DOS feature corresponds to the higher mass and is that Mn alloying does not significantly change the acoustic beneficial for enhancing the Seebeck coefficients, which is also phonon properties of rhombohedral GeTe despite the in- SCIENCES

consistent with the increased effective SPB m* after Mn alloying. duced substantial structure disorder. Furthermore, theoretical APPLIED PHYSICAL Fig. 6 shows the calculated electronic band structures for both calculations of κlat based on the Debye–Callaway model are the pure and Mn-doped GeTe supercell with spin-orbital cou- basically consistent with the experimental observations, which pling (SOC). The primitive band structure of both the rhombo- in turn indicates that Mn alloying can simply be regarded as hedral and cubic GeTe are essentially similar to the previously the point-defect scattering centers. In contrast, Murphy et al. reported ones (34). For rhombohedral and cubic pristine GeTe argued that soft optical mode transitions in Pb1−xGexTe (Fig. 6 A and C), the most beneficial feature is the multiple va- maximize the anharmonic acoustic–optical coupling and re- lence bands with relatively small band offset. Mn doping signif- sult in low κlat (52). Due to the presence of imaginary fre- icantly increases the nH and the corresponding Fermi level is quencies in the phonon dispersion of β-GeTe (SI Appendix,Fig. pushed downward into the multiple valence band, resulting in S5), it cannot provide a qualitative picture of the effect of Mn the multiple valence band contribution to carrier conducting. alloying on phonon transport in β-GeTe. Moreover, Mn alloying in rhombohedral GeTe realigns the bands, resulting in the contribution of the multiple band at dif- ferent points (Fig. 6B). This underlines the higher DOS, which is also beneficial for achieving high S (49), as demonstrated in various systems, such as PbTe (11), SnTe (50), Mg2Si (10, 13), etc. The calculated band structure without SOC can also support this conclusion (SI Appendix, Fig. S4). We have also shown the spin-polarized band structure in SI Appendix, Fig. S4 since the Mn alloying leads to a magnetic system (magnetic moment = + 5 μB/Mn) corresponding to the high spin state of Mn2 , which is consistent with the previously measured electron paramagnetic + resonance result (47). It should be noted that magnetic Mn2

A 10 B 10 α β 8 -GeTe-sc 8 -GeTe-sc α-Ge Mn Te β-Ge Mn Te 6 0.875 0.125 6 0.875 0.125

4 4 DOS (arb. units)

DOS (arb. units) 2 2

0 0 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 0.0 0.2 0.4 Energy (eV) Energy (eV)

Fig. 5. Comparison of the difference of the calculated DOS between pure Fig. 6. The calculated electronic band structures with SOC of (A) rhom-

GeTe and Mn-alloyed GeTe for (A) rhombohedral structure and (B) cubic bohedral structure α-GeTe, (B) α-Ge0.875Mn0.125Te, (C) cubic struc- structure. Black and red lines represent the pristine GeTe and Ge0.875Mn0.125Te, ture β-GeTe, and (D) β-Ge0.875Mn0.125Te. The dashed line represents the respectively. Fermi level.

Liu et al. PNAS | May 22, 2018 | vol. 115 | no. 21 | 5335 Downloaded by guest on October 2, 2021 Due to the balance between the decreased PF and the sup- A 2.0 B pressed κtot, the highest peak ZT at 773 K is almost unchanged Ref. 31 Ref. 33 x=0 x=0.05 3 1.6 This work This work 0.12

— α x=0.1 x=0.15 Te

after Mn alloying they are all about 1.5 for -Ge0.96Bi0.04Te, 2 Te

1.5 Te α β — x=0.2 x=0.3 1.2 Se

-Ge Mn Bi Te, and -Ge Mn Bi Te but the low- 0.04 0.86 0.1 0.04 0.81 0.15 0.04 0.04 Ref. 9 0.88 Ref. 50 ave Te/Bi Bi ZT A Bi temperature is enhanced somewhat (Fig. 8 ). In appli- 1.0 Te 0.1 Te 0.13 0.15 0.05 ZT 0.8 0.1

ZT (ZT) 0.09 Pb

cations, the average over the working temperature range Mn Se Mn Sb 0.87 0.9 0.86 0.95 0.5 Mn determines the conversion efficiency of a device (53, 54). For 0.4 0.81 0.91 -Ge

ZT -Ge -Ge

rhombohedral GeTe-based materials, the highest average -Ge α α α PbTe 0.0 0.0 β Sn from 300 to 773 K in our work is comparable with those of 300 400 500 600 700 800 previous reports (Fig. 8B) (31, 33). It should be highlighted that Temperature (K) the highest average ZT of cubic Mn-doped GeTe is higher than that of the current state-of-the-art p-type PbTe- (0.9) and SnTe- Fig. 8. (A) Temperature-dependent ZT of Ge0.96−xMnxBi0.04Te and (B)com- B parison of average ZT (from 300 to 773 K) of rhombohedral and cubic (0.4) based materials (Fig. 8 ) (9, 50). Therefore, we have Mn-doped GeTe as well as the state-of-the-art p-type rhombohedral GeTe, demonstrated the high performance of bulk cubic GeTe-based cubic PbTe, and SnTe (9, 31, 33, 50). materials, for which there is no phase transition over the whole temperature range from 300 to 773 K. Additionally, Mn alloying in the GeTe system also reduces the cost of raw materials since 0, 0.01, 0.05, 0.1, 0.15, 0.2, and 0.3), loaded into a stainless-steel jar in a glove less Ge is used. Both characteristics are beneficial for promoting box under argon atmosphere, and then subjected to ball milling for 5 h. The the GeTe system for energy harvesting. ball-milled powder was loaded into a die and hot pressed at 773 K for 2 min under a pressure of 90 MPa. Conclusions Phase and Property Characterizations. XRD analysis was performed using a In summary, we succeeded in suppressing the phase-transition ’ ∼ ∼ PANalytical multipurpose diffractometer with an X celerator detector temperature from 700 K to below 300 K to achieve cubic (PANalytical X’Pert Pro). Bar samples were cut from the pressed disks GeTe without phase transition from 300 to 773 K by a simple Bi and used for simultaneous measurement of electrical resistivity (ρ)and doping and Mn alloying on the Ge site. The suppression of the Seebeck coefficient (S) on a commercial system (ULVAC ZEM-3). The κ = phase transition to below room temperature is significant for any thermal conductivity was calculated using DCpd,whereD, Cp,andd are thermoelectric applications. Bi doping reduces the hole con- the thermal diffusivity, specific heat capacity, and density, respectively. centration and thus enhances ZT of the rhombohedral GeTe. The thermal diffusivity coefficient (D) was measured on a laser flash system (Netzsch LFA 457). The specific heat capacity (Cp) was measured on Mn alloying induced significant valence band modification and a DSC thermal analyzer (Netzsch DSC 404 C). The density (d)around increases the hole effective mass for both the rhombohedral and 6.2 g cm−3 was determined by the Archimedes method. The room-tem-

cubic GeTe, leading to a much higher Seebeck coefficient. The perature Hall coefficient RH was measured using the Physical Properties strong point-defect scattering for phonons caused by Bi and Mn Measurement System (Quantum Design). The Hall carrier concentration = μ largely reduces the lattice thermal conductivity, which leads to a (nH) was obtained by nH 1/eRH and the Hall carrier mobility ( H)was calculated by σ = eμ n , where e is the electronic charge and σ is the electrical peak ZT ∼1.5 at 773 K for cubic Ge0.81Mn0.15Bi0.04Te. Our work H H opens the door for further studies of phase transition in other conductivity. The uncertainty for the electrical conductivity is 3%, the Seebeck thermoelectric materials. coefficient is 5%, and the thermal conductivity is 7%, so the combined uncertainty for the PF is 13% and that for ZT value is 20%. To increase the Experimental Section readability of the curves, error bars were not shown in the figures. Synthesis. Appropriate raw materials, including Ge disks, Mn disks, Bi chunks, First-Principles Calculations. The electronic band-structure calculations were and Te chunks from Alfa Aesar, were weighed according to the nominal performed by adopting the generalized gradient approximation of the = = compositions Ge1−xBixTe (x 0, 0.04, and 0.08) and Ge0.96−xMnxBi0.04Te (x Perdew–Burke–Ernzerhof functional for the exchange-correlation poten- tial and the projector augmented wave method as implemented in the Vienna Ab initio Simulation Package (VASP) (55–57). The valence electrons included for Ge, Te, and Mn are 4s24p2,5s25p4,and3p64s23d5, respectively. A B The electron wave function was expanded in a plane-wave basis set with an energy cutoff of 400 eV. The convergence of the calculations were tested with dense k-point meshes. The structures were fully relaxed until the force − − on each atom was less than 10 5 eV Å 1 for both pure and Mn-doped GeTe. The effects of Mn doping were considered through a substitution of one Mn with one Ge atom in a 2 × 2 × 2 supercell that was built based on the original primitive cell in both cubic and rhombohedral phases. This yields a

composition of Ge0.875Mn0.125Te. The spin polarization was included with an initial magnetic moment of 5 μB on Mn. The supercell band structures were unfolded to the primitive Brillouin zone high-symmetry path using C D the BandUP code (58, 59). Phonons calculations were obtained within the harmonic approximation and using the finite displacement method based on the forces calculated via the Hellmann–Feynman theorem (60). A 2 × 2 × 2 supercell was set up for both pristine and Mn-doped rhombohedral phases, which consists of 128 atoms. The nonanalytical correction is applied by including the Born effective charges and dielectric constants calculated using the density functional perturbation theory.

ACKNOWLEDGMENTS. The work performed at the University of Houston and the University of Missouri is supported by the US Department of Energy under Award DE-SC0010831, as well as by US Air Force Office of Scientific Fig. 7. Temperature-dependent (A) κ and (B) κ of Ge − Mn Bi Te tot lat 0.96 x x 0.04 Research Grant FA9550-15-1-0236, the T. L. L. Temple Foundation, the John J. κ samples. (B, Inset) Room-temperature lat dependence on Mn concen- and Rebecca Moores Endowment, and the State of Texas through the Texas – tration, where the solid line is calculated by the Debye Callaway Center for Superconductivity at the University of Houston. J. Sui acknowl- model (43, 51). (C) Phonon dispersions and (D)PDOSofα-GeTe and edges support from the National Natural Science Foundation of China α-Ge0.875Mn0.125Te. (Grant 51622101).

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