Transport properties of p-type CaMg2Bi2 thermoelectrics

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Transport properties of p-type CaMg2Bi2 thermoelectrics Cheng Sun a, Xuemin Shi a, Liangtao Zheng a, Bo Chen b, c, **, Wen Li a, * a

Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai, 201804, China School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai, 201804, China c Key Laboratory of Performance Evolution and Control for Engineering Structures of Ministry of Education, Tongji Univ., 1239 Siping Rd., Shanghai, 200092, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 July 2019 Received in revised form 23 August 2019 Accepted 5 September 2019 Available online xxx

Due to the complex crystal structure for low lattice thermal conductivity and the tunable valence bands for superior electronic performance, CaAl2Si2-structured AB2C2 Zintl compounds have been frequently proven as promising p-type thermoelectric materials. In this work, thermoelectric properties of CaMg2Bi2 are systematically investigated in a broad carrier concentration (1018e1020 cm
3) through Agdoping for comprehensively evaluating its potential for thermoelectric applications. The broad carrier concentration enables a well assessment of the carrier transport properties by single parabolic band with acoustic phonon scattering and a revelation of the carrier transport by multiple valence orbitals when the carrier concentration higher than ~2  1019 cm
3, leading to a significant enhancement in electronic performance. With the help of additional point defect phonon scattering introduced by BaMg2Bi2alloying, a reduction in lattice thermal conductivity in the entire temperature range and the lowest one of ~0.7 W/m-K are achieved, leading to a 100% enhancement in average zTave. in addition to the contribution of a multiband transport. This work not only demonstrates CaMg2Bi2 as a promising thermoelectric material, but also provides a well understanding of its underlying material physics. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Recycling of waste heat has become a worldwide topic for increasing efficiency of the fossil fuel and relieving environmental pollution. Thermoelectrics have been widely considered as a top solution for these issues due to the capability of direct conversion between heat and electricity based on the Seebeck effect. The conversion efficiency is determined by the thermoelectric dimensionless figure of merit (zT), which is strongly related to the Seebeck coefficient (S), absolute temperature (T), resistivity (r), and electronic (kE) and lattice (kL) components of thermal conductivity via zT ¼ S2T/r(kEþkL). It is well-known that the electronic parameters including S, r and kE are strongly coupled with each other via carrier concentration, band structure and scattering of carrier. This leads the individual optimization on one parameter to be difficult. Therefore,

* Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai, 201804, China. E-mail addresses: [email protected] (B. Chen), [email protected] (W. Li). Peer review under responsibility of The Chinese Ceramic Society.

minimizing kL, the only one independent parameter determining zT, has long acted as the key strategy for enhancing zT. This can be effectively achieved by introducing various defects, such as point defects [1,2], dislocation [3e5] and nanostructures [6e8], for strengthening phonon scattering due to the mass and strain fluctuation. In addition, the materials with complex crystal structure [9], soft chemical bonds [10e12], low sound velocity [10], strong lattice anharmonicity [13,14], liquid-like ions [15e17] and vacancy [18] always show an intrinsically low kL. Alternatively, the strategies of band convergence [19]/nestification [20] for increasing band degeneracy (Nv) and resonant states [21] for distorting band structure near Fermi level, has successfully decoupled the interrelation among electronic parameters, leading to a significant enhancement of power factor (PF]S [2]/r) due to the increased density of state effective mass (m*). This has frequently led to zT-breakthrough in various materials such as PbTe [22,23], half-Heusler [24e26], SnTe [27e29], Mg2Si [30,31], GeTe [32e34], Zintl phase [35,36] and Te [20]. Moreover, it has also been proven that the low inertial effective mass (mI*) for high carrier mobility and deformation potential coefficient (Edef) for weak electron-phonon coupling are beneficial for the high PF [37,38]. Recently, CaAl2Si2-structured AB2C2 (A ¼ Eu, Yb, Mg, Ca, Sr, Ba;

https://doi.org/10.1016/j.jmat.2019.09.002 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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B]Mg, Zn, Cd; C]Sb, Bi) Zintl compounds [39] have been promoted to be promising candidates for p-type thermoelectric applications, which largely results from the tunable valence bands for superior electronic performance [40]. In more detail, a multiplevalence-band at the Brillouin zone center (G) consists of a doubly degenerate pxy and nondegenerate pz band orbitals due to the crystal field splitting [40]. Importantly, the energy offsets DE ¼ E [G(pxy)] -E[G(pz)] between the two orbitals are commonly lower than ~0.2 eV40. This opens a possibility for increasing Nv and then PF by decreasing the DE between two orbitals or by deepening the Fermi level into tow orbitals [6,36,41]. With the help of low intrinsic kL due to the complex crystal structure and further kL-reduction, competitive zTs have been achieved in AB2C2 thermoelectrics [6,36,40,42,43]. Pristine CaMg2Bi2 comes with a small DE of only 0.1 eV, implying a possibility of PF-enhancement [40]. Na-doping at Ca site and YbMg2Bi2-alloying for enhancing thermoelectric performance of CaMg2Bi2 have been studied and a peak zT up to 0.9 has been achieved at 850 K6, 44
45. These motivate the current work to focus on a comprehensive investigation of its transport properties for fully assessing the potential for thermoelectric applications and a revelation of the fundamental physical parameters (effective mass, deformation potential coefficient, elastic properties and critical lattice vibrational parameters) for further exploring the potential of these materials for diversified applications beyond thermoelectricity. In this work, transport properties of CaMg2Bi2 are systematically investigated in a carrier concentration range of 2  1018e1  1020 cm
3 by Ag-doping at Ca site. Such a broad carrier concentration enables an approximation of electrical properties by the single parabolic band (SPB) model with acoustic phonon scattering. The increased carrier concentration deepens the Fermi Level into the low energy valence orbital, leading to an increased overall band degeneracy and therefore power factor. Moreover, the underlying physical parameters for CaMg2Bi2 are revealed. With the help of further alloying with BaMg2Bi2 for introducing additional phonon scattering by point defects, a reduced lattice thermal conductivity of 0.7 W/m-K is obtained at 700 K. 2. Materials and methods All the polycrystalline samples were synthesized using the stoichiometric quantity of high purity elements (>99.9%) by sealing in graphite crucible and stainless steels tube under Ar atmosphere and then in vacuum quartz ampoules, melting at 1423K for 2 h, quenching in cold water and eventually annealing at 1073K for 2 days. The obtained ingots were hand ground into fine powders for hot press, which was carried out by an induction-heating hot press system [46] at 1073K for 0.5 h under uniaxial pressure of ~70 MPa. Dense pellets with theoretical density >97% and diameter of ~12 mm were obtained. The electrical properties including resistivity, Seebeck coefficient and Hall coefficient were measured in the temperature range of 300e700 K under helium atmosphere. The Seebeck coefficient was obtained from the slope of thermopower versus temperature gradient within 0e5 K. The resistivity and Hall coefficient were measured by a four-probe Van Der Pauw technique under a reversible superconductor magnetic field of 1.5 T. The thermal conductivity (к) is determined by к ¼ dCpD, where d is the density estimated by mass/volume method, Cp is the heat capacity of the Dulong-Petit limit assumed to be temperature-independent, D is the thermal diffusivity measured by a laser flash technique (Netzsch LFA 457). The uncertainty in measurements of S, r, k and Hall coefficient is about 5%. Phase composition and microstructure are characterized by X-

Ray diffraction (XRD) and Scanning Electronic Microscopy (SEM, PHenom Pro) equipped with an Energy Dispersive Spectrometer (EDS). The longitudinal (vL) and transverse (vT) sound velocities were measured using ultrasonic pulse-receiver (Olympus-NDT) equipped with an oscilloscope (Key-sight) at room temperature. The optical reflectance was measured by Fourier Transform Infrared Spectroscopy (FTIR, Bruker Tensor II) equipped with a Diffuse Reflectance attachment at room temperature. 3. Results and discussion Room temperature XRD patterns for all the samples are shown in Fig. 1aeb. Majority of the diffraction peaks can be well indexed to the CaAl2Si2-structured CaMg2Bi2. Trace amount of Bi impurity is detected presumably resulting the loss of Ca and Mg during sample synthesis. According to a Rietveld refinement of XRD result of CaMg2Bi2 (Fig. S1), the concentration of Bi precipitate is about 3.4%. Theoretically, the existence of Bi precipitates generally introduces additional scattering on the charge carriers and phonons, and eventually lead to a reduction in carrier mobility as well as resistivity and lattice thermal conductivity, respectively. This is further confirmed by the SEM observation and EDS mapping analysis, as shown in Fig. 1c. The matrix is found to be almost homogeneous and with high purity. Bi precipitate has also been observed in Mg3Bi2-YbMg2Bi2 solid solutions [47]. Based on the optical measurements, as shown in Fig. 2a, the estimated optical band gap (Eg) for all the samples is about ~0.25 eV, which is comparable to that of the existing thermoelectric materials with high performance (SnTe [48], PbTe [49], CoSb[350]). Moreover, the Eg is found to be independent of the composition. Due to the intrinsically low carrier concentration (n) of ~3  1018 cm
3 for pristine CaMg2Bi2, Ag-doping at Ca site is utilized to increase n. Room temperature n is effectively increased up to ~1  1020 cm
3 by Ag-doping (Fig. 2b). Such a broad n enables a systematic investigation on the electronic transport properties and a well understanding of the fundamental physical parameters. The thermoelectric properties for CaMg2Bi2 based materials is unstable as the measurement temperature up to 750 K (Fig. S2), while these are highly reproducible as the measurement temperature lower than 700 K (Fig. S3). Therefore, the investigation of the thermoelectric properties for the obtained samples is focused on the temperature range of 300e700 K. Temperature dependent Hall carrier concentration (nH) and Hall mobility (mH) for the samples Ag-doped is shown in Fig. 2b. A bipolar conduction can be seen at T > 500 K for pristine CaMg2Bi2 (Fig. 2b). Moreover, the nearly constant nH, as well as Hall coefficient (RH ¼ 1/enH, e is the electron charge) in the entire temperature range for the doped/alloyed samples, suggests a conduction by a single type of carriers. Temperature-dependent mH for all the samples well follows the relationship of mH ~ T
1.5, indicating the charge carrier scattering dominated by acoustic phonons. Interestingly, the mH remains nearly unchanged even with the increased nH due to Ag-doping. Due to the small DE of only 0.1 eV between pxy and pz valence orbitals [40], a multiband transport behavior is expected in CaMg2Bi2-based materials for a superior electronic performance [42]. According to the discussions of the RH and mH, a single parabolic band (SPB) model with acoustic phonon scattering is utilized to predict the electronic transport properties. nH dependent Seebeck coefficient is shown in Fig. 2c. For the samples with nH lower than 1  1019 cm
3, nH-dependent Seebeck coefficient is found to be well predicted by the SPB model using a constant density of states effect mass (m*) of ~0.4 me. However, an increased m* of ~0.5 me is required for the prediction on nH versus Seebeck coefficient for the samples with nH higher than 2  1019 cm
3. The increase in m* suggests the multiband transport behavior in the samples with

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Fig. 1. Room temperature powder XRD patterns for Ca1-xAgxMg2Bi2 and Ca0.7-xBa0.3AgxMg2Bi2 (a) and Ca1-yBayMg2Bi2 (b); SEM image and corresponding EDS mappings for Ca0.993Ag0.007Mg2Bi2.

high nH, resulting from the deepening of the Fermi level into lowenergy valence band by the increased nH. The similar results have also been observed in BaCd2Sb2-based materials [42]. As shown in Fig. 2d, it is found that the nH-dependent mH for the samples with nH<~1019 cm
3 and >~2  1019 can be well predicted by the SPB model using m* of 0.4 me and 0.5 me, respectively. Such an increase in the mH for the sample with high nH can be well understood by the involvement of a low-mass valence band. It can roughly infer that the valence bands at G position for CaMg2Bi2 consists of the low-energy doubly degenerate pxy band (Nv ¼ 2) and high-energy nondegenerate pz band (Nv ¼ 1)[51
52]. Therefore, an increased Nv of 3 could be achieved in the samples with high nH. Eventually, both increased Seebeck coefficient and mH lead to a significant enhancement in PF, as shown in Fig. 2e. Temperature dependent Seebeck coefficient and resistivity for Ca1-xAgxMg2Bi2 are shown in Fig. 3a and Fig. 3b, respectively. All the samples are labeled by Ag concentration and room temperature nH. The resistivity and Seebeck coefficient for pristine CaMg2Bi2 decrease at T > 500 K due to the thermally excited minority carriers, which is consistent with the Hall measurement (Fig. 2b). Moreover, both of them for Ag-doped samples show a continuously increase with increasing temperature, indicating a degenerate semiconducting behavior. The decrease of Seebeck coefficient with increasing Ag content dominantly results from the increased nH, which the decreased resistivity stems from both increased nH and mH. Temperature dependent total (k) and lattice thermal conductivity (kL) for Ca1-xAgxMg2Bi2 are shown in Fig. 3ced, respectively. kL

is estimated by subtracting the electronic thermal conductivity (ke) via Wiedemann-Franz law, ke ¼ LT/r, where L is Lorenz number is determined by the SPB model with acoustic scattering [53e55]. The increase in k with increasing Ag content stems from the increased ke due to nH-increase (Fig. 3c. The slight decrease in kL (Fig. 3d) can be understood by the point defects introduced by Ag-doping. Amorphous limits (kmin L , dash line in Fig. 3d of ~0.35 W/m-K and ~0.13 W/m-K are estimated according to the Cahill model [56] and the newly developed model taking the periodic boundary conditions into account [57] (named as Bvk-Pei model [58]), respectively. Both of them are much lower than the measured results in this work. This suggests that it still remains a big room for enhancing zT by further kL-reduction such as further introducing point defects. Therefore, BaMg2Bi2-allloying is used to further reducing kL of CaMg2Bi2. Room temperature XRD patterns and composition dependent lattice parameters for Ca1-yBayMg2Bi2 are shown in Fig. 1b and Fig. S4a, respectively. The solid solutions of CaMg2Bi2-BaMg2Bi2 are formed with x up to 0.5, as shown in Fig. 1b. However, it is found that the sample with x ¼ 0.5 is unstable, therefore, the investigation of thermoelectric properties is focused on the samples with x  0.4. The linear increase in the lattice parameters (Fig. S4a) can be understood by the larger atomic size of Ba than that of Ca. As shown in Fig. S4b, the pristine and alloyed samples show a comparable nH and an unchanged mechanism of charge carrier scattering. The details of the transport properties are shown in Fig. 2 and Fig. S5. All of them show an intrinsic semiconducting behavior. Importantly, the kL for Ca1-yBayMg2Bi2 is found to continuously decrease in the

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Fig. 2. Normalized optical absorption versus photon energy (a), Temperature dependent Hall carrier concentration (nH) and Hall mobility (mH) (b), Hall carrier concentration dependent Seebeck coefficient (c), Hall mobility (d) and power factor (e), and temperature dependent density of state effective mass (m*) and deformation potential coefficient (Edef) (f) for the samples with and without doping/alloying at room temperature.

entire temperature range with increasing x due to the strengthened phonon scattering by Ca/Ba substitutional defects, as shown in Fig. 4a. To quantitatively understand the effect of point defects on kL, sound velocities (vs) including longitudinal (vl) and transverse (vt) branches are measured at room temperature with a deviation lower than 5% and listed in Table S1. Both vl and vt are found to be nearly unchanged for Ag-doped and BaMg2Bi2-alloyed samples, therefore the effect of the sound velocity on kL is excluded. Using the measured sound velocities, the physical parameters including Debye Temperature (qD), Poisson ratio (ε), shear (G) and bulk (B) modulus and Grüneisen parameter (g) are estimated [2,11,59,60] as listed in Table S1. Additionally, composition-dependent kL at different temperatures for Ca1-yBayMg2Bi2 is seen to be well predicted by Debye-Callaway model [53e55,61], as shown in Fig. 4b, which suggests Ca/Ba substitutional point defects are dominantly responsible for the kL-reduction.

Furthermore, Ag-doping for nH-optimization is carried out on Ca0.7Ba0.3Mg2Bi2 for enhancing the electronic performance and then zT. The transport properties of Ag-doped Ca0.7Ba0.3Mg2Bi2 are shown in Fig. 2 and Fig. S6. The nH for Ca0.7Ba0.3Mg2Bi2 is effectively increased by Ag-doping as well (Fig. 2b, and all the samples show a degenerated semiconducting behavior (Figs. S6a and S6b). The prediction of the electronic properties by the SPB model with m* of 0.5 me (Fig. 2c and d) reveals an enhanced PF in Ag-doped Ca0.7Ba0.3Mg2Bi2 (Fig. 2e). Temperature dependent k and kL are shown in Figs. S6c and S6d, respectively. The estimated physical parameters for Ag-doped Ca0.7Ba0.3Mg2Bi2 are also included in Table S1, which suggest the reduction in kL dominantly resulting from additional phonon scattering by Ca/Ag and Ca/Ba substitutional point defects. As a result, the lowest kL of ~0.7 W/m-K is obtained at 700 K. Temperature dependent zT for Ag-doped CaMg2Bi2 and Ca0.7Ba0.3Mg2Bi2 are shown in Fig. 5a. Due to the increased nH leading to a multiband transport behavior in Ag-doped CaMg2Bi2, an enhanced

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Fig. 3. Temperature dependent Seebeck coefficient (a), resistivity (b), total (c) and lattice (d) thermal conductivities for Ca1-xAgxMg2Bi2, with a comparison to these of pristine CaMg2Bi45 2 .

Fig. 4. Temperature dependent lattice thermal conductivity (a) and composition dependent lattice thermal conductivity at different temperatures (b) for Ca1-yBayMg2Bi2.

peak zT is achieved. With the help of BaMg2Bi2-alloying for further reducing kL, a peak zT of ~0.7 is obtained at 700 K for Ca0.68Ba0.3Ag0.02Mg2Bi2. Additionally, the average zTave in the temperature range of 300e700 K is shown in Fig. 5b. It is found that 100% enhancement in zTave is achieved in this work comparing to that of pristine CaMg2Bi2. 4. Conclusion In summary, Ag-doping is found to effectively increase carrier concentration of CaMg2Bi2 from ~1018 cm
3 up to ~1020 cm
3. Such

a broad carrier concentration enables a full assessment of its thermoelectric properties by SPB model with acoustic phonon scattering, and provides a well understanding of the underlying physical parameters. Due to the existence of multiple valence bands, a multiband transport behavior is realized by a heavy Agdoping, leading to a significant enhancement in electronic performance. With the help of BaMg2Bi2-alloying for introducing point defects with high concentration, a reduced lattice thermal conductivity of ~0.7 W/m-K is obtained. The combined electronic and thermal engineering lead to both increased zT and zTave, indicating it as a promising p-type thermoelectric material.

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Fig. 5. Temperature dependent figure of merit (zT) (a) and average zTave at 300e700 K (b) for Ag-doped CaMg2Bi2 and Ca0.7Ba0.3Mg2Bi2, with a comparison to them from literatures [44,45].

Conflicts of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is supported by Shanghai Natural Science Foundation (19ZR1459900) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.09.002. References [1] Zhang J, Xu B, Wang LM, Yu D, Liu Z, He J, Tianb Y. Great thermoelectric power factor enhancement of CoSb3 through the lightest metal element filling. Appl Phys Lett 2011;98(7):072109. [2] Shi X, Yang J, Salvador JR, Chi MF, Cho JY, Wang H, Bai SQ, Yang JH, Zhang WQ, Chen LD. Multiple-filled skutterudites: high thermoelectric figure of merit through separately optimizing electrical and thermal transports. J Am Chem Soc 2011;133(20):7837e46. [3] Chen ZW, Ge BH, Li W, Lin SQ, Shen JW, Chang YJ, Hanus R, Snyder GJ, Pei Y,Z. Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics. Nat Commun 2017;8:13828. [4] Kim SI, Lee KH, Mun HA, Kim HS, Hwang SW, Roh JW, Yang DJ, Shin WH, Li XS, Lee YH, Snyder GJ, Kim SW. Thermoelectrics. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics. Science 2015;348(6230):109e14. [5] Wu Y, Chen Z, Nan P, Xiong F, Lin S, Zhang X, Chen Y, Chen L, Ge B, Pei Y. Lattice strain advances thermoelectrics. Joule. 2019. [6] Shuai J, Geng H, Lan Y, Zhu Z, Wang C, Liu Z, Bao J, Chu CW, Sui J, Ren Z. Higher thermoelectric performance of Zintl phases (Eu0.5Yb0.5)1-xCaxMg2Bi2 by band engineering and strain fluctuation. Proc Natl Acad Sci U S A 2016;113(29): E4125e32. [7] Poudel B, Hao Q, Ma Y, Lan YC, Minnich A, Yu B, Yan XA, Wang DZ, Muto A, Vashaee D, Chen XY, Liu JM, Dresselhaus MS, Chen G, Ren ZF. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008;320(5876):634e8. [8] Xie W, He J, Kang H, Tang X, Zhu S, Laver M. Identifying the specific nanostructures responsible for the high thermoelectric performance of (Bi,Sb)2Te3 nanocomposites. Nano Lett 2010;10(9):3283e9. [9] Snyder GJ, Toberer ES. Complex thermoelectric materials. Nat Mater 2008;7(2):105e14. [10] Li W, Lin S, Ge B, Yang J, Zhang W, Pei Y. Low sound velocity contributing to the high thermoelectric performance of Ag8SnSe6. Adv Sci (Weinh) 2016;3(11):1600196. [11] Ying P, Li X, Wang Y, Yang J, Fu C, Zhang W, Zhao X, Zhu T. Hierarchical chemical bonds contributing to the intrinsically low thermal conductivity in

a-MgAgSb thermoelectric materials. Adv Funct Mater 2017;27(1):1604145. [12] Li W, Lin S, Weiss M, Chen Z, Li J, Xu Y, Zeier WG, Pei Y. Crystal structure induced ultralow lattice thermal conductivity in thermoelectric Ag9AlSe6. Adv Energy Mater 2018;8(18):1800030. [13] Li CW, Hong J, May AF, Bansal D, Chi S, Hong T, Ehlers G, Delaire O. Orbitally driven giant phonon anharmonicity in SnSe. Nat Phys 2015;11(12):1063e9. [14] Chang C, Zhao L-D. Anharmoncity and low thermal conductivity in thermoelectrics. Mater Today Phy 2018;4:50e7. [15] Liu H, Shi X, Xu F, Zhang L, Zhang W, Chen L, Li Q, Uher C, Day T, Snyder GJ. Copper ion liquid-like thermoelectrics. Nat Mater 2012;11(5):422e5. [16] Liu H, Yuan X, Lu P, Shi X, Xu F, He Y, Tang Y, Bai S, Zhang W, Chen L, Lin Y, Shi L, Lin H, Gao X, Zhang X, Chi H, Uher C. Ultrahigh thermoelectric performance by electron and phonon critical scattering in Cu2Se1-xIx. Adv Mater 2013;25(45):6607e12. [17] Zhao K, Qiu P, Song Q, Blichfeld AB, Eikeland E, Ren D, Ge B, Iversen BB, Shi X, Chen L. Ultrahigh thermoelectric performance in Cu 2
ySe0.5S0.5 liquid-like materials. Mater Today Phy 2017;1:14e23. [18] Li W, Lin SQ, Zhang XY, Chen ZW, Xu XF, Pei YZ. Thermoelectric properties of Cu2SnSe4 with intrinsic vacancy. Chem Mater 2016;28(17):6227e32. [19] Pei Y, Shi X, LaLonde A, Wang H, Chen L, Snyder GJ. Convergence of electronic bands for high performance bulk thermoelectrics. Nature 2011;473(7345): 66e9. [20] Lin S, Li W, Chen Z, Shen J, Ge B, Pei Y. Tellurium as a high-performance elemental thermoelectric. Nat Commun 2016;7(10287):10287. [21] Heremans JP, Wiendlocha B, Chamoire AM. Resonant levels in bulk thermoelectric semiconductors. Energy Environ Sci 2012;5:5510e30. [22] Chen ZW, Jian ZZ, Li W, Chang YJ, Ge BH, Hanus R, Yang J, Chen Y, Huang MX, Snyder GJ, Pei YZ. Lattice dislocations enhancing thermoelectric PbTe in addition to band convergence. Adv Mater 2017;29(23). [23] Pei Y, LaLonde AD, Heinz NA, Snyder GJ. High thermoelectric figure of merit in PbTe alloys demonstrated in PbTeeCdTe. Adv. Energy Mater. 2012;2(6): 670e5. [24] Zhu T, Fu C, Xie H, Liu Y, Zhao X. High efficiency half-heusler thermoelectric materials for energy harvesting. Adv Energy Mater 2015;5(19):1500588. [25] Fu C, Zhu T, Pei Y, Xie H, Wang H, Snyder GJ, Liu Y, Liu Y, Xinbing Z. High band degeneracy contributes to high thermoelectric performance in p-type halfheusler compounds. Adv Energy Mater 2014;4:1400600. [26] He R, Zhu H, Sun J, Mao J, Reith H, Chen S, Schierning G, Nielsch K, Ren Z. Improved thermoelectric performance of n-type half-Heusler MCo1-xNixSb (M ¼ Hf, Zr). Mater Today Phy 2017;1:24e30. [27] Li W, Wu Y, Lin S, Chen Z, Li J, Zhang X, Zheng L, Pei Y. Advances in environment-friendly SnTe thermoelectrics. ACS Energy Lett 2017;2(10): 2349e55. [28] Li W, Chen ZW, Lin SQ, Chang YJ, Ge BH, Chen Y, Pei YZ. Band and scattering tuning for high performance thermoelectric Sn1-xMnxTe alloys. J Materiomics 2015;1(4):307e15. [29] Li W, Zheng L, Ge B, Lin S, Zhang X, Chen Z, Chang Y, Pei Y. Promoting SnTe as an eco-friendly solution for p-PbTe thermoelectric via band convergence and interstitial defects. Adv Mater 2017;29(17). [30] Liu W, Tan X, Yin K, Liu H, Tang X, Shi J, Zhang Q, Uher C. Convergence of conduction bands as a means of enhancing thermoelectric performance of ntype Mg2Si1-xSnx solid solutions. Phys Rev Lett 2012;108(16):166601. [31] Liu X, Zhu T, Wang H, Hu L, Xie H, Jiang G, Snyder GJ, Zhao X. Low electron scattering potentials in high performance Mg2Si0.45Sn0.55 based thermoelectric solid solutions with band convergence. Adv. Energy Mater. 2013;3(9): 1238e44. [32] Li J, Zhang X, Chen Z, Lin S, Li W, Shen J, Witting IT, Faghaninia A, Chen Y, Jain A, Chen L, Snyder GJ, Pei Y. Low-symmetry rhombohedral GeTe thermoelectrics. Joule 2018;2(5):976e87.

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C. Sun et al. / Journal of Materiomics xxx (xxxx) xxx [33] Hazan E, Ben-Yehuda O, Madar N, Gelbstein Y. Functional graded germaniumlead chalcogenide-based thermoelectric module for renewable energy applications. Adv Energy Mater 2015;5(11):1500272. [34] Hong M, Chen ZG, Yang L, Zou YC, Dargusch MS, Wang H, Zou J. Realizing zT of 2.3 in Ge1-x-ySbxIny Te via reducing the phase-transition temperature and introducing resonant energy doping. Adv Mater 2018;30(11). [35] Zhang J, Song L, Pedersen SH, Yin H, Hung LT, Iversen BB. Discovery of highperformance low-cost n-type Mg3Sb2-based thermoelectric materials with multi-valley conduction bands. Nat Commun 2017;8:13901. [36] Wang X, Li J, Wang C, Zhou BQ, Zheng LT, Gao B, Chen Y, Pei YZ. Orbital alignment for high performance thermoelectric YbCd2Sb2 alloys. Chem Mater 2018;30(15):5339e45. [37] Pei Y, LaLonde AD, Wang H, Snyder GJ. Low effective mass leading to high thermoelectric performance. Energy Environ Sci 2012;5(7):7963e9. [38] Wang H, Pei Y, LaLonde AD, Snyder GJ. Weak electron-phonon coupling contributing to high thermoelectric performance in n-type PbSe. Proc Natl Acad Sci U S A 2012;109(25):9705e9. [39] Shuai J, Mao J, Song SW, Zhang QY, Chen G, Ren ZF. Recent progress and future challenges on thermoelectric Zintl materials. Mater Today Phy 2017;1:74e95. [40] Zhang J, Song L, Madsen GK, Fischer KF, Zhang W, Shi X, Iversen BB. Designing high-performance layered thermoelectric materials through orbital engineering. Nat Commun 2016;7:10892. [41] Wood M, Aydemir U, Ohno S, Snyder GJ. Observation of valence band crossing: the thermoelectric properties of CaZn2Sb2eCaMg2Sb2 solid solution. J Mater Chem 2018;6(20):9437e44. [42] Wang X, Li W, Zhou B, Sun C, Zheng L, Tang J, Shi X, Pei Y. Experimental revelation of multiband transport in heavily doped BaCd2Sb2 with promising thermoelectric performance. Mater Today Phy 2019;8:123e7. [43] Zevalkink A, Zeier WG, Cheng E, Snyder J, Fleurial J-P, Bux S. Nonstoichiometry in the Zintl phase Yb1
dZn2Sb2 as a route to thermoelectric optimization. Chem Mater 2014;26(19):5710e7. [44] Shuai J, Liu Z, Kim HS, Wang Y, Mao J, He R, Sui J, Ren Z. Thermoelectric properties of Bi-based Zintl compounds Ca1
xYbxMg2Bi2. J Mater Chem 2016;4(11):4312e20. [45] Shuai J, Kim HS, Liu Z, He R, Sui J, Ren Z. Thermoelectric properties of Zintl compound Ca1
xNaxMg2Bi1.98. Appl Phys Lett 2016;108(18):183901. [46] LaLonde AD, Ikeda T, Snyder GJ. Rapid consolidation of powdered materials by induction hot pressing. Rev Sci Instrum 2011;82(2):025104. [47] Zhou T, Mao J, Jiang J, Song S, Zhu H, Zhu Q, Zhang Q, Ren W, Wang Z, Wang C, Ren Z. Large reduction of thermal conductivity leading to enhanced thermoelectric performance in p-type Mg3Bi2eYbMg2Bi2 solid solutions. J Mater Chem C 2019;7(2):434e40. [48] Rogers L. Valence band structure of SnTe. J Phys D Appl Phys 1968;1(7):845. [49] Crocker AJ, Rogers LM. Valence band structure of PbTe. J Phys Colloq 1968;29(C4):129e32. C4. [50] Sofo JO, Mahan GD. Electronic structure of CoSb3: a narrow-band-gap semiconductor. Phys Rev B 1998;58(23):15620e3. [51] Sun JF, Shuai J, Ren ZF, Singh DJ. Computational modelling of the thermoelectric properties of p-type Zintl compound CaMg2Bi2. Mater Today Phy

7

2017;2:40e5. [52] Zhang J, Song L, Madsen GKH, Fischer KFF, Zhang W, Shi X, Iversen BB. Designing high-performance layered thermoelectric materials through orbital engineering. Nat Commun 2016;7:10892. [53] Callaway J, Vonbaeyer HC. Effect of point imperfections on lattice thermal conductivity. Phys Rev 1960;120(4):1149e54. [54] Klemens PG. Thermal resistance due to point defects at high temperatures. Phys Rev 1960;119(2):507e9. [55] Abeles B. Lattice thermal conductivity of disordered semiconductor alloys at high temperatures. Phys Rev 1963;131(5):1906e11. [56] Cahill DG, Watson SK, Pohl RO. Lower limit to the thermal conductivity of disordered crystals. Phys Rev B 1992;46(10):6131e40. [57] Chen ZW, Zhang XY, Lin SQ, Chen LD, Pei YZ. Rationalizing phonon dispersion for lattice thermal conductivity of solids. Natl Sci Rev 2018;5(6):888e94. [58] Jeffrey Snyder G, Agne Matthias T, Gurunathan R. Thermal conductivity of complex materials. Nat Sci Rev 2019;6(3):380e1. [59] Bozhko VV, Novosad ОV, Parasyuk OV, Kozer VR, Vertelis V, Nekrosius A, Ka zukauskas V. Influence of cation-vacancy defects on the properties of CuInSe2eZnIn2Se4 solid solutions. J Alloy Comp 2015;618:712e7. [60] Sanditov DS, Belomestnykh VN. Relation between the parameters of the elasticity theory and averaged bulk modulus of solids. Tech Phys 2011;56(11): 1619e23. [61] Klemens PG. The scattering of low-frequency lattice waves by static imperfections. Proc Phys Soc 1955;A68(12):1113e28.

Wen Li is an assistant researcher at Tongji University, China. His research is focused on developing new thermoelectric materials and understanding the material parameters that determine the thermoelectric properties, as well as engineering of these parameters for further enhancement of performance. He received a M.E. and a B.E. both from Zhengzhou University, China and a Ph. D from Shizuoka University, Japan.

Cheng Sun, received a B.E. in College of Chemistry, Chemical Engineering and Material Science from Suzhou university. He is now a master student at Tongji University under the supervision of Prof. Wen Li. His research focuses on thermoelectric Zintl phases.

Please cite this article as: Sun C et al., Transport properties of p-type CaMg2Bi2 thermoelectrics, Journal of Materiomics, https://doi.org/10.1016/ j.jmat.2019.09.002