Research Article | Open Access

Wan Yu Lyu, Min Hong, Wei Di Liu, Meng Li, Qiang Sun, Sheng Duo Xu, Jin Zou, Zhi-Gang Chen, "Rare-Earth Nd Inducing Record-High Thermoelectric Performance of (GeTe)_{85}(AgSbTe_{2})_{15}", *Energy Material Advances*, vol. 2021, Article ID 2414286, 8 pages, 2021. https://doi.org/10.34133/2021/2414286

# Rare-Earth Nd Inducing Record-High Thermoelectric Performance of (GeTe)_{85}(AgSbTe_{2})_{15}

#### Abstract

As a promising midtemperature thermoelectric material with both higher thermoelectric performance and mechanical property, Tellurium Antimony Germanium Silver (TAGS-*x*), written as (GeTe)* _{x}*(AgSbTe

_{2})

_{1-x}, especially (GeTe)

_{0.85}(AgSbTe

_{2})

_{0.15}(TAGS-85), has attracted wide attention. Herein, we innovatively use Nd doping to synergistically decrease the carrier concentration to the optimal level leading to enhanced dimensionless figure of merit, zT. Our density-functional theory calculation results indicate that Nd-doping reduced carrier concentration should be attributed to the enlargement of band gap. The optimized carrier concentration results in an ultrahigh power factor of ~32

*μ*W cm

^{-1}K

^{-2}at 727 K in Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te. Simultaneously, the lattice thermal conductivity of Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te retained as low as ~0.5 at 727 K. Ultimately, a record-high zT of 1.65 at 727 K is observed in the Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te. This study indicates rare-earth Nd doping is effective in boosting the thermoelectric performance of TAGS-85 and approached a record-high level via synergistic effect.

#### 1. Introduction

Fossil fuel overexploitation and the corresponding environment pollution have called for growing development of sustainable energy-utilization technologies [1]. Thermoelectrics, which can achieve the reciprocal energy conversion between heat and electricity, are one promising candidate [2, 3]. However, the wide commercialization of thermoelectrics is limited by the low thermoelectric energy conversion efficiency, evaluated by the dimensionless figure of merit, . Here, is the Seebeck coefficient, is the electrical conductivity, and represents the total thermal conductivity (accumulation of electron () and lattice () parts) [4]. is defined as the power factor to evaluate the overall electrical performance. To enhance zT, both high and low are required. Multiple strategies have been developed to enhance of thermoelectric materials, such as carrier concentration (, for holes) optimization [5, 6], resonance level engineering [7], band convergence [8–10], energy filtering [11], quantum confinement [12], and Rashba effect [13]. Alternatively, hierarchical phonon scattering centers, including point defects, dislocations, stacking faults, dense grain boundaries [14, 15], and phase interfaces, can effectively decrease and to achieve high zT values [16, 17].

IV-VI tellurides (Ge/Sn/Pb-Te) are promising midtemperature thermoelectric materials due to their relatively high zT values [5, 18–21]. For example, PbTe-based thermoelectric material has approached the high zT value of 2.2 at 915 K in PbTe-4 mol.% SrTe with 2 mol.% Na [18, 22]. However, the high toxicity of Pb limits its practical application. Alternatively, of SnTe is relatively higher which leads to lower zT value of only 1.51 at 800 K [23] comparing with PbTe [18] and GeTe [24]. Therefore, nontoxic and high-performance GeTe-based thermoelectric materials, with the peak zT values higher than 2 [24, 25], have attracted extensive attention. To boost the thermoelectric performance of GeTe with good mechanical property and make it suitable for application, rhombohedral GeTe is alloyed with cubic AgSbTe_{2} and forms a continuous solid solution, called as Tellurium Antimony Germanium Silver (TAGS-*x*, written as (GeTe)* _{x}*(AgSbTe

_{2})

_{1-x}, where ranges from 0.7 to 0.9) [26, 27]. TAGS-85, which can also be written as Ge

_{0.74}Ag

_{0.13}Sb

_{0.13}Te, has approached the highest peak zT (zT

_{max}) of 1.36 at 700 K with good mechanical properties (elastic modulus of 50 GPa and Poisson’s ratio of 0.24) [27]. However, the thermoelectric performance of TAGS-85 is still lower than other state-of-the-art thermoelectric materials, such as PbTe [28]. Thus, recent studies of TAGS-85 focus on further improving the thermoelectric performance. Self-doping achieved by tuning the ratio of Ag/Sb ratio can effectively enhance the zT

_{max}value of TAGS-85 up to 1.6 at 750 K [29]. Levin et al. [30, 31] reported that the large atomic size and localized magnetic moment of rare-earth elements Dy, Yb, and Ce can increase zT values of TAGS-85. For example, 1% Ce and 1% Yb substitution in Te site can boost up by 16% to 205

*μ*V K

^{-1}[30], attributed to the localized paramagnetic moment, leading to an enhanced zT

_{max}of 1.5 at 700 K.

To better understand the influence of rare-earth element doping on thermoelectric performance of TAGS-85, we selected Nd as the dopant to optimize thermoelectric performance of TAGS-85. As a rare-earth element, Nd as dopant can affect transport properties of thermoelectric materials in many ways: (a) Nd doping in TAGS-85 can result in local lattice distortion due to larger atomic size; (b) multielectron Nd^{3+} can affect carrier concentration due to different valence electron counts or influence of the chemical bond [32]; (c) different with large localized magnetic moments of Ce (4f^{1}5d^{1}6s^{2}) and Yb (4f^{13}6s^{2}), Nd (4f^{4}6s^{2}) has small localized magnetic effect, which can induce interesting change to [30, 31]. Considering the complex crystal structure of TAGS-85, three cation sites (Ge, Ag, and Sb) can be substitute by Nd (Figure 1(a)). Figure 1(b) compares the formation energy of Nd substitution at different cation sites of TAGS-85. As can be seen, Nd can preferentially substitute Sb due to the lowest formation energy. Here, we design the sample with nominal composition as Ge_{0.74}Ag_{0.13}Sb_{0.13-x}Nd* _{x}*Te (, 0.02, and 0.04). Figure 1(c) schematically shows the experiment process, where all samples are prepared by melting, quenching, annealing, and spark plasma sintering (SPS). Our results show that Nd doping can effectively reduce of TAGS-85 close to the optimal level due to band gap enlargement. Correspondingly, zT

_{max}can be boosted to as high as 1.65 in Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te.

#### 2. Experimental Methods

##### 2.1. Synthesis

Polycrystalline Nd-doped TAGS-85 was prepared in the formulae of Ge_{0.74}Ag_{0.13}Sb_{0.13-x}Nd* _{x}*Te (, 0.02, and 0.04). Precursors, including Ge (99.999%, Sigma-Aldrich, Australia), Te (99.999%, Sigma-Aldrich, Australia), Ag (99.99%, Sigma-Aldrich, United States), Sb (99.999%, Alfa Aesar, United States), and Nd (99.999%, Alfa Aesar, United States), were weighed following the nominal compositions. The weighted precursors were sealed in quartz ampules and heated in 1223 K for 12 hours followed by water quenching. The obtained samples were further annealed at 873 K for 2 days and quenched again. This two-step quenching method is designed to reduce Ge vacancies and inhibit the formation of the second phase, Ag

_{8}GeTe

_{6}[33]. The resultant products were further ground into powders and sintered into pellets (SPS, 65 MPa and 673 K) for performance measurement.

##### 2.2. Characterization

The crystal structures of as-prepared samples were characterized by X-ray diffraction (XRD, Bruker, United States, Cu K_{α} radiation with a wavelength of 1.5418 Å, , step with 0.02°). A field-emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL, Japan) equipped with energy-dispersive spectrum (EDS) detector was employed to explore the morphologies and compositions. To further understand the structural information, the Ge_{0.74}Ag_{0.13}Sb_{0.11}Nd_{0.02}Te sample was characterized by a transmission electron microscope (TEM, Tecnai F20, Philips, United States).

##### 2.3. Thermoelectric Property Measurement

and of the sintered pellets were simultaneously measured by ZEM-3 (ULVAC Technologies, Inc., Japan). is calculated by , where is the specific heat estimated from the Dulong-Petit approximation [34], is the pellet densities measured by Archimedes method, the densities are shown in supporting information Table S1, and is the thermal diffusivity coefficient measured by the laser flash diffusivity apparatus (LFA 457, Netzsch, Germany). The Hall coefficient () was measured based on the Van der Pauw technique [35]. and hole mobility () were through the equation and , where is the electron charge.

##### 2.4. Density Functional Theory Calculations

Band structures were calculated by using the plane-wave self-consistent field code [36]. A uniform mesh of -points was used in sampling integrations over the Brillouin zone. The band structures of pristine TAGS-85 and Nd-doped TAGS-85 were calculated based on Ge_{19}Ag_{4}Sb_{4}Te_{27} and Ge_{19}Ag_{4}Sb_{3}Nd_{1}Te_{27} with the uniform mesh of -points, respectively.

#### 3. Results and Discussions

Figure 2(a) shows room temperature X-ray diffraction (XRD) patterns of the as-sintered (Ge_{0.74}Ag_{0.13}Sb_{0.13-x}Nd* _{x}*Te) (, 0.02, and 0.04) pellets. All main peaks can be indexed as R-GeTe (space group R3m) whose structure is a slightly distorted rock-salt structure along the [111] direction [20]. Ge precipitates (purple triangle) were detected as normal due to the nature of easily formed Ge vacancies in GeTe-based thermoelectric materials [37–39]. When the Nd doping level reaches as high as , NdTe

_{2}(yellow diamond) and NdTe

_{3}(blue square) impurities can be found. Figure 2(b) shows the backscattered scanning electron microscopy (SEM) image and corresponding energy-dispersive spectrum (EDS) maps of the Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te sample. As can be seen, Ge, Te, Ag, Sb, and Nd are homogeneously distributed in the matrix, indicating that Nd has successfully doped into TAGS-85. Figures 2(c) and 2(d) show high-resolution (HR) transition electron microscopy (TEM) image of Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te pellet along the [110] zone axis (indexed from the selected area electron diffraction (SEAD) pattern as shown in the inset of Figure 2(c)). Figure 2(d) shows its enlarged HRTEM image in the marked square area of Figure 2(c), where the dislocations caused by Nd doping-induced dense point defects can be clearly observed.

Figure 3(a) shows temperature-dependent of the as-sintered Ge_{0.74}Ag_{0.13}Sb_{0.13-x}Nd* _{x}*Te pellets. All samples show degenerated semiconducting behavior, and of Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te decreases from 1178.10 at room temperature to 687.45 S cm

^{-1}at 673 K. With increasing the Nd doping level, at room temperature reduces from 1424.45 to 1108.89 S cm

^{-1}. The observed reduced should be attributed to reduced as shown in Figure 3(b). Figure 3(c) shows temperature-dependent of the as-sintered Ge

_{0.74}Ag

_{0.13}Sb

_{0.13-x}Nd

*Te pellets. With increasing the Nd doping level, room temperature of Ge*

_{x}_{0.74}Ag

_{0.13}Sb

_{0.13-x}Nd

*Te slightly increases from 92 to 104*

_{x}*μ*V K

^{-1}due to the reduced . of Ge

_{0.74}Ag

_{0.13}Sb

_{0.09}Nd

_{0.04}Te can approach as high as 210

*μ*V K

^{-1}at 678 K. Figure 3(d) illustrates temperature-dependent of the as-sintered Ge

_{0.74}Ag

_{0.13}Sb

_{0.13-x}Nd

*Te pellets. An enhanced of 32*

_{x}*μ*W cm

^{-1}K

^{-2}at 727 K can be observed in Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te.

To understand the change of electrical transport properties, we calculated the electronic band structure of pristine TAGS-85 (Ge_{19}Ag_{4}Sb_{4}Te_{27}) and Nd-doped TAGS-85 (Ge_{19}Ag_{4}Sb_{3}Nd_{1}Te_{27}). Figure 4(a) compares the calculated electronic band structures of low-temperature (LT) Ge_{19}Ag_{4}Sb_{4}Te_{27} and Ge_{19}Ag_{4}Sb_{3}Nd_{1}Te_{27}. After Nd doping, the valence band maximum (VBM) shifts result in changed band gap from indirect to direct type. The band gap increases from 0.18 eV of Ge_{19}Ag_{4}Sb_{4}Te_{27} to 0.26 eV of Ge_{19}Ag_{4}Sb_{3}Nd_{1}Te_{27}. Nd doping also effectively enlarges the band gap of high-temperature (HT) TAGS-85 from 0.1 eV of Ge_{19}Ag_{4}Sb_{4}Te_{27} to 0.31 eV of Ge_{19}Ag_{4}Sb_{3}Nd_{1}Te_{27}, as shown in Figure 4(b). The enlarged band gap of both LT and HT TAGS-85 after Nd doping can well explain the decrease of .

Figures 5(a)–5(c) show temperature-dependent thermal transport properties of the as-sintered Ge_{0.74}Ag_{0.13}Sb_{0.13-x}Nd* _{x}*Te pellets. As shown in Figure 5(a), of TAGS-85 slightly decreases after Nd doping. However, with increasing the content of Nd to 0.04, the room temperature increases to 1.76 W m

^{-1}K

^{-1}. To better understand the effect of Nd doping on the thermal transport properties of TAGS-85, and were further calculated as shown in Figures 5(b) and 5(c). was estimated by subtracting from . and the Lorenz number () were calculated based on the classic single parabolic band model. As can be seen, decreases from 0.85 to 0.65 W m

^{-1}K

^{-1}at room temperature with increasing the Nd content due to reduced . With 2% Nd doping, shows no obvious change. When the Nd doping level further increases to 4% to surpass the solubility limit, the room temperature increases from 0.70 to 1.11 W m

^{-1}K

^{-1}due to additional impurity phases NdTe

_{2}and NdTe

_{3}[40]. Figure 5(d) plots temperature-dependent zT values of the as-sintered Ge

_{0.74}Ag

_{0.13}Sb

_{0.13-x}Nd

*Te pellets and shows increased zT values with increasing temperature. The zT*

_{x}_{max}value of Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te can reach 1.65 at 727 K. Figure 5(e) compares the zT

_{max}values of this study with previously reported Ce-, Yb-, and Dy-doped TAGS-85 [30, 31]. In our study, Nd tends to substitute the Sb site, which is different from Ce, Yb (Te site), and Dy (Ge site). Different occupation sites can result in various effects on electrical and thermal properties. Thus, Nd doping in the Sb site can decrease both the carrier concentration and thermal conductivity. This results in the highest zT value of 1.65 at 727 K. Figure 5(f) presents the reproductivity of Ge

_{0.74}Ag

_{0.13}Sb

_{0.11}Nd

_{0.02}Te in three cycling tests, suggesting high stability of the Nd-doped TAGS-85.

#### 4. Conclusion

In this study, we achieve a record-high zT of 1.65 at 727 K in Ge_{0.74}Ag_{0.13}Sb_{0.11}Nd_{0.02}Te mainly due to reduction while maintaining high . Reduced should be attributed to reduced due to decreased . Although reduced deteriorates , reduced also slightly increases , leading to nearly unchanged . Our DFT calculation suggests that reduced is caused by the Nd doping-induced band gap enlargement. Comparing with other rare-earth elements, Nd is more effective in enhancing the thermoelectric performance of TAGS-85. The Nd-doped TAGS-85 also shows high stability.

#### Data Availability

All data required to support this study are presented in the paper and the supporting document. Additional data are available upon request from the authors.

#### Conflicts of Interest

The authors declare that they have no conflicts of interest.

#### Acknowledgments

This work was financially supported by the Australian Research Council, Innovation Centre for Sustainable Steel project, and University of Southern Queensland strategic research grant. LWY thanks the Chinese Scholarship Council for providing the Ph.D. stipend.

#### Supplementary Materials

Figure S1: Hall mobility () of the Nd-doped TAGS-85 at the room temperature. Table S1: the density of as-prepared samples.* (Supplementary Materials)*

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