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[Airan Li](https://orcid.org/0009-0004-7318-4821), [Longquan Wang](https://orcid.org/0009-0009-9910-9770), [Xinzhi Wu](https://orcid.org/0000-0002-5545-8460), Jiankang Li, [Xinyuan Wang](https://orcid.org/0000-0002-0218-8452), [Gang Wu](https://orcid.org/0009-0007-0201-3787), [Zhao Hu](https://orcid.org/0000-0003-4252-6572), [Takao Mori](https://orcid.org/0000-0003-2682-1846)

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[Semiconductor-metal transition powers high-efficiency MgAgSb thermoelectrics](https://mdr.nims.go.jp/datasets/8b6a0ba4-38a1-4f1d-99ba-25f462028bd3)

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Semiconductor-metal transition powers high-efficiency MgAgSb thermoelectricsLi et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e1 of 9C H E M I S T R YSemiconductor-metal transition powers high-efficiency MgAgSb thermoelectricsAiran Li1†, Longquan Wang1†, Xinzhi Wu1†, Jiankang Li1,2, Xinyuan Wang1,2, Gang Wu1,  Zhao Hu1,2, Takao Mori1,2*Because of the inferior thermoelectric performance of metals, the semiconductor-to-metal transition in thermo-electric materials is always avoided. Here, we demonstrate that α-to-β semiconductor-metal transition in MgAgSb is actually not detrimental but can be strategically exploited to create α/β-MgAgSb junction, enabling 150% en-hancement in output power while maintaining high conversion efficiency. This power enhancement lies in the notably reduced internal resistance induced by semiconductor-to-metal transition, which is independent of di-mensional changes. Consequently, α/β-MgAgSb can simultaneously achieve high maximum conversion efficiency exceeding 10% (9%) and maximum power density above 1 (0.9) W cm−2 by simulation (experiment), outperform-ing most p-type materials under identical conditions. In addition, a two-pair thermoelectric module combining α/β-MgAgSb with n-type Mg3Sb0.6Bi1.4 achieves an unprecedented power density, representing notable advance-ments over existing Mg3(Sb,Bi)2/MgAgSb two-pair system. These findings highlight the immense potential of α/β-MgAgSb for thermoelectric applications and provide insights into the design of high-power thermoelectrics.INTRODUCTIONThe increasing consumption of fossil fuels has led to vast amounts of carbon dioxide being released into the atmosphere, accompanied by notable waste heat generation. This not only exacerbates climate change but also poses a formidable challenge to achieving a carbon-neutral society. Addressing these issues necessitates innovative tech-nologies capable of efficiently utilizing wasted energy resources. Thermoelectric (TE) technology, which directly converts heat into electricity without moving parts, represents a highly promising so-lution (1, 2). In addition to waste heat power generation by harvest-ing heat from the environment, TE devices can provide a sustainable power source for electronic devices and numerous sensors, where decentralized and energy-efficient power systems are critical. This demands TE devices having both high conversion efficiency η and high power density ω (3, 4).Traditionally, the primary focus in TE research has been on achieving a high figure of merit zT of the material, as it is directly correlated with maximum conversion efficiency ηmax of TE devices (5–7). zT is defined as zT = S2σ/κ × T, where S, σ, κ, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Over the past few decades, substantial progress has been made in developing multifunctional high-performance TE materials and strategies to enhance their zT (8–11). As a result, the ηmax of state-of-the-art TE devices has reached ~7 to 10% near the room temperature range and 10 to 15% in the mid to high-temperature range (12–18).While high ηmax is critical, achieving high maximum power den-sity ωmax is equally important for practical applications (19, 20). High ωmax directly determines the output power capability and overall en-ergy utilization rate of TE devices, making it indispensable for real-world applications. Enhancing the power factor (PF  = S2σ) of TE materials is vital for achieving high ωmax. The PF is determined by carrier transport behavior and can be improved through band struc-ture engineering and carrier density optimization (21, 22). However, while certain metals could exhibit high PF due to their excellent elec-trical properties, their inherently high κ poses a challenge in achiev-ing high zT and, consequently, high ηmax (23). Nonetheless, it should be mentioned that metals can often find a niche as TE interface ma-terials (TEiMs), where high σ and κ are desirable to minimize energy loss during heat and electricity transport (24, 25).For typical TE materials, the intricate interplay among S, σ, and κ makes it particularly challenging to simultaneously achieve high S, high σ, and low κ, which are essential for achieving both high ηmax and ωmax. Among the wide array of TE materials, Bi2Te3-based com-pounds stand out for their high PF and high zT at room tempera-ture, making them the only TE materials commercialized, primarily for cooling applications (26–28). However, their use in power gen-eration remains limited due to a notable decrease in zT above room temperature, resulting in relatively low ηmax. This limitation is now being challenged by the emergence of n-type Mg3(Sb,Bi)2 and p-type MgAgSb (29, 30). MgAgSb stands out as one of the most prom-ising p-type TE materials for applications in the room temperature range, offering its excellent TE properties, air robustness, and eco-friendliness (31–33). Advancements such as phase purity control (34, 35), doping optimization (36, 37), global softening (38), and grain size regulation (39) have notably improved the zT of MgAgSb, reaching values between 1.0 and 1.6. In addition, the development of TEiMs has further accelerated its application in power generation (24, 29, 40, 41). However, despite achieving ηmax exceeding 7%, the ωmax of its corresponding TE modules remains low, requiring fur-ther enhancements to meet diverse power demands.A unique feature of MgAgSb is its multiple phase transitions with temperature (31). Among these phases, only the semicon-ducting α-MgAgSb exhibits excellent TE performance, whereas both β-MgAgSb and γ-MgAgSb show much lower zT values (42). As a result, the application of MgAgSb is limited to its α-phase range. Notably, γ-MgAgSb is highly stable once formed, posing a major challenge to realizing the high performance of MgAgSb (31, 35). In comparison, 1Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044 Japan. 2Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8671 Japan.*Corresponding author. Email: mori.​takao@​nims.​go.​jp†These authors contributed equally to this work.Copyright © 2025 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025mailto:mori.​takao@​nims.​go.​jphttp://crossmark.crossref.org/dialog/?doi=10.1126%2Fsciadv.adx7115&domain=pdf&date_stamp=2025-07-04Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e2 of 9β-MgAgSb remains less extensively studied. Reports indicate that β-MgAgSb exhibits metallic behavior, characterized by high σ but low S and high κ (31). These properties result in a low zT for β-MgAgSb, rendering it undesirable in MgAgSb-based applications. Con-sequently, the α-to-β semiconductor-metal transition is typically avoided to prevent the introduction of the low-zT β-MgAgSb, limit-ing the operation temperature of MgAgSb-based TE devices below 573 to 593 K (29).In this work, contrary to previous understanding, we demonstrate that the α-to-β phase transition in MgAgSb can be leveraged to enhance the performance of MgAgSb-based TE devices. This semiconductor-metal transition allows the creation of an α/β-MgAgSb junction, which not only maintains high efficiency but also achieves a notable 150% in-crease in output power compared to pristine α-MgAgSb. Simulation (and experiment) show that α/β-MgAgSb can achieve ηmax exceeding 10% (9%) and ωmax above 1 (0.9) W cm−2, outperforming other repre-sentative p-type TE materials under identical conditions. We investi-gate the α-to-β phase transition and find that α-MgAgSb can transform into β-MgAgSb and revert between room temperature and 623 K, with β-MgAgSb demonstrating exceptional stability, enabling continuous power generation without degradation even at 623 K. We then fabri-cate two-pair TE modules with MgAgSb and n-type Mg3(Sb,Bi)2. The MgAgSb/Mg3Sb0.6Bi1.4 two-pair module achieves ηmax of 8.6% and record-high ωmax of 0.5 W cm−2 (10 by 10 mm2 base), advancing exist-ing MgAgSb/Mg3(Sb,Bi)2 modules. This work underscores the potential of MgAgSb for efficient power generation and provides valuable insights for designing future high-performance TEs with both high output power and high efficiency.RESULTSSemiconducting α-MgAgSb and metallic β-MgAgSbMgAgSb exhibits three distinct phases: α-MgAgSb, β-MgAgSb, and γ-MgAgSb (31). With increasing temperature, α-MgAgSb transi-tions to β-MgAgSb first. As shown in Fig. 1A, both α-MgAgSb and β-MgAgSb share a tetragonal crystal structure but differ in their -2-1012300 400 500 6000.10.020.4(mohm·cm)T (K)max (%) Rin (m ohm)Pmax (mW)MgAgSbV0 (mV)MgAgSbE(eV)cbaABCEX P N MS|S0EFSemiconductorMetal11.110.826%50%X M Z R ZAEFRef. (38)Ref. (47)Ref. (36)DFRef. (32)Ref. (45)Ref. (39) Ref. (37)Ref. (34)Ref. (35)Ref. (42)Ref. (13)Ref. (46)0.060.100.140.1860 80 100 120 140 160 180 2001/Rin (ohm-1)Pmax(W)5.47.2103 154TH = 573 K57 55TH = 623 K300 6000.01.6zTT (K)5100.2 0.4 0.6 0.8 1.0 1.2Simulationmax (W cm-2)max(%) GeTe(Bi,Sb)2Te3ZintlBiCuSeOCu2SePbTep-type TE leg (same dimension)TH = 623 KFig. 1. Semiconducting α-MgAgSb and metallic β-MgAgSb. (A) Crystal structure of semiconducting α-MgAgSb and metallic β-MgAgSb. (B) Band structures of α-MgAgSb and β-MgAgSb. (C) T-dependent ρ for α-MgAgSb and β-MgAgSb, with the inset showing their corresponding T-dependent zT values. (D) Simulated ηmax, Pmax, Rin, and V0 for α-MgAgSb and α/β-MgAgSb single TE legs. The inset is the schematic diagram of the α-MgAgSb leg and α/β-MgAgSb junction leg when TH = 573 and 623 K, respectively. (E) Comparison of simulated 1/Rin versus Pmax of α-MgAgSb and α/β-MgAgSb in this work with the literature (13, 32, 34–39, 42, 45–47). (F) Comparison of simulated ηmax versus ωmax of α/β-MgAgSb with representative p-type TE materials, including (Bi,Sb)2Te3 (48), GeTe (15), PbTe (49), Zintl (18), BiCuSeO (50), and Cu2Se (51).Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e3 of 9space groups: α-MgAgSb belongs to I-4c2, while β-MgAgSb adopts P4/nmm. Previous studies suggest that the α-to-β phase transition is driven by Ag atom rearrangement, which induces a semiconductor-to-metal transition in MgAgSb (43). Specifically, α-MgAgSb is a typi-cal semiconductor with a bandgap of ~0.18 eV, while β-MgAgSb, as a metal, lacks a bandgap (Fig. 1B). The temperature dependence of resistivity ρ further confirms this behavior: α-MgAgSb shows semi-conductive characteristics, with its ρ decreasing due to bipolar con-duction, whereas β-MgAgSb exhibits metallic transport behavior, as indicated by its increasing ρ with temperature (Fig. 1C). Generally, metals have high carrier concentrations, resulting in a very high σ (low ρ) but a very low S. In addition, the high σ of metals typically leads to high κ according to Wiedemann-Franz law, resulting in low zT values. Figure S1 displays the σ, S, and κ of MgAgSb. Because of its low S and high κ, the metallic β-MgAgSb has a much lower zT compared to the semiconducting α-MgAgSb, as shown in the inset of Fig. 1C.For a long time, the α-to-β semiconductor-metal transition was avoided to prevent the formation of β-MgAgSb due to its low zT. However, changing the viewpoint, the metallic nature of β-MgAgSb can actually make it a promising candidate as a TEiM, especially when used alongside α-MgAgSb to form an α/β-MgAgSb junction. Unlike previous junctions made from different materials with different compositions, this junction is formed by materials with the same composition but two different crystallographic phases, which may help mitigate atomic diffusion. A simple method to create the α/β-MgAgSb junction is by increasing the hot-side temperature TH. When the TH of α-MgAgSb rises to 623 K, an α/β-MgAgSb junc-tion forms naturally, as shown in the inset of Fig. 1D. Compared to α-MgAgSb at TH = 573 K, the increase in the temperature gradient (ΔT = TH – TC, where TC is the cold-side temperature) could benefit both the ηmax and the maximum output power Pmax, as both are pos-itively correlated with ΔT (44).However, the emergence of β-MgAgSb also alters the overall TE properties of MgAgSb. The potential increase in ηmax may be offset by a reduced average figure of merit zTavg, but the Pmax may be notably enhanced due to the semiconductor-metal transition. To accurately as-sess the performance of the α/β-MgAgSb junction, finite-element anal-ysis was used. As shown in Fig. 1D, although the ηmax of α/β-MgAgSb is slightly reduced, it remains comparable to that of α-MgAgSb (97%). Crucially, Pmax is notably enhanced (~150%) owing to the reduced in-ternal resistance Rin induced by the α-to-β semiconductor-metal tran-sition. Moreover, it should be noted that open-circuit V0 remains nearly identical for both α-MgAgSb and α/β-MgAgSb. This highlights the pivotal role of the semiconductor-metal transition in minimizing Rin, thereby notably improving the Pmax. In addition to electrical ben-efits, the metallic nature and high κ of β-MgAgSb may also help buffer against temperature fluctuations, a critical factor for the practical de-ployment of MgAgSb-based TE modules.It is crucial to emphasize that the enhancement in Pmax results from the decreased Rin is similar but different from modifying the physical dimensions of the TE leg. On one hand, the emergence of the β-MgAgSb phase partially replaces the original α-MgAgSb, ef-fectively shortening the length of α-phase. At the same time, because of the metallic nature of β-MgAgSb, this substitution substantially reduces the internal resistance, thereby enhancing the output power. On the other hand, unlike conventional approaches that increase Pmax by adjusting the TE leg’s height or cross-sectional area, the improvement achieved through the α-to-β semiconductor-metal transition is independent of dimensional changes. As shown in figs. S2 to S3, regardless of variations in the TE leg’s height or area, once the α/β-MgAgSb junction forms, an ~150% increase in Pmax can be achieved while maintaining ηmax. This further emphasizes the intrinsic ability of the α-to-β transition to reduce Rin and en-hance power output. Detailed simulated current I dependence of output voltage V, heat flow, Q output power P, and conversion effi-ciency η for single MgAgSb TE legs with varying dimensions are provided in figs. S4 to S8.Contrary to previous understanding, it is evident that the emer-gence of β-MgAgSb is not detrimental; instead, it presents an op-portunity to substantially enhance the output power. To further demonstrate the benefits of α-to-β semiconductor-metal transition in MgAgSb, we simulated the Rin and Pmax of various α-MgAgSb reported in literature (2, 13, 32, 34–39, 45–47) under identical di-mensions (3.8 mm by 3.8 mm by 6 mm). The detailed I-dependent V and P are provided in figs. S9 to S12. Figure 1E summarizes the relationship between 1/Rin and Pmax of a single MgAgSb TE leg in this work compared to literature (2, 13, 32, 34–39, 45–47). The re-sults demonstrate that the α-to-β transition enables the α/β-MgAgSb junction to achieve a lower Rin and record-high Pmax, underscoring the superior potential of α/β-MgAgSb over α-MgAgSb alone.In terms of both ηmax and ωmax, α/β-MgAgSb can simultane-ously achieve a high ηmax exceeding 10% and ωmax above 1 W cm−2. To benchmark its performance, we compared α/β-MgAgSb with other representative p-type TE materials by simulation, including (Bi,Sb)2Te3 (48), GeTe (15), PbTe (49), Zintl (18), BiCuSeO (50), and Cu2Se (51), under identical conditions (TE leg dimensions: 3.8 mm by 3.8 mm by 6 mm; TH = 623 K). The detailed I-dependent V, P, Q, and η characteristics of these materials are shown in figs. S13 to S18, and Fig. 1F summarizes the ηmax versus ωmax. It can be seen that α/β-MgAgSb exhibits excellent ηmax and ωmax compared to others, further demonstrating its strong potential for applications.α-to-β phase transitionDespite the promising potential of α/β-MgAgSb, the formation and stability of β-MgAgSb require thorough investigation for its practical applications. The α-to-β phase transition was studied first. As shown in Fig. 2A, the x-ray diffraction (XRD) patterns of MgAgSb dur-ing heating reveal that the phase remains indexed to α-MgAgSb at 573 K. However, upon reaching 623 K, β-MgAgSb begins to emerge. In addition, minor amounts of Sb and Ag3Sb impurities appear dur-ing heating, consistent with previous reports (31, 52, 53). Once β-MgAgSb forms, it persists at 573 K during cooling but reverts to α-MgAgSb when the temperature is further reduced to 523 K, as depicted in Fig. 2B. A hysteresis phenomenon is observed between the α-to-β and β-to-α phase transitions. This hysteresis in phase transition temperature is also reflected in the TE transport prop-erties, including S, σ, and κ, as shown in  Fig.  2C and  Fig.  2E, respectively. Notably, when β-MgAgSb reverts to α-MgAgSb, the TE properties nearly return to their original values, indicating good reversibility between α and β-phase. In contrast, for γ-MgAgSb, once it forms, it will persist even at room temperature, adversely affecting the performance of α-MgAgSb (52). However, β-MgAgSb can revert to α-MgAgSb without notably compromising its performance de-spite the persistence of trace Sb and Ag3Sb impurities upon cooling (Fig. 2B).The good reversibility between α-MgAgSb and β-MgAgSb also reflects the good stability of MgAgSb, which can be further supported Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e4 of 9by the homogeneous elemental distribution of Mg, Ag, and Sb revealed by energy dispersive x-ray spectroscopy (EDS) mappings after the heating and cooling (Fig. 2F). Furthermore, Fig. 2 (G and H) shows the thermogravimetric–differential scanning calorimetry (TG-DSC) curves of MgAgSb, which illustrate the reversible phase transitions between α-MgAgSb and β-MgAgSb, along with the corresponding mass changes. No mass changes were detected after heating and cool-ing, further confirming the good stability of MgAgSb with heating and cooling. This robustness is also evident in the subsequent heating and cooling cycle, where identical endothermic and exothermic peak positions were observed, and no mass variations were detected. This highlights the suitability of MgAgSb for practical applications, even with the α-to-β phase transition.Highly stable β-MgAgSbAlthough the phase transition between α-MgAgSb and β-MgAgSb is reversible and repeatable, the long-term thermal stability of β-MgAgSb is crucial for practical applications. As shown in Fig. 3A, repeated measurements of S and σ for β-MgAgSb over six cycles are consistent. Furthermore, extended stability tests at 623 K show no noticeable degradation, with the normalized values of σ/σ0 and S/S0 remaining around 1.0 (Fig. 3B). This demonstrates the outstanding thermal stability of β-MgAgSb, even at temperatures higher than the 573 K used for α-MgAgSb, making it highly suitable for long-term applications. It is also worth noting that although small amounts of Sb and Ag3Sb form during heating, their metallic nature and low concentrations have minimal impact on the stability and metallic trans-port properties of β-MgAgSb.To further demonstrate its potential, a MgAgSb single TE leg was fabricated, with the cold-side temperature fixed at 293 K, while the hot-side temperature was varied at 373, 473, 573, and 623 K. Figure S19 shows the corresponding measured I dependence of V, P, Q, and η. When the TH reached 623 K, β-MgAgSb naturally and in  situ formed, bonding with α-MgAgSb. As shown in Fig. 3C, the ηmax of 20 40 60 80 20 40 60 80300 400 500 6000.91.01.10100200300010203040300 400 500 600012345Intensity(a.u.)2 (°)A Heating298 K323 K373 K473 K523 K573 K623 KSb Ag3Sb2 (°)B Cooling623 K573 K523 K473 K373 K323 K298 KSb Ag3Sb2nd heating2nd cooling1st heating1st coolingHeatflow(a.u.)GFSEMMgAg SbExo.584 K532 KMassT (K)HeatingCoolingS(–1)C(104Sm–1)DHeatingCooling(Wm–1K–1 )T (K)EHeatingCoolingFig. 2. Reversible and repeatable α-to-β phase transition. T-dependent XRD patterns of MgAgSb during (A) heating and (B) cooling, showing the reversible α-to-β and β-to-α phase transitions. T dependence of (C) S, (D) σ, and (E) κ of MgAgSb during heating and cooling within room temperature and 623 K. (F) Scanning electron micros-copy (SEM) image and EDS mappings of Mg, Ag, and Sb in MgAgSb after a complete heating and cooling cycle. (G) TG-DSC curves of MgAgSb during the first and second heating and cooling tests. a.u., arbitrary unit.Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e5 of 9the α-MgAgSb TE leg at 573 K is 9.2%, while for the α/β-MgAgSb TE leg at 623 K, ηmax slightly decreased to 9.0% due to the reduced overall zT value. The trend of the measured results aligns well with theoretical predictions, although slightly lower.As discussed earlier, the α-to-β semiconductor-metal transition enables the α/β-MgAgSb junction to achieve a low Rin and record-high Pmax. This improvement can further be seen in Fig. 3D, where the Pmax of the α/β-MgAgSb TE leg is notably higher than that of the α-MgAgSb TE leg. In addition, Fig. 3 E and F shows a substantial reduction in Rin and a slight increase V0 for α/β-MgAgSb (623 K) compared to α-MgAgSb (573 K). It is worth noting that the previous simulation did not account for the contact resistance between α- and β-MgAgSb. The agreement between experimental and simulated results in Fig. 3E suggests that the contact resistance between α-MgAgSb and β-MgAgSb is minimal. Furthermore, the metallic nature of β-MgAgSb also ensures good electrical contact with the electrode. These findings highlight the excellent reliability and performance of α/β-MgAgSb, offering low Rin, high ηmax, and enhanced Pmax. The stability of the α/β-MgAgSb TE leg was further evaluated through multiple measurement cycles at a set TH of 623 K and TC of 293 K. As shown in Fig. 3 (G and H), the α/β-MgAgSb TE leg consistently maintained its high ηmax, and its Pmax remained stable over repeated cycles. This high stability can be attributed to the exceptional thermal stability of β-MgAgSb at elevated temperatures, ensuring the reliable performance of α/β-MgAgSb for practical TE applications. In addi-tion to the intrinsic stability of the material, the close similarity in 0 1 2 3 4 5 625303540202530350.51.00.01.5300 400 500 600051015300 400 500 6000.00.10.2300 400 500 60010200510150 2 4 6 8 10 12 140.00.10.26810120 5 10 150.100.15300 400 500 6000204060800 10 20 300.51.00.0SMeasured timesA(104Sm-1)573 K623 K573 KBC D E FGHS-1)0At 623KAt 623KTheoreticalMeasuredmax(%)TH (K)293 K623 K Pmax(W)TH (K)TheoreticalMeasuredRin(mohm)TH (K)TheoreticalMeasured(%)9.0 %TC = 293 K TH = 623 KP(W)Measured times125 mWTC = 293 K TH = 623 Kmax(%)Pmax(W)Measured timesV0(mV)TH (K)TheoreticalMeasuredS/S0Measured timesFig. 3. Stability of β-MgAgSb and α/β-MgAgSb single TE leg. (A) T dependence of S and σ of β-MgAgSb over six repeated measurements. (B) Ratios of σ/σ0 and S/S0 for β-MgAgSb during multiple measurements at 623 K. TH dependence of measured and theoretical (C) ηmax, (D) Pmax, (E) Rin, and (F) V0 of MgAgSb single TE leg. Measured (G) η, ηmax and (H) P, Pmax of MgAgSb single TE leg under multiple measurements with a set TH of 623 K and TC of 293 K.Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e6 of 9thermal expansion between α-MgAgSb and β-MgAgSb also plays an important role in preserving the mechanical integrity of the leg (fig. S20). Moreover, the uniform elemental distribution across the α/β-MgAgSb leg after repeated tests further confirms the good sta-bility of α/β-MgAgSb (fig. S21).Besides, we also examined the post-test microstructure of α/β-MgAgSb (fig. S22). Hot and cold sides of α/β-MgAgSb exhibit simi-lar transgranular fracture patterns with small intragrain pores. Notably, the fracture surface at hot side reveals notably larger grains, consistent with thermally driven grain growth at ~573 K reported previously (39). This grain growth can help reduce charge carrier and phonon scattering, thereby potentially improving the σ and κ of β-MgAgSb. These findings further confirm the effective role of β-MgAgSb when applied at high temperatures above 593 K. To our knowledge, this finding also enables the MgAgSb constituent device to be used above 593 K for the first time, unlocking possibilities for efficient waste heat recovery in steelworks.Excellent power generation employing β-MgAgSbThe boosted Pmax, maintained ηmax, and demonstrated high stability of α/β-MgAgSb highlight its strong potential for practical applica-tions. Typically, TE devices are composed of both n-type and p-type TE legs. In this study, p-type MgAgSb is coupled with n-type Mg3(Sb,Bi)2 to fabricate two-pair modules. To fully demonstrate the advantages of α/β-MgAgSb, two types of two-pair modules were as-sembled: one using Bi-rich Mg3Sb0.6Bi1.4 and the other using Sb-rich Mg3Sb1.5Bi0.5. Figure 4 (A and B) illustrates the I dependence of η and P, respectively, in Bi-rich Mg3Sb0.6Bi1.4/MgAgSb two-pair mod-ule, with I dependence of V and Q shown in fig. S23. Similarly, when β-MgAgSb forms at the hot side of α-MgAgSb (TH = 623 K), both maintained ηmax and improved Pmax are observed compared to α-MgAgSb alone (TH = 573 K). Moreover, when the operating tem-perature exceeds the α-β transition point of MgAgSb, the effective temperature gradient across the n-type Mg3Sb0.6Bi1.4 leg also in-creases, further enhancing the overall power output and conversion efficiency of the device. Similar results are also evident in modules using Sb-rich Mg3Sb1.5Bi0.5 (Fig. 4, C and D, and fig. S24). These find-ings further strengthen the capability of α/β-MgAgSb to enhance P while maintaining high η. Figure 4E summarizes the ratios of ηmax and Pmax in single-leg and two-pair modules using α-MgAgSb and α/β-MgAgSb. This comparison further emphasizes that α/β-MgAgSb can achieve a similar ηmax to α-MgAgSb while delivering notably enhanced Pmax.Figure  4F shows the ΔT dependence of ηmax in the two-pair Mg3(Sb,Bi)2/MgAgSb modules, alongside comparisons to values re-ported in the literature (14, 24, 30, 38, 40, 54–58). The high ηmax of 8.6% achieved in the Mg3Sb0.6Bi1.4/MgAgSb module is comparable to the best-reported results. As shown in Fig. 4G, the Mg3Sb0.6Bi1.4/MgAgSb module achieves a record-breaking ωmax of 0.5 W cm−2, outperforming other modules (14, 24, 30, 38, 40, 54–58). Figure 4H illustrates the relationship between ηmax versus ωmax, and it can be found the two-pair TE module using α/β-MgAgSb as the p-type TE legs and Mg3Sb0.6Bi1.4 as the n-type TE legs not only achieves a higher ωmax but also maintains a high ηmax compared to one using α-MgAgSb. Its performance is particularly noteworthy among Mg(Sb,Bi)2/MgAgSb two-pair modules offering both high ωmax and high ηmax (14, 24, 30, 38, 40, 54–58). It should be emphasized that ωmax is calculated based on ωmax = Pmax/A, where A represents the total cross-sectional area of the TE device. The total cross-sectional area used here is the area of the ceramic base (10 mm by 10 mm). If the cross-sectional area of TE legs (3.8 mm by 3.8 mm × 2 pairs) is used, then ωmax would be 0.9 W cm−2. These results highlight the notable potential of α/β-MgAgSb to replace α-MgAgSb. The inte-gration of β-MgAgSb within α-MgAgSb is anticipated to expand its applicability across various TE systems.DISCUSSIONIn this study, we systematically investigated the reversible and repeat-able α-to-β semiconductor-metal transition in MgAgSb. We demon-strated that α-MgAgSb, with a semiconductive nature, transitions to metallic β-MgAgSb at elevated temperatures. Although the β-MgAgSb exhibits lower zT values, its metallic nature makes it promising to cou-ple with α-MgAgSb to form a nondiffusive α/β-MgAgSb junction. The incorporation of β-MgAgSb in the α/β-MgAgSb junction notably en-hances its power output without sacrificing conversion efficiency, veri-fied both theoretically and experimentally. Notably, the α/β-MgAgSb leg demonstrated excellent thermal stability during repeated tests, with no observable degradation of output performance. To further leverage α/β-MgAgSb, two-pair modules are fabricated, pairing with n-type Bi-rich Mg3Sb0.6Bi1.4 or Sb-rich Mg3Sb1.5Bi0.5. Mg3Sb0.6Bi1.4 and α/β-MgAgSb module achieves ηmax of 8.6% and reaches a record-breaking ωmax of 0.51 W cm−2, highlighting their superior performance com-pared to conventional Mg3(Sb,Bi)2/MgAgSb two-pair systems. These findings not only demonstrate the feasibility of α/β-MgAgSb for high-performance TE systems but also pave the way for exploring similar phase transition–based TE materials and devices in the future.MATERIALS AND METHODSMaterials synthesisMgAgSb containing 0.625 wt% C18H36O2 (referred to as MgAgSb in the main text and below), Mg3.2In0.02Sb0.595Bi1.4Te0.005 (referred to as Mg3Sb0.6Bi1.4 in the main text and below), and Mg3.2In0.005Sb1.5Bi0.49Te0.01 (referred to as Mg3Sb1.5Bi0.5 in the main text and below) were pre-pared using high-purity Mg turnings (99.95%), Ag powders (99.99%), Sb shots (99.999%), Bi shots (99.999%), Te shots (99.999%), and In powders (99.99%). Raw materials were weighed according to their stoichiometric ratios and mechanically alloyed for 5 hours using a SPEX-8000D mill under an argon atmosphere. The resulting powders were consolidated into bulk samples via vacuum spark plasma sinter-ing. MgAgSb was sintered at 573 K and 60 MPa for 5 min (SPS-322Lx, Dr. Sintering), while Mg3Sb0.6Bi1.4 and Mg3Sb1.5Bi0.5 were sintered at 973 K and 60 MPa for 10 min (SPS-1080 System, SPS SYNTEX INC).Characterization and measurementsThe phase and phase transitions of MgAgSb were investigated using a variable-temperature x-ray diffractometer (Rigaku SmartLab 9 kW) with Cu Kα radiation at 45 kV and 200 mA. The measurements were performed on MgAgSb powders as the temperature increased from room temperature to 623 K and then cooled back to room tempera-ture at a rate of 5 K min−1 under a dry N2 gas flow of 2 liter min−1. The electrical transport properties of MgAgSb, including S and the σ, were measured by ZEM-3 (Advance Riko) with ±5% uncertainty. The thermal transport property κ of MgAgSb was obtained by multi-plying thermal diffusivity D via LFA467 (Netzsch), sample density ρvia Archimedes method, and heat capacity Cp via Dulong-Petit law. The thermal analysis TG-DSC of MgAgSb was performed using a Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e7 of 9STA 449 (Netzsch). Samples were loaded into aluminum crucibles, heated to 623 K, and then cooled to 323 K at a rate of 5 K min−1. The composition distribution of MgAgSb after the heating and cooling cycle was examined using field-emission scanning electron micros-copy (Hitachi SU8000) equipped with an energy-dispersive spectrom-eter (EDS, XFlash FlatQUAD 5060 F).TE device fabrication, measurement, and simulationThe MgAgSb TE leg was fabricated by sandwiching two layers of Sb powders as interface materials, followed by SPS (SPS-322Lx, Dr. Sintering) at 573 K and 60 MPa for 5 min. The sintered MgAgSb TE legs were then cut into dice with dimensions of ~3.8 mm by 3.8 mm by 6  mm. When the hot-side temperature was below 573 K, the MgAgSb leg remained in the α-MgAgSb phase, while at 623 K, it transitioned to the α/β-MgAgSb phase. Two-pair TE modules were constructed using these p-type MgAgSb legs and n-type Bi-rich Mg3Sb0.6Bi1.4 or Sb-rich Mg3Sb1.5Bi0.5 TE legs, with 304 stainless steel serving as the TEiM. The dimensions of the two-pair TE mod-ules are 10 mm by 10 mm by 7.67 mm, with the height of the TE legs, AlN ceramic plate, and copper electrode being 6, 0.635, and 0.2 mm, respectively. The output power P and heat flow Q of both the single-leg and two-pair TE devices were measured using Mini-PEM (Ad-vance Riko) in a vacuum. The conversion efficiency η was calculated based on the formula: η = P/(P + Q). Finite-element simulations were performed using COMSOL Multiphysics to model both the single-leg and two-pair TE devices.First-principles calculationsThe band structure of α-MgAgSb and β-MgAgSb was calculated by using Vienna ab initio Simulation Package (VASP) software (59, 60). Generalized gradient approximation - Perdew-Burke-Ernzerhof type was used and modified Becke-Johnson (mBJ) exchange-correlation 0 2 4 6 8 100.00.20.40.60 2 4 6 8 100369120 1 2 3 4 5 6 70.00.20.40.60 1 2 3 4 5 6 70369120 100 200 300036912100 200 3000.00.20.40.60.0 0.1 0.2 0.3 0.4 0.54681012373 K473 K573 K623 KP(W)I (A)TH(%)I (A)Two-pair module with Mg3Sb0.4Bi1.6373 K473 K573 K623 KP(W)I (A)TH(%)I (A)Two-pair module with Mg3Sb1.5Bi0.5This work (Sb-rich)This work (Bi-rich)Ref. (58) (Bi2Te3)max(%)T (K)Ref. (14)Ref. (24)Ref. (30)Ref. (38)Ref. (40)Ref. (54)Ref. (55)Ref. (56)Ref. (57)Amax(W)T (K)Ref. (14)Ref. (24)Ref. (30)Ref. (38)Ref. (40)Ref. (54)Ref. (55)Ref. (56)Ref. (57)Ref. (58) (Bi2Te3)FThis workmax(W)max (W)BC DGEH0.5 1.0 1.5PPPSingle TE legTwo-pair module with Mg3Sb1.5Bi0.5Two-pair module with Mg3Sb0.4Bi1.6, P /PExperimentsMg3(Sb,Bi)2/MgAgSb two-pair moduleRef. (24)Ref. (14)Ref. (38)Ref. (58) (Bi2Te3-based)Ref. (54)Ref. (57)Ref. (56)Ref. (30)Ref. (40)Ref. (55)This workFig. 4. Two-pair TE modules based on n-type Mg3(Sb,Bi)2 and p-type MgAgSb. I dependence of (A) η and (B) P in Bi-rich Mg3Sb0.6Bi1.4/MgAgSb two-pair module, (C) η and (D) P in Sb-rich Mg3Sb1.5Bi0.5/MgAgSb two-pair module. (E) Summary of the ratios of ηmax and Pmax for single TE leg and two-pair TE modules using α-MgAgSb and α/β-MgAgSb. ΔT dependence of the (F) ηmax and (G) ωmax achieved in this study compared to values reported in the literature. (H) Comparison of ηmax and ωmax of the Mg3(Sb,Bi)2/MgAgSb two-pair module in this work with values reported in the literature (14, 24, 30, 38, 40, 54–58).Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025Li et al., Sci. Adv. 11, eadx7115 (2025)     4 July 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e8 of 9functionals to get more accurate bandgap of MgAgSb (61, 62). Ge-ometry relaxation was conducted first to get the relaxed structure of α-MgAgSb and β-MgAgSb for the next self-consistent static calcula-tions. The plane-wave energy cutoff was set to 500 eV. Hellmann-Feynman force on each atom energy was set to 0.001 eV Å−1, and the convergence criterion was set to 10−8 eV. For geometry relaxation and self-consistent static calculations, k = 30/L and k = 60/L gamma-centered k-point sampling were adopted, respectively, where the L is the lattice parameter of MgAgSb. 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Supervision: T.M. Project administration: T.M. Funding acquisition: T.M. Competing interests: T.M. and A.L. have filed one Japanese patent application (2024-111372) submitted by National Institute for Materials Science (Japan). The other authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.Submitted 25 March 2025 Accepted 2 June 2025 Published 4 July 2025 10.1126/sciadv.adx7115Downloaded from https://www.science.org at National Institute for Materials Science on July 05, 2025 Semiconductor-metal transition powers high-efficiency MgAgSb thermoelectrics INTRODUCTION RESULTS Semiconducting α-MgAgSb and metallic β-MgAgSb α-to-β phase transition Highly stable β-MgAgSb Excellent power generation employing β-MgAgSb DISCUSSION MATERIALS AND METHODS Materials synthesis Characterization and measurements TE device fabrication, measurement, and simulation First-principles calculations Supplementary Materials This PDF file includes: REFERENCES AND NOTES Acknowledgments