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Unraveling Non-polymorphic Phase Evolution in Mg-Ag-Sb for Designing Thermally Recoverable Thermoelectrics

Shizhen Zhi1,*, Xiaojing Ma1,2,*, Shanghao Chen1,*, Tianyu Zhang1, Yao Xu1, Jiang Chen1, Sheng Ye1, Chenhao Lin1, Linmao Wen1 et al.

Jinxuan Cheng1, Rongpei Shi1, Xingjun Liu1, Feng Cao3, Lijun Zhang4, Yuhao Fu5,†, Qian Zhang1,6,7,‡, and Jun Mao1,6,7,§

  • 1School of Materials Science and Engineering, and Institute of Materials Genome and Big Data, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China
  • 2State Key Laboratory of Tropic Ocean Engineering Materials and Materials Evaluation, School of Materials Science and Engineering, Hainan University, Haikou, 570228, People’s Republic of China
  • 3School of Science, and Ministry of Industry and Information Technology Key Lab of Micro-Nano Optoelectronic Information System, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China
  • 4State Key Laboratory of Integrated Optoelectronics, Key Laboratory of Automobile Materials of MOE, International Center of Computational Method and Software, College of Materials Science and Engineering, Jilin University, Changchun 130012, People’s Republic of China
  • 5State Key Laboratory of High Pressure and Superhard Materials, Key Laboratory of Material Simulation Methods and Software of Ministry of Education, College of Physics, Jilin University, Changchun 132000, People’s Republic of China
  • 6State Key Laboratory of Precision Welding and Joining of Materials and Structures, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
  • 7Shenzhen Key Laboratory of New Materials Technology, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China

  • *These authors contributed equally to this work.
  • †Contact author: yuhao_fu@jlu.edu.cn
  • ‡Contact author: zhangqf@hit.edu.cn
  • §Contact author: maojun@hit.edu.cn
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Phys. Rev. X 16, 031035 – Published 12 August, 2026

DOI: https://doi.org/10.1103/7nxm-t64f

Abstract

P-type MgAgSb with excellent room-temperature thermoelectric performance holds great promise for cooling and power generation. However, its practical application has been significantly limited by phase transitions, which are long considered a polymorphic transformation where the compositions remain identical but the crystal structures differ. Here we report the non-polymorphic nature of the complex phase transitions in Mg-Ag-Sb during thermal cycling, yielding four compositionally distinct ternary compounds that crystallize in three different structures. During heating, the tetragonal α-phase MgAgSb decomposes into the tetragonal β-phase Mg5Ag4.32Sb4.5, and subsequently into the cubic γH-phase Mg5Ag2.11Sb4, with the progressive precipitation of Ag3Sb and Sb phases. Upon cooling, the γH-phase Mg5Ag2.11Sb4 converts to the cubic γL-phase Mg5Ag1.18Sb3.63, accompanied by the α phase with substantial Ag3Sb and Sb phases. The electron localization function and bonding analysis indicate that the Mg-Ag-Sb compound is characterized by ionic interactions, wherein the Ag-Sb bond is notably weaker compared to the Mg-Sb bond. Molecular dynamics simulations reveal that significant Mg and Ag migration within the Sb sublattices underlies the phase evolution. Surprisingly, the rapid atomic diffusion also enables the restoration of the α-phase MgAgSb upon annealing at low temperatures. Successful recovery of the α phase can be realized even after 500 thermal cycles between 423–673 K, demonstrating unprecedented thermal recoverability.

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Popular Summary

Thermal degradation, typically occurring through phase transitions or decomposition, is a common failure mechanism that limits material reusability. For high-performance p-type MgAgSb thermoelectric material, thermal instability and the difficulty in addressing it, has long constrained operating temperature to below 573 K. This limiting temperature hinders practical deployment. We found that significant performance degradation is driven by non-polymorphic transitions tied to Mg/Ag migration within Sb sublattices, accompanied by the precipitation of Ag3Sb and Sb phases. This contrasts with the previous assumption of a polymorphic phase transition, where the composition remains fixed during the structural rearrangement. We found that low-temperature annealing reverses the changes we observed, enabling full recovery of the material’s high performance even after 500 thermal cycles (at 423–673 K). Clarifying the mechanism behind the phase transitions of this material establishes a foundational basis for the design of self-healing thermoelectrics.

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Phys. Rev. X 16, 031035– Published 12 August, 2026
  • Received 19 December 2025
  • Accepted 22 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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