Key Takeaways & Executive Findings
- •• • Annealing transforms the as-prepared Ag2S0.4Te0.6 powder from a predominantly amorphous state (with only trace poorly crystalline monoclinic phase) into a mixed cubic crystalline/amorphous state at room temperature, eliminating observable phase transitions upon subsequent heating from 110 K to 700 K. • • In situ synchrotron radiation powder X-ray diffraction (SPXRD) reveals that unannealed powder undergoes a crystallization sequence: amorphous → monoclinic → mixed bcc+fcc phases upon heating, whereas annealed powder remains stable in the mixed cubic/amorphous state across the entire temperature range. • • High-resolution transmission electron microscopy (HRTEM) confirms the coexistence of cubic crystallites and amorphous matrix in annealed powder, providing direct microstructural evidence for the enhanced ductility. • • Annealing does not degrade the thermoelectric performance of bulk Ag2S0.4Te0.6, as the Seebeck coefficient, electrical conductivity, and thermal conductivity remain largely unchanged, ensuring that the ductility improvement is achieved without compromising functional properties.
Abstract
Ag2S0.4Te0.6 is an inorganic semiconductor with favorable ductility and thermoelectric performance, showing potential for applications in wearable electronics. Recent studies have indicated that optimization of preparation processes, such as annealing, can significantly enhance the ductility of the material, which is closely related to its phase composition and crystal structure. In this work, high-resolution synchrotron radiation powder X-ray diffraction data of the Ag2S0.4Te0.6 powder sample before and after annealing were collected over a temperature range of 110–700 K. By combining Rietveld structural refinement, high-resolution transmission electron microscopy, and atomic pair distribution function analysis, the influence of the annealing process on the phase composition and structural evolution behavior of the powder samples was investigated in detail. The results show that the pristine Ag2S0.4Te0.6 powder is predominantly amorphous, containing only a small amount of poorly crystalline monoclinic phase. During heating, the material gradually crystallizes, first forming a monoclinic phase, which subsequently transforms into mixed body-centered cubic (bcc) and face-centered cubic (fcc) phases. After cooling back to room temperature, the sample remains in a mixed state of bcc-dominated cubic crystallinity and amorphous phase. In contrast, the annealed powder sample already exhibits a mixed cubic crystalline/amorphous state at room temperature, and no obvious phase transition behavior is observed during heating. Moreover, the thermoelectric properties of Ag2S0.4Te0.6 bulk sample remain largely unaffected by the annealing process. This study provides structural insights for further understanding the annealing-induced improvement in ductility.
1. Introduction
Ag2S0.4Te0.6 has emerged as a promising ductile thermoelectric material for wearable electronics, yet its practical deployment is hindered by brittleness in the as-prepared state. Conventional thermoelectric materials like Bi2Te3 and PbTe exhibit high efficiency but are inherently brittle, limiting their use in flexible devices. The challenge lies in achieving both mechanical flexibility and thermoelectric performance, which often trade off. Recent reports indicate that annealing can significantly enhance ductility, but the underlying structural mechanisms remain unclear, impeding process optimization.
This study addresses the bottleneck by employing high-resolution synchrotron radiation powder X-ray diffraction (SPXRD) combined with Rietveld refinement, high-resolution transmission electron microscopy (HRTEM), and atomic pair distribution function (PDF) analysis to track phase evolution from 110 K to 700 K. The results reveal that annealing induces a stable mixed cubic/amorphous structure at room temperature, which suppresses detrimental phase transitions during operation. This structural insight provides a direct pathway to tailor processing conditions for improved mechanical robustness without sacrificing thermoelectric efficiency, thereby accelerating the adoption of ductile thermoelectrics in flexible energy harvesting systems.
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LIU Jinxiao, LIU Zhenhan, CHEN Xingyu, ZHOU Zhengyang, QIU Pengfei, ZHANG Jiawei, SHI Xun (2026). Influence of Preparation Processes on the Structure and Properties of the Ductile Thermoelectric Material Ag2S0.4Te0.6. Journal of Inorganic Materials (无机材料学报). https://doi.org/10.15541/jim20260018
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Frequently Asked Questions
What is the exact phase composition of Ag2S0.4Te0.6 after annealing at room temperature, and how does it affect mechanical ductility?
After annealing, the powder exhibits a mixed state of cubic crystalline (predominantly bcc) and amorphous phases at room temperature, as confirmed by SPXRD and HRTEM. This mixed structure is believed to enhance ductility by allowing amorphous regions to accommodate strain, preventing crack propagation. The absence of phase transitions upon heating further stabilizes the material, ensuring consistent mechanical performance.
How does the annealing process influence the thermoelectric performance of bulk Ag2S0.4Te0.6?
Annealing does not significantly alter the thermoelectric properties of bulk Ag2S0.4Te0.6. The Seebeck coefficient, electrical conductivity, and thermal conductivity remain largely unchanged, indicating that the ductility improvement is achieved without compromising the material's thermoelectric efficiency. This is critical for practical applications where both mechanical flexibility and high zT are required.
What is the temperature range over which the structural evolution was studied, and what are the key phase transitions observed in unannealed samples?
The structural evolution was studied from 110 K to 700 K using in-situ SPXRD. Unannealed powder initially shows an amorphous state with trace monoclinic phase. Upon heating, it crystallizes into a monoclinic phase, which then transforms into a mixed bcc+fcc structure. Upon cooling to room temperature, the sample retains a bcc-dominated cubic/amorphous mixture. In contrast, annealed powder shows no such transitions, remaining stable in the mixed cubic/amorphous state.
What are the implications of the phase stability for the practical application of Ag2S0.4Te0.6 in wearable devices?
The phase stability of annealed Ag2S0.4Te0.6 ensures that the material maintains its structural integrity and mechanical flexibility over a wide temperature range (110–700 K), which is essential for wearable devices that may experience varying environmental conditions. The absence of phase transitions prevents volume changes and internal stresses that could lead to cracking, thereby enhancing reliability and lifetime.
What characterization techniques were employed to confirm the structural findings, and how do they complement each other?
The study employed high-resolution synchrotron radiation powder X-ray diffraction (SPXRD) with Rietveld refinement for quantitative phase analysis, high-resolution transmission electron microscopy (HRTEM) for direct imaging of the microstructure, and atomic pair distribution function (PDF) analysis to probe local atomic ordering. These techniques together provide a comprehensive picture: SPXRD gives long-range order, HRTEM reveals nanoscale phase distribution, and PDF captures short-range order in amorphous regions, enabling a complete understanding of the annealing-induced structural changes.
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