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Inorganic plastic thermoelectric materials can break the constraints of intrinsic brittleness inherent in conventional inorganic thermoelectric compounds and the low electrical transport properties of organic thermoelectric materials, while simultaneously achieving exceptional room-temperature deformability and outstanding thermoelectric performance. These materials hold great promise for applications in fields such as flexible electronics and the recovery and utilization of waste heat from unconventional heat sources. In earlier studies, the Shanghai Institute of Ceramics, Chinese Academy of Sciences, discovered semiconductor materials with metal-like ductility at room temperature, including Ag2S (Nature Materials, 2018) and two-dimensional layered single crystals such as InSe (Science, 2020), SnSe2 (Advanced Science, 2022), and MoS2 (Nature Communications, 2022). Based on these findings, a series of high-performance inorganic plastic thermoelectric materials have been developed (Energy & Environmental Science, 2019; Advanced Materials, 2021; Advanced Energy Materials, 2021; Science, 2022, etc.), thus opening up a new research direction in inorganic plastic thermoelectric materials. However, currently, the power factor (PF) and thermoelectric figure of merit (zT) of inorganic plastic thermoelectric materials still lag significantly behind those of traditional rigid thermoelectric materials, severely limiting the development and application of high-efficiency flexible thermoelectric devices.

Recently, Researcher Qiu Pengfei, Researcher Shi Xun, and Researcher Chen Lidong from the Shanghai Institute of Ceramics, in collaboration with Associate Professor Zhao Kunpeng from Shanghai Jiao Tong University, discovered that a pseudo-binary solid solution of Ag2Se-Ag2S exhibits a morphotropic phase boundary (MPB) similar to that found in PZT ferroelectric materials. By precisely controlling the phase structure near the MPB, they simultaneously achieved both excellent plasticity and high thermoelectric performance. The power factor and thermoelectric figure of merit reached 22 μWcm⁻¹K⁻² and 0.61, respectively—both significantly higher than those reported for previously known inorganic plastic thermoelectric materials.

Ag2Se and Ag2S can form a continuous solid solution. When x ≤ 0.2, Ag2Se1-xSx exhibits an orthorhombic structure at room temperature; when x ≥ 0.4, Ag2Se1-xSx adopts a monoclinic structure at room temperature; and when 0.2 < x < 0.4, the phase structure of Ag2Se1-xSx becomes extremely complex. In this work, we used a differential scanning calorimetry system to investigate the phase transition characteristics of Ag2Se1-xSx. Combined with X-ray diffraction analysis, we constructed a pseudo-binary phase diagram of the Ag2Se-Ag2S system over the temperature range from 300 K to 480 K. When x ≤ 0.2 and x ≥ 0.4, Ag2Se1-xSx undergoes only one phase transition within this temperature range, corresponding respectively to an orthorhombic-to-cubic transition and a monoclinic-to-cubic transition. However, when 0.2 < x < 0.4, Ag2Se1-xSx experiences two phase transitions within this temperature range: as the temperature increases, the material first transforms from the orthorhombic structure to the monoclinic structure and then to the cubic structure. Notably, the orthorhombic-to-monoclinic phase boundary is a sloping line rather than the typical straight line, similar to the rhombohedral-tetragonal morphotropic phase boundary observed in PZT ferroelectric materials. When x ≥ 0.4, the temperatures for the cubic-to-monoclinic phase transition during both cooling and heating processes are close to each other; whereas when x < 0.4, the cubic-to-orthorhombic phase transition temperature during cooling is lower than that during heating, resulting in significant thermal hysteresis. Theoretical calculations indicate that when the sulfur content is between 0.3 and 0.4, the energies of the orthorhombic and monoclinic phases become comparable, thus forming a morphotropic phase boundary. Furthermore, the arrangement of Se/S anions in the cubic structure is similar to that in the monoclinic structure but significantly different from that in the orthorhombic structure, which leads to the pronounced thermal hysteresis observed during the orthorhombic-to-cubic phase transition.

The pseudobinary solid solution Ag2Se1-xSx exhibits intriguing thermoelectric and mechanical properties near the morphotropic phase boundary. Previous studies have shown that the monoclinic phase of Ag2Se1-xSx possesses good ductility at room temperature but has a relatively low PF/zT; whereas the orthorhombic phase of Ag2Se1-xSx boasts a high PF/zT but is brittle at room temperature. Consequently, for a long time, it has been widely believed that the pseudobinary solid solution Ag2Se1-xSx could not simultaneously achieve both excellent ductility at room temperature and a high PF/zT. In this study, we found that the mechanical properties of Ag2Se1-xSx depend on the sulfur content rather than the crystal structure. When the sulfur content x ranges from 0.25 to 0.30, the material undergoes a "brittle-to-ductile" transition, enabling the orthorhombic phase of Ag2Se1-xSx near the morphotropic phase boundary to also exhibit good ductility—its three-point bending strain exceeds 15%, allowing the material to be bent into various shapes without cracking. This "brittle-to-ductile" transition arises from the formation of a conductive "percolation" network composed of multi-center, diffuse Ag-S bonds. Meanwhile, the effective mass of charge carriers at the conduction-band minimum in the orthorhombic phase of Ag2Se1-xSx is only about one-third that of the monoclinic phase, resulting in significantly higher carrier mobility and superior thermoelectric performance compared to the monoclinic phase. Therefore, the orthorhombic phase of Ag2Se1-xSx near the morphotropic phase boundary can simultaneously achieve both excellent ductility at room temperature and a high PF/zT. Notably, the phase space of the orthorhombic phase of Ag2Se1-xSx can also be tuned to some extent through material processing techniques such as heating or cooling, greatly expanding the design scope of inorganic ductile thermoelectric materials. At room temperature, the orthorhombic phase of Ag2Se0.69S0.31, which exhibits excellent ductility, achieves a power factor as high as 22 μWcm⁻¹K⁻² and a thermoelectric figure of merit of 0.61—both representing the highest values reported so far for ductile materials. This work provides new material support for the research on flexible thermoelectric technologies.

The relevant research findings, titled “Modulation of the morphotropic phase boundary for high-performance ductile thermoelectric materials,” were published in Nature Communications (2023, doi:10.1038/s41467-023-44318-4). Liang Jiasheng, a doctoral graduate from the Shanghai Institute of Ceramics, and Liu Jin, a doctoral student, are co-first authors of the paper.

The research was funded and supported by projects including the National Key Research and Development Program, the National Natural Science Foundation, and the Shanghai Municipal Special Zone for Basic Research.

Link: https://doi.org/10.1038/s41467-023-44318-4

Ag2Se – Ag2S pseudo-binary phase diagram

The Relationship Between Three-Point Bending Strain and S Content in the Pseudo-Binary Solid Solution Ag2Se1-xSx

Thermoelectric Properties of the Pseudo-binary Solid Solution Ag2Se1-xSx Near the Quasi-Isomorphic Phase Boundary