As a new type of inorganic ductile semiconductor, silver sulfide (α-Ag₂S) has garnered a plethora of interests in recent years due to its promising applications in flexible electronics. However, the lack of detailed defect calculations and chemical intuition has largely hindered the optimization of material's performance. In this study, we systematically investigate the defect chemistry of extrinsic doping in α-Ag₂S using first-principles calculations. We computationally examine a broad suite of 17 dopants and find that all aliovalent elements have extremely low doping limits (<0.002%) in α-Ag₂S, rendering them ineffective in tuning the electron concentrations. In contrast, the isovalent elements Se and Te have relatively high doping limits, being consistent with the experimental observations. While the dopant Se or Te itself does not provide additional electrons, its introduction has a significant impact on the band gap, the band-edge position, and especially the formation energy of Ag interstitials, which effectively improve the electron concentrations by 2–3 orders of magnitudes. The size effects of Se and Te doping are responsible for the more favorable Ag interstitials in Ag₂S_0.875Se_0.125 and Ag₂S_0.875Te_0.125 with respect to pristine Ag₂S. This work serves as a theoretical foundation for the rational design of Ag₂S-based functional materials.

Ductile Ag₂(Te,S) pseudobinary compounds have attracted great attention in thermoelectric community since they can be fabricated into high-performance flexible and hetero-shaped thermoelectric devices. However, in spite of the numerous studies, the ‘brittle–ductile’ transition boundary in Ag₂(Te,S) is still unclear. In this work, a series of Te-rich Ag₂(Te,S) pseudobinary compounds have been prepared. The structure characterizations confirm they belong to the new-concept of meta-phase. The systematically investigation on the mechanical properties demonstrate that the ‘brittle–ductile’ transition boundary appears around x = 0.1. Unexpected good ductility is observed in the Te-rich Ag₂Te_1-xS_x crystalizing in the Ag₂Te room-temperature monoclinic structure and high-temperature cubic structure, which are thought to be brittle before. Likewise, Ag content is found to be a very critical parameter determining the ductility of Te-rich Ag₂Te_1-xS_x. Very slight Ag-deficiency can greatly deteriorate the ductility. The thermoelectric properties of these ductile Te-rich Ag₂Te_1-xS_x pseudobinary compounds are investigated. A maximum thermoelectric figure-of-merit of 0.6 is obtained for Ag₂Te_0.9S_0.1 at 600 K. This work sheds light on the future investigation of Ag₂(Te,S) pseudobinary compounds.

Exploring new prototypes for a given chemical composition is of great importance and interest to several disciplines. As a famous semiconducting binary compound, InSe usually exhibits a two-dimensional layered structure with decent physical and mechanical properties. However, it is less noticed that InSe can also adopt a monoclinic structure, denoted as mcl-InSe. The synthesis of such a phase usually requires high-pressure conditions, and the knowledge is quite scarce on its chemical bonding, lattice dynamics, and thermal transport. Here in this work, by developing a facile method combining mechanical alloying and spark plasma sintering, we successfully synthesize mcl-InSe bulks with well-crystallized nanograins. The chemical bonding of mcl-InSe is understood as compared with layered InSe via charge analysis. Low cut-off frequencies of acoustic phonons and several low-lying optical modes are demonstrated. Noticeably, mcl-InSe exhibits a low room-temperature thermal conductivity of 0.6 W·m⁻¹·K⁻¹, which is smaller than that of other materials in the In–Se system and many other selenides. Low-temperature thermal analyses corroborate the role of nanograin boundaries and low-frequency optical phonons in scattering acoustic phonons. This work provides new insights into the non-common prototype of the InSe binary compound with potential applications in thermoelectrics or thermal insulation.

By virtue of the excellent plasticity and tunable transport properties, Ag₂S-based materials demonstrate an intriguing prospect for flexible or hetero-shaped thermoelectric applications. Among them, Ag₂S_1-xTe_x exhibits rich and interesting variations in crystal structure, mechanical and thermoelectric transport properties. However, Te alloying obviously introduces extremely large order-disorder distributions of cations and anions, leading to quite complicated crystal structures and thermoelectric properties. Detailed composition-structure-performance correlation of Ag₂S_1-xTe_x still remains to be established. In this work, we designed and prepared a series of Ag₂S_1-xTe_x (x = 0–0.3) materials with low Te content. We discovered that the monoclinic-to-cubic phase transition occurs around x = 0.16 at room temperature. Te alloying plays a similar role as heating in facilitating this monoclinic-to-cubic phase transition, which is analyzed based on the thermodynamic principles. Compared with the monoclinic counterparts, the cubic-structured phases are more ductile and softer in mechanical properties. In addition, the cubic phases show a degenerately semiconducting behavior with higher thermoelectric performance. A maximum zT = 0.8 at 600 K and bending strain larger than 20% at room temperature were obtained in Ag₂S_0.7Te_0.3. This work provides a useful guidance for designing Ag₂S-based alloys with enhanced plasticity and high thermoelectric performance.