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.

Indium selenide (InSe) crystals are reported to show exceptional plasticity, a new property to two-dimensional van der Waals (2D vdW) semiconductors. However, the correlation between plasticity and specific prototypes is unclear, and the understanding of detailed plastic deformation mechanisms is inadequate. Here three prototypes of InSe are predicted to be plastically deformable by calculation, and the plasticity of polymorphic crystals is verified by experiment. Moreover, distinct nanoindentation behaviors are seen on the cleavage and cross-section surfaces. The modulus and hardness of InSe are the lowest ones among a large variety of materials. The plastic deformation is further perceived from chemical interactions during the slip process. Particularly for the cross-layer slip, the initial In-Se bonds break while new In-In and Se-Se bonds are newly formed, maintaining a decent interaction strength. The remarkable plasticity and softness alongside the novel physical properties, endow InSe great promise for application in deformable and flexible electronics.

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.