Kai Huang

We present a systematic first-principles study on the electronic and mechanical properties of both monolayer and bulk X2Si2Y= (X = As, Sb, Bi; Y = Se, Te) compounds. Our calculations reveal that all investigated materials are mechanically, dynamically and thermally stable. The monolayers are indirect band gap semiconductors with band gaps ranging from 0.81 to 1.08 eV, whereas the bulk counterparts exhibit direct band gaps that decrease monotonically with increasing atomic mass of the group-V element. Charge density and charge density difference analyses uncover a distinct bonding hierarchy: strong covalent Si–Si bonds, mixed covalent-ionic Si–Y bonds, and primarily ionic X–Y interactions. The X2Si2Y6 monolayers possess low elastic constants (C11=35–60 N/m), indicating high deformability, while the bulk materials exhibit pronounced mechanical anisotropy with Young’s modulus varying by up to an order of magnitude depending on crystallographic direction. Notably, several compositions display auxetic behavior with negative Poisson’s ratios. The average polycrystalline moduli obtained from the Voigt-Reuss-Hill scheme reveal systematic trends with chemical substitution, where tellurides exhibit higher shear stiffness but lower bulk moduli compared to selenides.

Dozens of layered V ₂ IV ₂ VI ₆ (V=P, As, Sb, Bi; IV=Si, Ge, Sn, Pb; VI=S, Se, Te) materials are investigated, several of which have been successfully synthesized in experiment. Among them, nine strong topological insulators (TIs), two strong Z ₂ topological metals (TMs), and nearly twenty trivial insulators are predicted at their equilibrium structures. The TIs are in the (1;111) topological class, with energy gaps ranging from 0.04 to 0.2 eV. The strong Z ₂ TMs and the trivial insulators belong to the (1;111) and (0;000) topological classes, respectively. Small compressive strains easily turn some of the trivial insulators into strong TIs. This study enriches not only the family of topological materials but also the family of van der Waals layered materials, providing promising candidates for the future spintronic devices.