High-throughput computational screening of auxetic two-dimensional metal dichalcogenides and dihalides

Auxetic materials hold tremendous potential for many advanced applications, but candidates are quite scarce, especially at two dimensions. Here, we focus on two-dimensional (2D) metal dichalcogenides and dihalides with the chemical formula MX2 by screening structures sharing the P4m2 space group among 330 MX2 compounds from the computational 2D materials database. Via high-throughput first-principles computations, 25 stable MX2 (M = Mg, Ca, Mn, Co, Ni, Cu, Zn, Ge, Cd, Sn; X = F, Cl, Br, I, O, S, Se) systems with in-plane negative Poisson's ratios (NPRs) are successfully identified. Within these structures, 2D NiCl2 has the largest NPR value of -0.34, with a magnitude significantly higher than those of black phosphorene (-0.027) and SnO2 (-0.1). The distinct auxetic effect in MX2 originates from both the unique local corner-sharing tetrahedral structural motif under the low-dimensional effect and the strong orbital interaction between the d orbitals of M and the p orbitals of halogen/chalcogen atoms. As a result, Poisson's ratio can be effectively tuned by enhancing the d-p interaction through an external biaxial strain. We reveal that these auxetic materials exhibit rich electronic and magnetic properties, covering nonmagnetic, ferromagnetic, or anti-ferromagnetic metals, semiconductors, and insulators. The extraordinary auxetic behaviors in combination with rich physical properties could lead to multifunctional nanomechanical, optoelectronic, and spintronic applications.