Unusual anisotropic thermal expansion in multilayer SnSe leads to positive-to-negative crossover of Poisson’s ratio

An unusual anisotropic thermal expansion occurs in monolayer SnSe when it is subjected to heating, i.e., the long lattice constant contracts, and the short one expands, causing a rectangular-to-square-lattice phase transition at a critical temperature (T_c). An even more prominent monolayer-to-bulk thermal-expansion behavior has been discovered by density-functional-theory calculations combined with Grüneisen’s theory within the quasiharmonic approximation[1]. The unusual thermal expansion survives in multilayer SnSe, while the thermal expansion coefficients of different layers are almost unchanged. This phenomenon can be explained by a delicate balance between the elastic stiffness coefficient and Grüneisen parameters. Finally, the Poisson’s ratio of multilayer SnSe decreases at elevated temperatures and turns negative, indicating a crossover point of diminished Poisson’s ratio at some intermediate temperature. These findings provide not only new understanding of the thermal properties of monochalcogenides, but also a feasible approach to design zero-Poisson’s-ratio materials.