Prediction of the ferroelastic and negative Poisson's ratio of a two-dimensional α-CaX (X = S, Se) monolayer

Two-dimensional ferroelastic semiconductor materials have attracted wide attention due to their unique properties. Along these lines, in this work, a series of new types of two-dimensional materials called monolayer alpha-CaX (X = S, Se) were proposed and thoroughly investigated by performing first-principles calculations. Different from the hexagonal lattice structure of beta-CaX that was previously reported, the alpha-CaX structure holds a tetragonal lattice. By considering the calculated phonon spectra and the respective Born criteria, good stability was demonstrated for the alpha-CaX monolayer. The values of 3.41 eV (alpha-CaS) and 3.11 eV (alpha-CaSe) were calculated for the indirect band gap of the alpha-CaX (X = S, Se) structures, respectively. The mechanical properties of the proposed alpha-CaX monolayer were proved to exhibit distinct anisotropy.Particularly,the existence of a negative Poisson's ratio was detected for the case of alpha-CaSe. Additionally, strong ferroelastic signal and moderate energy barriers were found for both monolayer alpha-CaS and alpha-CaSe. All these intriguing properties render the alpha-CaX structures quite promising candidates for applications in shape memory devices and impact resistance materials.