Superelasticity and anomalous mechanical response of two-dimensional SiO2 under tensile and bending deformations

Silica displays a rich phenomenology of mechanical response to deformations in its 3D form. It recrystallizes by dislocation creep and shows non-linear elastic behavior under large strains. In this study, we discuss results from first-principles calculations, that indicate that the recently synthetized 2D form of silica (2D-SiO₂) displays as rich a reponse to mechanical deformations as its 3D counterpart. Under tensile uniaxial strain of up to 30% in both armchair (AC) and zigzag (ZZ) directions, 2D-SiO2 shows a non-linear elastic response, and the stress-strain curves do not display the plastic deformation platoo. At larger than 30% strains, the response is highly anisotropic: while in the AC direction the lattice breaks, in the ZZ direction it recovers the broken bonds and undergoes a process akin to a 15^o rotation of the lattice. For even larger strains another effective rotation (by 30^o) of the lattice takes place. The effective rotation is shown to occur also in the presence of vacancies in the SiO₂ layer. Moreover, 2D-SiO₂ shows an intrinsic non-linear response to bending deformations even for small deformations. We identify a metastable structure of a 2D-SiO₂ layer that is connected with the anomalous bending response of 2D silica.