Stress-sign-tunable Poisson's ratio in monolayer blue phosphorus oxide

Negative Poisson's ratio (NPR) materials have attracted tremendous interest due to their unusual physical properties and potential applications. Certain two-dimensional (2D) monolayer materials have also been found to exhibit NPR and the corresponding deformation mechanism varies. In this study, we found, based on first-principles calculations, that the Poisson's ratio (PR) sign of monolayer blue phosphorus oxide (BPO) can be tuned by strain: the PR is positive under uniaxial strain but becomes negative under   >  0. The deformation mechanism for BPO under strain depends on the mutual competition between the P–P attraction and P–O repulsion effect, and these two factors induce two different deformation pathways (one with positive PR, and the other with NPR). Moreover, with increasing of strain, both the decreased strength of P–P attraction and the increased strength of P–O repulsion effect modulates the PR of BPO from positive to negative. (paper)