  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 <= -1% 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 modulate the
PR of BPO from positive to negative.
