Concurrence of auxetic effect and topological phase transition in a 2D phosphorous nitride

The auxetic effect and topological phase transition are interesting mechanical and electronic properties of some materials, respectively. Although each has been extensively studied separately, no material has been identified to possess both properties simultaneously. Here, we report that a two-dimensional phosphorous nitride monolayer simultaneously possesses auxetic behavior and undergoes a topological phase transition under tensile strain. The monolayer has a normal-auxeticity mechanical phase transition when a tensile strain above 0.055 is applied along the P–P zigzag direction. The negative Poisson ratio can even approach as abnormally high as −0.60. Furthermore, the material is an intrinsic Dirac material, but a phase transition from the semi-Dirac material to Dirac material is observed at nearly the same critical tensile strain as that in auxetic phase transition. An electronic orbital analysis reveals that the simultaneity of the normal-auxeticity phase transition and topological phase transition originates from the variation of orbital hybridization around the Fermi level.