Phosphorenes with Non-Honeycomb Structures: A Much Extended Family*

We predict a new class of monolayer phosphorous allotropes, namely, $\varepsilon $-P, $\zeta $-P, $\eta $-P and $\theta $-P. Distinctly different from the monolayer $\alpha $-P (black) and previously predicted $\beta $-P (Phys. Rev. Lett. \textbf{112}, 176802 (2014)), $\gamma $-P and $\delta $-P (Phys. Rev. Lett. \textbf{113}, 046804 (2014)) with buckled honeycomb lattice, the new allotropes are composed of P$_{\mathrm{4}}$ square or P$_{\mathrm{5}}$ pentagon units that favor tricoordination for P atoms. The new four phases, together with 5 hybrid phases, are confirmed stable by first-principles calculations. In particularly, the $\theta $-P is shown to be equally stable as the $\alpha $-P (black) and more stable than all previously reported phosphorene allotropes. Prediction of nonvolatile ferroelastic switching and structural transformation among different phases under strains points out their potential applications via strain engineering.