Nine New Phosphorene Polymorphs with Non-Honeycomb Structures: A Much Extended Family

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