Metastable interlayer Frenkel pair defects in black phosphorus

The low formation energy of atomic vacancies in black phosphorus allows it to serve as an ideal prototypical system for exploring the dynamics of interlayer interstitial-vacancy (I-V) pairs (i.e., Frenkel defects) which account for Wigner energy release. Based on a few-layer model of black phosphorus, we conduct discrete geometry analysis and investigate the structural dynamics of intimate interlayer Frenkel pairs from first-principles calculations. We reveal a highly metastable I-V pair state driven by anisotropic dipolelike strain fields which can build strong connections between neighboring layers. In the two-dimensional limit (monolayer), the intimate I-V pair exhibits a relatively low formation energy of 1.54 eV and is energetically favored over its isolated constituents by up to 1.68 eV. The barrier for annihilation of the Frenkel pair is 1.46 eV in the bilayer, which is remarkably higher than that of similar defects in graphite. The findings reported in this work suggest that there exist rich bridging pathways in black phosphorus, leading to stable dimensional reduction and structural condensation on exposure to moderate electron excitation or thermal annealing. This study paves the way for creating novel dimensional-hybrid polymorphs of phosphorus via the introduction of such metastable interlayer I-V pair defects.