  We investigate the deformation and failure mechanisms of phosphorene sheet
and nanoribbons under uniaxial tensile strain along the zigzag direction using
the density functional tight-binding method. Surprisingly, twin-like
deformation occurs homogenously across the phosphorene sheet, which
significantly increases its failure strain. Vacancies within the sheet lead to
the heterogeneous nucleation of twins at a lower critical strain which,
subsequently, propagate across the entire sheet. Twin-like deformation always
occurs heterogeneously in phosphorene nanoribbons (with or without vacancies).
Propagation of the twins is interrupted by fracture which initiates along the
ribbon edge. The underlying mechanism is bond breaking between the atoms within
phosphorene puckers and simultaneous bond formation between the atoms in
neighboring puckers. This unusual deformation behavior in phosphorene may be
exploited in novel nano-electronic-mechanical applications.
