  ZnS is a brittle material but shows extraordinary plasticity during
mechanical tests performed in complete darkness. This phenomenon is known as
the photoplastic effect, whose underlying mechanisms have long been unclear. We
study the impact of light, via photoexcited charge carriers, on the dislocation
core structure and mobility using first-principles calculations. We calculate
the core structure and the charge-dependent Peierls barriers of the glide set
of Shockley partial dislocations in ZnS. Our findings reveal that locally
charged dislocations capture excess carriers in the system, leading to core
reconstructions that alter the Peierls barrier, resulting in higher barriers
and lower mobility for these dislocations. This altered and asymmetric
mobility, depending on dislocation character (edge or mixed) and local
stoichiometry (Zn or S rich), is responsible for the brittle behavior of ZnS
under light exposure and will be reversed in complete darkness.
