Light: A new handle to control the structure of cesium lead iodide

${\mathrm{CsPbI}}₃$ is of high interest for photovoltaic applications owing to its low band gap, provided it could be stabilized in its $\ensuremath{\gamma}$ perovskite phase. Instead, it energetically prefers to adopt a so-called yellow, $\ensuremath{\delta}$ phase, with a much larger band gap and reduced photovoltaic properties. Here, using an original constrained density functional theory method, we mimic the effect of thermalized photoexcited carriers, and show that larger concentrations in photoexcited carriers (i.e., larger optical pump fluences) effectively reduce the energy difference between the $\ensuremath{\delta}$ nonperovskite ground state and the $\ensuremath{\gamma}$ perovskite phase. Even further, the stability of the phases could be potentially reversed and therefore the $\ensuremath{\gamma}$ phase stabilized under strong illumination. We also report large photostriction, i.e., large photoinduced strain for all phases in this material, making ${\mathrm{CsPbI}}₃$ suitable for other applications such as photodriven relays and photoactuators.