  We present a perspective on recent observations of the photoinduced phase
separation of halides in multi-component lead-halide perovskites. The
spontaneous phase separation of an initial homogeneous solid solution under
steady-state illumination conditions is found experimentally to be reversible,
stochastic, weakly dependent on morphology, yet strongly dependent on
composition and thermodynamic state. Regions enriched in a specific halide
species that form upon phase separation are self-limiting in size, pinned to
specific compositions, and grow in number in proportion to the steady-state
carrier concentration until saturation. These empirical observations of
robustness rule out explanations based on specific defect structures and point
to the local modulation of an existing miscibility phase transition in the
presence of excess charge carriers. A model for rationalizing existing
observations based on the coupling between composition, strain and charge
density fluctuations through the formation of polarons is reviewed.
