The mechanism we proposed aligns with the observed characteristics of the B↔Y phase transition in CsPbI 3 and other perovskite systems, showcasing distinct non-martensitic phase transition behaviors . Specifically, the transition does not occur simultaneously across all regions within a short timeframe. Instead, it exhibits clearly distinguishable nucleation and propagation processes , with the latter proceeding along the perpendicular direction of the B/Y interface 20 .