Herein, the convenient interplay between Eu incorporation and morphology to form the perovskite (PSK) γ‐phase of CsPbI3 at 80 °C (low temperature [LT] γ‐phase) is unveiled. In contrast, CsPbI3 without Eu exists in a mixture of γ‐phase and non‐PSK δ‐phase at 65 °C or in a fully δ‐phase at 80 °C. Based on experimental and theoretical findings, an argument about a double beneficial role of Eu is presented. On one hand, it assists in the formation of the γ‐phase either by substituting Pb or by occupying interstitial positions in the CsPbI3 lattice. On the other hand, it indirectly promotes the formation of a fine‐grained layer at LT wherein the high surface‐to‐volume ratio makes the establishment of the δ‐phase unfavorable. Strain accommodation in the fine‐grained matrix and the formation of a gluing intergrain‐self‐material during the kinetics of reaction (snowplow effect) cooperate in extending the lifetime of the …