Strain analysis of black‐to‐yellow phase transitions in CsPbI₃

All‐inorganic halide perovskite cesium lead iodide (CsPbI 3 ) has emerged as a promising semiconductor and optoelectronic material. However, its optically active black phases easily transform into the non‐optically active yellow phase, limiting practical applications. Here, strain variations during phase transitions in CsPbI 3 are investigated using a strain/order parameter coupling model, based on lattice parameters measured over the temperature range of 100–650 K. Structural and thermal analyses suggest that the large positive ( e 1 – e 2 ) strain is the primary driver responsible for the transformation of black CsPbI 3 into the yellow phase. The impact of this strain on the phase stability of CsPbI 3 ‐based films is further validated by comparing films on different substrates and with varying compositions. Our findings uncover the key metric that determines the phase stability of CsPbI 3 , providing insights for the design of stable, optically active all‐inorganic halide perovskite.