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 CsPbI₃ into the yellow phase. The impact of this strain on the phase stability of CsPbI₃-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.