Thermochromism in Bismuth Halide Perovskites with Cation and Anion Transmutation

Abstract Bismuth halide perovskites are highly promising for various applications owing to their lower toxicity and greater environmental stability. The temperature‐dependent color tunability of these materials renders them a strong contender for use in smart window applications and temperature sensors. To delve deeper into their properties, the thermochromism and fundamental lattice dynamics are investigated in bismuth halide perovskites through the transmutation of cation and anions, specifically focusing on Cs 3 Bi 2 Br 9 , MA 3 Bi 2 Br 9 , Cs 2 AgBiBr 6 , and Cs 2 AgBiCl 6 perovskites. Absorption spectroscopy and powder‐XRD patterns as a function of temperature reveal that 2D‐layered Cs 3 Bi 2 Br 9 and MA 3 Bi 2 Br 9 perovskites exhibit significantly greater bandgap changes and thermal expansion compared to 3D connected Cs 2 AgBiCl 6 and Cs 2 AgBiBr 6 perovskites attributed to more space in the 2D connected lattice. The reduction in bandgap and color variation in these perovskites stems from the intricate interplay between electron‐phonon interactions and thermal expansion. Notably, it is found that the type of cation and anion play a crucial role in influencing the degree of thermochromism, as the electron‐phonon coupling depends on this transformation. This study sheds new light on the fundamental mechanisms underlying the thermochromic behavior of bismuth halide perovskites and paves the way for the rational design and engineering of novel thermochromic materials with tailored properties.