Reversible and Irreversible Thermochromism in Copper‐Based Halide Perovskites

Abstract Copper‐based layered perovskites have shown abundant phase transitions and thermochromic behaviors. However, there are very few cases of perovskites with irreversible transitions. Here, the thermochromic behavior of two perovskites (CEA) 2 CuCl 4 and (BEA) 2 CuCl 4 is discussed, where CEA = 2‐chloroethylammonium and BEA = 2‐bromoethylammonium, the latter of which is reported for the first time. These materials exhibit a reversible and irreversible thermochromic behavior determined by the nature of the organic cation. Using nuclear magnetic resonance, X‐ray diffraction, and optical absorption, it was possible to investigate the mechanism for this transition: a topochemical exchange reaction between the organic and inorganic halides in (BEA) 2 CuCl 4 to yield the mixed‐halide perovskite (BEA) 1.1 (CEA) 0.9 CuCl 3.1 Br 0.9 . These materials and the fundamental principles shown herein have potential use as low‐cost irreversible thermochromic sensors and their design.