Two-dimensional lead-free double perovskite ferroelastics with dynamic thermochromism.

Ferroelastics, as a key branch of ferroic materials, are vital for mechanical switches, energy conversion, etc. While research on ferroelastic organic-inorganic hybrid perovskites has laid a preliminary foundation, in-depth exploration is still needed to expand their material system and functional integration. In this work, we constructed two perovskite ferroelastics using cyclobutylmethanaminium (CBA+) as the cationic template. First, the lead-based (CBA)2PbBr4 exhibits a ferroelastic phase transition at Curie temperature (T c) = 383.4 K (mmmF2/m). To address lead toxicity, we developed the lead-free double perovskite (CBA)4AgBiBr8 (T c = 346.7 K, 4/mmmF2/m), which not only achieves lead-free design and increases domain states from 2 to 4, but also features a narrower bandgap (2.22 eV vs. 2.97 eV in (CBA)2PbBr4). Most notably, (CBA)4AgBiBr8 shows thermochromic behavior, which is rarely observed in hybrid double perovskites. These findings not only expand the perovskite ferroelastic family but also provide a strategy for integrating lead-free design with functional properties like thermochromism.