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.