Reversible Thermal Color Switching in Two-Dimensional Lead-Free Double Perovskites

Temperature-induced color switching is a fascinating phenomenon in semiconductors, enabling applications in thermochromic windows and display technologies. Recently, halide perovskites have emerged as soft-lattice semiconductors exhibiting reversible thermochromism. Their structural diversity and lattice dimensionality play a critical role in tuning thermochromic responses. In this work, we explore thermochromism in 2D layered lead-free double perovskites and investigate the underlying electronic and phononic mechanisms. Single crystals of (BA)2CsAgBiBr7 were grown via a slow-cooling method and were found to exhibit reversible thermochromic behavior. Temperature-dependent absorption spectroscopy and powder XRD measurements reveal a pronounced redshift of the absorption edge and lattice expansion with increasing temperature. Complementary Raman spectroscopy indicates temperature-driven changes in vibrational modes, highlighting the role of enhanced phonon contributions, which is further supported by specific heat analysis. The reduction in band gap and the accompanying color variation stem from the cooperative influence of electron–phonon interactions and lattice thermal expansion. This work elucidates the fundamental mechanisms driving thermochromism in 2D layered lead-free double perovskites and offers a framework for the rational design of advanced thermochromic materials with tunable structural and optical properties.