Reversible Thermochromism in All-Inorganic Lead-Free Cs3Sb2I9 Perovskite Single Crystals

Here, lattice expansion modulated reversible thermochromism (TCM) in all-inorganic lead-free Cs3Sb2I9 perovskite single crystals (SCs) is demonstrated. Upon temperature change, Cs3Sb2I9 SC exhibits a reversible color change between red and dark-brown. The corresponding optical band-edge modulation and structural phase transition are monitored by temperature dependent UV-vis absorption spectroscopy and X-ray diffraction study. This observed reversible TCM is attributed to the lattice expansion and strain relaxation at elevated temperatures. Additionally, Raman spectroscopy confirms enhanced electron-phonon coupling originating from stretched Sb-I terminal bonds at elevated temperature. This work validates that the all-inorganic Cs3Sb2I9 perovskites have the potential in smart thermochromic windows and temperature sensors applications.