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

Abstract 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.