Transition Metal Compounds as Thermochromic Materials: Classification, Properties, Limitations, and Applications

ABSTRACT Thermochromic materials are gaining increasing interest with the merits of adaptive optical response to temperature/heat, with potential applications in thermal indicators, energy‐efficient smart windows, thermal distribution monitors, laser warning markers, etc. Transition metal compounds are the most classic inorganic pigments known since prehistoric culture. In this review, we summarize the recent research progress of inorganic thermochromic materials, especially focused on transition metal compounds. The performance of inorganic thermochromic materials is closely related to their crystal structure and electronic structure, especially the d‐orbital electrons of transition metals, which play a key role in the response to temperature changes. The classification based on the composition, mechanism, and working principles of the thermochromic materials is clarified first. Then, the materials based on different transition metal compounds are summarized for varied d‐electron configurations of transition metal cations in binary and multinary metal oxides, halides, sulfides, and salts. The mechanisms of thermochromism are summarized accordingly, including phase transition, charge transfer, and bandgap change. Then, the applications in the fields such as smart windows, radiative cooling, and temperature sensing are reviewed. Finally, we propose a perspective on the challenges and future development directions in the design, fabrication, and applications of thermochromic materials.