Reversible Thermochromism in Cs4PbBr6 Microcrystals/CsPbBr3 Nanocrystals Based on the Synergistic Interaction between Cesium Ions and PbBr6 Octahedra

Zero-dimensional perovskites have received intensive attention due to their low formation energy and soft ionic nature. Although thermochromism has been observed in perovskite materials, little is known about the effects of A-site cations and exciton-phonon coupling on the color-switching mechanism. Here, thermochromism in a composite perovskite where CsPbBr₃ nanocrystals are embedded in a matrix of Cs₄PbBr₆ microcrystals has been reported. Reversible color switching occurs with a progressive change from yellow-green to orange in a wide temperature range of 295-495 K. It is found that the temperature-induced bandgap change can be attributed to the competing interaction between lattice thermal expansion and electron-phonon interaction. The entire heating and cooling process is accompanied by the movement of Cs⁺ and distortion of the [PbBr₆]^4- octahedron, while the former is more drastic through temperature-dependent Raman spectra and verified by Materials Studio calculations. The cation dynamics have been investigated, at the atomic scale, by using molecular dynamics simulations, which indicate Cs⁺ has more freedom to move in the lattice. This work provides insights to guide strategies for designing a material platform for diverse optical anticounterfeiting and temperature-indicating label applications and describes the emission color tuning mechanisms for further design of stimuli-responsive materials.