Regulation of the order–disorder phase transition in a Cs2NaFeCl6 double perovskite towards reversible thermochromic application

Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields.Herein,an environmentally-friendly,low-cost,high-throughput Cs2NaFeCl6 single crystal with exceedingly high thermal stabil-ity is designed and grown.It obtains a cubic lattice system in the temperature range of 80-500 K,accompanied by a com-pletely reversible chromatic variation ranging from yellow to black.Importantly,the intriguing thermochromism is proved to own extremely high reproducibility (over 1000 cycles) without a hysteretic effect,originating from its structural flexibility that in-cluding (i) the noteworthy distortion/deformation of[NaCl6]5-and[FeCl6]3-octahedra;(ii) order-disorder arrangement trans-ition of[NaCl6]5-and[FeCl6]3-octahedra as the function of temperature.This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs2NaFeCl6 perovskite.