Remarkable Thermochromism in the Double Perovskite Cs₂NaFeCl₆

Abstract Lead‐free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs 2 NaFeCl 6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light‐yellow to black over a temperature range of 10 to 423 K is investigated. First‐principles, density functional theory (DFT)‐based calculations indicate that the thermochromism in Cs 2 NaFeCl 6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs 2 NaFeCl 6 is up to 2.52 meVK −1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group‐IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.