Local structural distortions and thermochromic properties of Cs2NaFeCl6 halide double perovskite

The enhanced structural stability of halide double perovskites compared to hybrid perovskites has aided their use in photovoltaic research. In the present study, we explored temperature-dependent band-gap engineering, leading to reversible thermochromism in the lead-free halide double perovskite of Cs2NaFeCl6. The single crystal exhibited a color change from yellow at low temperature to brown at high temperature. It is well known that electron-phonon interactions are crucial in determining the structural, electronic and optical properties of such halide perovskites, which are scarcely studied. Here, we approached both qualitative and quantitative descriptions of the root cause of the electron-phonon interaction and its effect on the electronic structure of the novel material Cs2NaFeCl6, by employing comprehensive X-ray diffraction, Raman and X-ray absorption fine structure studies. Though no structural phase transition is seen in this direct-band-gap material, local octahedral distortion of the Fe atom due to charge localization and strong electron-phonon coupling is observed, leading to changes in the electronic picture of the lattice, which are responsible for the exotic thermochromic behavior.