Ferroelastic Transition and Local Structural Properties in Cs ₂ NaBiCl ₆ :Mn ²⁺ Double Perovskites

Halide double perovskites (DPs) have been extensively investigated for their optoelectronic properties, which make them suitable for optoelectronic applications such as solar cells and LEDs. The unique crystal structure of DPs can accommodate a wide range of transition metals, thereby enhancing their chemical diversity and properties. However, this also calls for in-depth studies to determine how the composition affects these materials. In this study, we comprehensively inspect the structural transitions of Cs2NaBiCl6:Mn2+ by synchrotron X-ray elastic scattering analysis at room temperature (290 K) and low temperature (10 K). Both X-ray powder diffraction and pair distribution function analysis indicate that the low-temperature polymorph crystallizes in the I4/m space group. The improper ferroelastic structural transition Fm3̅m → I4/m is triggered by the condensation of the primary Γ4+ (T1g) soft mode, and spontaneous strain analysis reveals its second-order nature. The underlying dynamics are mainly determined by the cooperative tilting of the octahedral framework, whereas cationic positions appear fixed.