Structural Propensities in Cs ₂ MBiX ₆ (M  = Na, Ag; X  = Cl, Br) Bismuth Halide Double Perovskites

A previously unreported low-temperature phase transition in bismuth halide double perovskite Cs2AgBiCl6 is reported, thereby establishing trends in the structural ground states across Cs2NaBiCl6, Cs2AgBiCl6, and Cs2AgBiBr6. Using the combined toolkit of variable-temperature synchrotron X-ray and neutron powder diffraction, Raman spectroscopy, and density-functional theory-based electronic structure modeling, we demonstrate a cubic Fm3¯m⁡→ tetragonal I4/m transition upon cooling with distinct onset temperatures. Neutron powder diffraction refinements and DFT calculations assign the low-temperature phase of Cs2NaBiCl6 to I4/m, rather than the previously reported I4/mmm ground state. Cs2AgBiCl6 is also found to transform to a structure crystallizing in the I4/m space group at low temperatures. Temperature-dependent Raman data and density-functional-theory-based modeling capture the softening and freezing of octahedral tilt modes and quantify relative instabilities. Solid-state nuclear magnetic resonance spectroscopy at room temperature completes the characterization and helps underpin the subtle differences in the covalency across the compounds. Trends in the phase transition temperature Ts and tilt magnitudes emerge from coupled effects of halide identity, M(I)–site bonding character, and a mismatch between interatomic distances. These results establish the structure–dynamics–bonding framework for tuning tilt-driven instabilities in halide double perovskites.