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 Cs 2 AgBiCl 6 is reported, thereby establishing trends in the structural ground states across Cs 2 NaBiCl 6, Cs 2 AgBiCl 6, and Cs 2 AgBiBr 6 . 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 F m 3 ¯ m ⁡ → tetragonal I 4/ m transition upon cooling with distinct onset temperatures. Neutron powder diffraction refinements and DFT calculations assign the low-temperature phase of Cs 2 NaBiCl 6 to I 4/ m, rather than the previously reported I 4/ mmm ground state. Cs 2 AgBiCl 6 is also found to transform to a structure crystallizing in the I 4/ 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 T s 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.