Fig. 4: Role of the central organic moiety in the observed phonon dynamics.

a Calculated minimum-energy path for the rotation of FA moiety between two stable symmetrically equivalent configurations. The results are shown for FAPbBr3 (maroon circle), FAPbBr1.5I1.5 (green diamond), and FAPbI3 (purple square). b Change in volume of the 2 × 2 × 2 supercell (used for our DFT calculation) as a function of Br fraction. c, d show the relaxed structures of FAPbBr3 and FAPbI3, respectively. The numbers indicate the framework cage size.