This PDF file includes:

• Tables of atomic parameters for the three structural phases
• Preferred orientation of FA⁺ cation in the hexagonal phase
• Synchrotron x-ray diffraction measurements
• Confirmation of thermal equilibrium during neutron measurement
• Kinetic trapping of the cubic structure by thermal quenching
• Details on DFT calculations
• table S1. Refined structural parameters of FAPbI₃ for 390 K.
• table S2. Refined structural parameters of FAPbI₃ for 220 K.
• table S3. Refined structural parameters of FAPbI₃ for 15 K.
• fig. S1. Orientation of FA⁺ cation.
• fig. S2. Refined crystal structure.
• fig. S3. Synchrotron x-ray powder diffraction data for FAPbI₃.
• fig. S4. Check for thermal equilibrium during elastic neutron scattering measurements.
• fig. S5. Temporal monitoring of the stability of the cubic phase at 8.2 K after thermal quenching.
• References (27–31)

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