Figure 1.

Local symmetry breaking in MAPbI₃ at 350 K

(A) Cubic nonequilibrium structure (free of distortions) and (B) Distortions from cubic symmetry generate anisotropic cavities and couple to motion of the MA cation, which was represented by off-centered and oriented along the long axis of the cavity.

(C) DFT-based lattice dynamic calculations show that the energy minimum at the R point at 350 K is displaced in a double-well potential that causes local symmetry breaking.

(D) Comparison of the experimental PDF (purple) to cubic (Pm$\overline{3}$m), centrosymmetric (I4/mcm), and noncentrosymmetric (I4cm) tetragonal models (blue) shows a superior fit for the low-symmetry models at low r (2–8 Å). However, the models perform oppositely at high r with the high-symmetry cubic structure giving the best agreement to the data in the 12–50 Å region. The residuals (orange) are scaled ×3 for clarity.

Credit Adapted from Beecher et al. (2016).