Figure 16.

(a) Temperature–composition phase diagram of the mixed FA_1–xCs_xPbBr₃ system. Symmetry notation: C - cubic, T - tetragonal, O - orthorhombic. (b) Room-temperature ¹⁴N MAS NMR spectra of the FA_1–xCs_xPbBr₃ compounds. (c) Schematic cartoon illustrating the FA cation arrangement in a pure FAPbBr₃ system. Such a “T”-type (red) long-range quadrupolar order is only possible in a two-dimensional plane. (d) Incorporated Cs⁺ ion disrupts the “T”-type arrangement, as neighboring FA cations point to the positive charge of Cs⁺ causing disordering of the next-nearest neighbor quadrupoles (black arrows). Reprinted (adapted) with permission from ref (172). Copyright 2020 American Chemical Society.