Fig. 1. (a) PXRD patterns of MAPbI_3−xBr_x (x = 0, 1, 1.5, 2, 3) revealing cubic perovskite lattices for all mixed samples and MAPbBr₃, while MAPbI₃ is in the tetragonal phase at room temperature. The zoom on the cubic 200 reflections between 27.5 and 31° 2θ highlights the sensitivity of the 2θ position (and the lattice constant) to the halide composition. (b) ²⁰⁷Pb MAS NMR spectra of a physical mixture of MAPbI₃ and MAPbBr₃, as well as the mixed halide perovskites MAPbI_3−xBr_x (x = 1, 1.5, 2). The grey shaded areas indicate the chemical shifts for different [PbI_6−xBr_x] environments.