Figure 1. Crystal growth process and powder X-ray diffraction.

(a) Schematic representation of the ITC apparatus in which the crystallization vial is immersed within a heating bath. The solution is heated from room temperature and kept at an elevated temperature (80 °C for MAPbBr₃ and 110 °C for MAPbI₃) to initiate the crystallization. (b,c) MAPbI₃ and MAPbBr₃ crystal growth at different time intervals. (d,e) Powder X-ray diffraction of ground MAPbBr₃ and MAPbI₃ crystals. Insets: pictures of the corresponding crystals grown within a non-constraining vessel geometry.