CH₃NH₃PbI₃: precise structural consequences of water absorption at ambient conditions

The crystal structure of the pristine (I) and aged (II) crystals of CH₃NH₃PbI₃ (hereafter MAPbI₃) hybrid organic-inorganic lead iodide has been studied at 293 K with high-precision single-crystal X-ray diffraction using a synchrotron light source. We show that (I) and (II) are characterized by an identical tetragonal unit cell but different space groups: I422 for (I) and P4₂2₁2 for (II). Both space groups are subgroups of I4/mcm, which is widely used for MAPbI₃. The main difference between (I) and (II) comes from the difference in hydrogen bonds between the MA⁺ cation and the PbI₃ framework which is the direct consequence of H₂O insertion in the aged crystal (II).